Does added stiffness always degrade ride quality?

[RPS]

Is there a valid theory suggesting added vertical stiffness always degrades a bicycle’s ride quality? When comparing equipment and our personal experiences it seems most of us presume added vertical stiffness always degrades ride quality, and conversely we presume greater vertical compliance is a good thing that always leads to a smoother ride. However, if this is not always true, can it mean that some of the random data we collect in our heads while out riding is erroneous?


From a European safety paper on vibration and devices meant to reduce its effect on humans:
All suspension devices have a frequency range over which they amplify rather than reduce vibration and an incorrect choice could easily result in increased exposure to vibration.
Can the above statement be applied to cycling? If so, how should we compare vertically stiff versus vertically compliant components when conditions may favor one and some times the other? When a guy says a new part/component rides great can we know for sure it will do the same for us?

[eddief]

As a career counselor and writer of killer resumes, I can honestly say those are really good questions. As a non-scientist-type, I can believe the quote. The challenge with applying to cycling the way we tend to do here is really difficult as there are so many variables in the opinions/information we are sharing.

road surface
rider weight
tube dimensions
tire size
tire quality
tire materials
tire inflation
saddle padding
shorts padding
butt fat
level of hangover
amount of sleep
amount of hunger
amount of sexual satisfaction

That is why I keep trying new things, cause I want to feel/believe each of the bikes I own rides just right...whatever that is.

[AndrewS]

I would have a hard time believing that the quoted statement could have much application to road cycling, where the amplitude is relatively low.


The test of your question could be a full suspension road bike. Mechanical suspension offers giant vertical compliance with minimal lateral flex. Are they inefficient or otherwise problematic?

In the end, I think that cycling itself is tiring, and road buzz and bumps are only a small component of that. So we can tolerate a pretty wide range of abuse before we declare a particular bicycle "too stiff".

[rice rocket]

The study of vibration frequency and harmonics has become a focus in many high levels of motorsport. That's chassis harmonics, engine/drivetrain harmonics, etc., although their focus is more making sure cars/engines don't rattle themselves apart.

Haven't put much thought into applying it into a bike setting, but the same theories apply. Intuition would say that ride quality isn't dependent on the aggregate amount of vertical compliance you have, but also being able to damp the natural frequency of the frame, wheels, etc.

Not sure how many small bike manufacturers are gonna do a harmonics study on their frames though.

[eddief]

I still wonder if a curved blade steel fork rides any differently than a straight blade steel fork? It looks like it should, but when built into the frame, tire, inflation equation could anyone actually detect/feel the difference? Or could a test bike equipped with a scientific measurement device detect any difference?

[Mark McM]

The quote from the paper sounds like it is referring to resonance, which occurs whan an input vibration (excitation) is close to the natural frequency of the system. Most suspension systems have some type of 'spring' to provide compliance, which in turn lowers the natural frequency of the system, perhaps into the range of the excitation frequency.

But I don't think that will be much of an issue on a rigid framed bicycle, because the stiffness/mass ratio puts the natural frequency very high, and likely above the 'excitation' frequency of road vibration.

Consider: A typical automobile has 20 times the mass of a bicycle plus its rider. Yet the automobile has a softer (less stiff) suspension than a rigid framed bike. This puts the natural frequency of the automobile suspension at a much lower frequency than a bike, and is why an automobile requires dampers (shock absorbers) to quell vibration, lest the automobile become uncontrollable at certain road speeds (i.e. certain excitation frequencies).

[Louis]

From a European safety paper on vibration and devices meant to reduce its effect on humans:

All suspension devices have a frequency range over which they amplify rather than reduce vibration and an incorrect choice could easily result in increased exposure to vibration.

When a guy says a new part/component rides great can we know for sure it will do the same for us?

This may be a trick question, but I'll bite:

1) In life you can rarely be 100% sure about anything, so there will always be variation in how you experience things compared to someone else, whether you're talking suspension or shoe fit.

2) A given design can be selected to behave in a certain manner over a range of rider characteristics. Adult cyclists typically range in weight from 100-200 lbs, so the designer can safely assume that he does not need to worry about a 25 lb rider on an adult-sized frame. That way he can be fairly certain that the range of frequencies over which the suspension system can be predicted, given a range of mass for the rider. That should allow both Rider A and Rider B to use the same bike and have both of them avoid the coincidence of resonant frequencies and forcing frequencies.

3) Of course, many suspensions can be adjusted for the rider to compensate for various preferences or built-in characteristics.

[Ken Robb]

I still wonder if a curved blade steel fork rides any differently than a straight blade steel fork? It looks like is should, but when built into the frame, tire, inflation equation could anyone actually detect/feel the difference? Or could a test bike equipped with a scientific measurement device detect any difference?

I think it was Grant Petersen who said the curved forks he loves wouldn't ride any softer than a straight-legged fork if they were designed and built with the same parameters. His "proof" was the fact that more than minimal flexing of the steel would cause the paint to flake off. :beer:

[Mark McM]

I think it was Grant Petersen who said the curved forks he loves wouldn't ride any softer than a straight-legged fork if they were designed and built with the same parameters. His "proof" was the fact that more than minimal flexing of the steel would cause the paint to flake off. :beer:

Which again demonstrates that Grant Petersen doesn't use a lot critical thinking in his product claims.

All this proves is that neither type of fork flexes enough to flake the paint off, but says nothing about if one flexes more than the other.

Theoretically, a curved fork should have a little more flex (to vertical loads) than an otherwise identical straight fork. But since the bending loads down near the axle are so small, and because an angular flex near the axle results in smaller total deflections than the same angular flex further up the fork, the reality is that the vast majority of the fork deflection is due to flex near and in the crown and steerer. There are many factors that influence fork flex, and the curve of the blades near the axle is one of the least significant.

[RPS]

This may be a trick question, but I'll bite:

2) A given design can be selected to behave in a certain manner over a range of rider characteristics. Adult cyclists typically range in weight from 100-200 lbs, so the designer can safely assume that he does not need to worry about a 25 lb rider on an adult-sized frame. That way he can be fairly certain that the range of frequencies over which the suspension system can be predicted, given a range of mass for the rider. That should allow both Rider A and Rider B to use the same bike and have both of them avoid the coincidence of resonant frequencies and forcing frequencies.
No Louis, it’s not a trick question at all. Maybe a little rhetorical, but I’m not out to get anyone.

Mostly I wanted to continue the technical discussion that evolved in the “Stiff Wheel Question” thread but didn’t want to hijack that thread or go too far off topic. However, 1centaur raised some good questions regarding perceptions vs. theory vs. reality. They don’t always agree well although they should to a greater degree. I’m also interested in knowing why one guy may think that a new carbon wheelset is the greatest thing since sliced bread and another guy thinks they suck.


BTW, you make an excellent point regarding the 2:1 weight range of riders not being all that great, but when combined with differences in forcing frequencies due to different roads and speeds, don’t you think that what a rider experiences when making incremental changes can vary from one person to the next? Also, is comparing 2:1 rider mass much different than an unloaded pickup truck riding harshly while the same truck when overloaded with its own weight would feel completely differently? Granted we are talking about natural frequencies that are completely different between a pickup and a bike, but if a mass ratio of 2:1 can affect the truck so much, why not the bike?

[RPS]

I would have a hard time believing that the quoted statement could have much application to road cycling, where the amplitude is relatively low.

The test of your question could be a full suspension road bike. Mechanical suspension offers giant vertical compliance with minimal lateral flex. Are they inefficient or otherwise problematic?

In the end, I think that cycling itself is tiring, and road buzz and bumps are only a small component of that. So we can tolerate a pretty wide range of abuse before we declare a particular bicycle "too stiff".
How much amplitude do you think is required before a person can notice a difference? At very high frequencies like we experience on rough chip seal roads amplitude has to be very low. It’s low enough that devices like gloves can make a difference. On the other hand at lower frequencies amplitude can be much higher, like when riding on cobbles. For that gloves aren’t very effective in reducing vibration because they don’t have enough thickness (i.e. – ability to give).

From the same paper:

Grip coatings made of rubber or of special elastic material can reduce the vibration transmission at the point of contact in a similar way to anti-vibration grips, but due to limits of material thickness, only high-frequency vibration can be reduced significantly (above 200 Hz).

….and……

In the early 1980s, so-called ‘anti-vibration’ gloves became available and a laboratory test procedure was developed that provides a uniform and reproducible basis for evaluating the vibration-insulation effects of such gloves (EN ISO 10819). For frequency ranges higher than 150 Hz, a vibration transmission factor of TRH ≤ 0.6 is typical for an effective glove, but for lower frequencies, anti-vibration protection gloves are not useful.

[AndrewS]

How much amplitude do you think is required before a person can notice a difference? At very high frequencies like we experience on rough chip seal roads amplitude has to be very low. It’s low enough that devices like gloves can make a difference. On the other hand at lower frequencies amplitude can be much higher, like when riding on cobbles. For that gloves aren’t very effective in reducing vibration because they don’t have enough thickness (i.e. – ability to give).

From the same paper:
None of those support the notion that the amplitude of bicycle related vibration could be increased by the addition of the type of suspension used in bicycles.

If one continuously road on rumble strips, that would be a frequency where bicycle suspension might just increase the vibration, but even cobblestones don't have the kind of continuous input that would make that sort of doubling likely. That comes from a very regular vibration and a suspension with a similar frequency.

My gut feeling is that you could have almost any amount of suspension in a bicycle and it wouldn't matter. On the other end of the scale, you could increase the stiffness of the frame as much as you want (1987 Cannondale) and it would still be rideable with the right tires.

The fact is that even a "rigid" road bike has multiple layers of suspension - tires, spokes, fork, frame, bars, tape, seat, legs and arms. All of them have different frequencies and all of them reduce vibration and shock. Making the frame vertically compliant isn't the first line of suspension and can only contribute so much when the effect of the other layers is factored.

[timto]

I think it was Grant Petersen who said the curved forks he loves wouldn't ride any softer than a straight-legged fork if they were designed and built with the same parameters. His "proof" was the fact that more than minimal flexing of the steel would cause the paint to flake off. :beer:

I just read an issue where jan heine puts some measuring devices on various forks and tests their deflection - and makes some conclusions about rake style, blade shape and straight vs raked.

The jist was he did measure marked effects on a imperial shaped blade with rake low near the hub. FWIW. I found it interesting reading. I could see how this may be a good thing for riding around but might not be what i'd actually want in a tight corner in a crit after having gone in a little hot...

[Wilkinson4]

I had a conversation about straight vs. curved with Matt Chester a year or so ago. His take:

"Blade selection makes a lot more difference regarding 'feel' than the shape of the curve on the tube. I agree that classic curves look better though."

mIKE

[Ken Robb]

Which again demonstrates that Grant Petersen doesn't use a lot critical thinking in his product claims.

All this proves is that neither type of fork flexes enough to flake the paint off, but says nothing about if one flexes more than the other.

Theoretically, a curved fork should have a little more flex (to vertical loads) than an otherwise identical straight fork. But since the bending loads down near the axle are so small, and because an angular flex near the axle results in smaller total deflections than the same angular flex further up the fork, the reality is that the vast majority of the fork deflection is due to flex near and in the crown and steerer. There are many factors that influence fork flex, and the curve of the blades near the axle is one of the least significant.

Geez, don't blast Grant when I paraphrase what I THINK he wrote. Actually he said pretty much what you did. He was asked if he used curved forks for a softer ride due to flexing at the curves and he said there wasn't a lot of flex at the curves but he really liked the look. Then to help the questioner "get it' he added the line about paint not sticking to steel that flexes much.

[RPS]

None of those support the notion that the amplitude of bicycle related vibration could be increased by the addition of the type of suspension used in bicycles.
Andrew, thanks for sharing your opinions on this subject matter, but I don’t know how you made the leap to bicycle road suspension. I’d rather not discuss that subject if for no other reason than very few riders have suspension road bikes, therefore most forum members won’t be able to relate to much of anything discussed in that context. Suspension effect from wheels, frame/forks, saddles, tires, etc… is another matter and more relevant in my opinion. I will only say I disagree with your general conclusion about suspension but won’t debate it in this thread.

[RPS]

On the subject of steel fork blades, let’s not forget that it may involve more than the shape of the fork blades affecting vertical stiffness. In addition to what has already been stated above, any “incremental” vertical suspension effect that comes from the blades flexing ever so slightly requires the front wheel to move back and forth (i.e. – horizontally) a greater amount than it moves vertically. I haven’t calculated it but suspect that a typical curved steel blade improves the ratio of vertical to horizontal movement. I think this could be significant in providing an incremental ride benefit due to other factors.

Beyond straight vs. curved fork blades themselves there is also head tube angle. A slack head tube angle places fork blades in greater bending which in theory would provide a greater suspension benefit. And since a slack HTA normally requires more fork rake the potential “suspension” difference is even greater. Reminds me of those city bikes in Europe with extremely low HTA and lots of curved rake.

And if we assume that fork blades flex ever so slightly to provide some incremental benefit, then we should also consider the effect of the steerer tube. The longer the head tube the more the steerer will flex thereby providing more incremental suspension effect. We could go on listing things that should theoretically provide incremental vertical compliance, but the question still remains whether they do any good. ;)

[RPS]

The quote from the paper sounds like it is referring to resonance, which occurs whan an input vibration (excitation) is close to the natural frequency of the system. Most suspension systems have some type of 'spring' to provide compliance, which in turn lowers the natural frequency of the system, perhaps into the range of the excitation frequency.

But I don't think that will be much of an issue on a rigid framed bicycle, because the stiffness/mass ratio puts the natural frequency very high, and likely above the 'excitation' frequency of road vibration.

Consider: A typical automobile has 20 times the mass of a bicycle plus its rider. Yet the automobile has a softer (less stiff) suspension than a rigid framed bike. This puts the natural frequency of the automobile suspension at a much lower frequency than a bike, and is why an automobile requires dampers (shock absorbers) to quell vibration, lest the automobile become uncontrollable at certain road speeds (i.e. certain excitation frequencies).
Mark, obviously the statement relates to avoiding resonance. And that’s really the issue I hoped to discuss as it relates to ordinary “rigid” road bikes. Suspension bikes are a different issue altogether. You raise a good question – really the key question to the discussion – of whether our bicycles’ natural frequency at times may match the excitation frequencies created by road conditions. If it did, it would confirm that our tweaking or fine tuning by replacing bike parts could actually make things better “or” worse. It would also imply that data we collect may not be transferable to others with equal results.

Although you think it’s unlikely, I suspect that most riders have encountered resonance at some point while riding. I have a few times and it’s not something one forgets quickly. The most severe happened about 12 years ago riding from Houston to Austin on an old bike (very stiff aluminum with very stiff aero 650C wheels) which suddenly came alive making me feel like I was sitting on a jackhammer. And all it took was a change in speed to reduce the severe vibration. There was a long stretch of concrete that had tooling marks from construction equipment and as I recall at a speed between 15 and 20 MPH the bike felt like it was going to come apart. Slowing down or speeding up made all the difference in the world. Based on a rough measurement of the tooling marks’ pitch I roughly estimated the forcing frequency was probably in the range of 100 to 150 Hz.

I think it is reasonable to assume that under different speeds and surfaces forcing frequencies in the range of 30 to over 200 Hz are not all that uncommon. And if so, at times we are going to get that extra “buzz” out of our bikes. I’d guess it is not that unusual to encounter resonance. Anyone who had to slow down or speed up to make their bikes ride smoother has probably been there.

[AndrewS]

Andrew, thanks for sharing your opinions on this subject matter, but I don’t know how you made the leap to bicycle road suspension. I’d rather not discuss that subject if for no other reason than very few riders have suspension road bikes, therefore most forum members won’t be able to relate to much of anything discussed in that context. Suspension effect from wheels, frame/forks, saddles, tires, etc… is another matter and more relevant in my opinion. I will only say I disagree with your general conclusion about suspension but won’t debate it in this thread.
I was talking about the suspension of flexing stays and fork blades, not "active" suspension of a mountain bike. I used the word "suspension" because that is the language of the article you quoted. A rigid road frame is a suspension device.

You bothered to quote something that suggested that road bicycles could conceivably be built with enough vertical compliance in their "suspension" to produce a type of resonance that they would increase road vibration, and that's what I was writing about. It seems entirely unlikely that this type of suspension could ever contribute to increased vibration for the reasons I listed. Mark responded pretty much in kind.

If you weren't talking about road bicycles, I don't see why you quoted that.

[auto_rock]

I think the quote in the OP was referencing spring-mass-damper suspension, which is very frequency dependant. However, a road bike, at least in the frame, has negligible damping and very high stiffness, so the same phenomena are not relevant. Vertical compliance, as far as I know, improves ride quality by allowing deflection under sudden vertical loading at the wheel. A design which increases compliance will increase the amount if deflection for a given vertical force, and vice versa.

Flick an old steel frame and you'll hear, as previously mentioned, it has a high resonance frequency.

-John

[RPS]

.....snipped......
I used the word "suspension" because that is the language of the article you quoted. A rigid road frame is a suspension device.
.....snipped.....
Sorry about the confusion – I quoted from the article exactly as written even though their choice of words was unusual at times.

[RPS]

I think the quote in the OP was referencing spring-mass-damper suspension, which is very frequency dependant. However, a road bike, at least in the frame, has negligible damping and very high stiffness, so the same phenomena are not relevant. .....snipped......

-John
John, I think fundamental theories still apply so they are still relevant, the magnitude of the numbers are just way different. If there are no dampers like shock absorbers but one relies on the material itself, then the damping coefficient for the material is used. That’s what is behind many claims that carbon items ride better – the damping coefficient is higher than say that of aluminum.

Also the very high relative stiffness of a frame and the very low mass just means the natural frequency of vibration will be much higher, but the general characteristics are pretty much the same. IMO vibration theories should hold fine as long as applied correctly.

[AndrewS]

John, I think fundamental theories still apply so they are still relevant, the magnitude of the numbers are just way different. If there are no dampers like shock absorbers but one relies on the material itself, then the damping coefficient for the material is used. That’s what is behind many claims that carbon items ride better – the damping coefficient is higher than say that of aluminum.

Also the very high relative stiffness of a frame and the very low mass just means the natural frequency of vibration will be much higher, but the general characteristics are pretty much the same. IMO vibration theories should hold fine as long as applied correctly.
How do you apply them, though? As I pointed out, the frame isn't even the primary "suspension" of a bicycle. On the scale of the 200 lbs. rider/bike unit at 20 mph, there is neither the level of regular vibration or frequent bumps that can cause a feedback loop (resonance) in a system with so many layers of suspension and damping.

When I flew helicopters vibration and resonance was constantly on my mind, because of all the different parts turning at radically different RPMs under forces that were large enough to tear the airframe apart. Bicycles are subject to incredibly tiny forces compared to the mass involved - so much so that we don't even balance our wheels, despite 50+ mph descents.

This discussion just seems like arguing about the aerodynamic drag of different handlebar tapes - the type of effects you're talking about don't happen at this scale.

[RPS]

This discussion just seems like arguing about the aerodynamic drag of different handlebar tapes - the type of effects you're talking about don't happen at this scale.
OK, in your opinion what physically happens at “this scale” to differentiate bikes?

Funny that there are literally 100s of threads questioning the ride quality effect of all kinds of things; including carbon vs. aluminum bars, frame or seatpost materials, tire size, tire pressure, wheels and more wheels, etc…. Not too many on aero benefit of bar tape but certainly on “damping” quality of bar tape, gloves, carbon seatstays, etc…. And they keep coming up so it must be a real issue.

If not as stand-alone changes, these items must at least make a difference in combination, right? Otherwise wouldn’t all bikes ride exactly the same? If the correct answer is that frames are too rigid to do anything useful, wheels don’t have enough vertical give compared to tires to make a difference, etc… then what does make a difference? Is it always going to come down to tires because they have the greatest amount of vertical compliance? :confused:

[AndrewS]

I wasn't implying that vibration is not a factor on bicycles. I, once again, was talking about the negative effects of suspension in the article you quoted. The various means of bicycle suspension (tires, frame, spokes, tape) are not going to have a the effect of amplifying road vibration for the reasons I stated. Those suspension amplifying effects are the ones that don't scale down to bicycles.


On the other end of the scale - can bicycles be too stiff? Of course, we all know that. BUT, the stiffness of any one component of a bicycle, including the frame, can be mitigated by the flexibility and damping of other components. An incredibly harsh old Cannondale's ride could be made up for by 28c tires and 36 spoke wheels with lower tension, thinner spokes. No one bothers to do this, but that doesn't mean it can't be done. You won't end up with identical rides, anymore than two instruments playing the same note are going to sound the same, but the net effect on rider effeciency and fatigue may be similar.

[RPS]

I wasn't implying that vibration is not a factor on bicycles. I, once again, was talking about the negative effects of suspension in the article you quoted. The various means of bicycle suspension (tires, frame, spokes, tape) are not going to have a the effect of amplifying road vibration for the reasons I stated. Those suspension amplifying effects are the ones that don't scale down to bicycles.
I don’t understand on what technical basis you assume this.

Any time there is forced vibration with little or no damping the amplitude of motion can easily exceed 5 or more times the forcing amplitude. All that is needed is for the forcing frequency (of the road) to come close to the natural frequency (of the bike or components). Under “typical” riding conditions we hope and expect our bikes to significantly diminish any road conditions before they get to our butts, hands, and feet, but when things don’t work like we want magnification is potentially a very real problem, not one some riders imagine.

I clearly understand that 5 or 10 times very little amplitude may still be very little amplitude (except for the case of zero), but is it not more than one started with and maybe more than the rider next to you is experiencing? And how do we know that someone can’t tell the difference when our own bike may not magnify anything at all under the same conditions?

Just stating a wheel or a frame is too stiff to deform enough to be perceived is not a good answer in my opinion.



OK, we can beat this to death and not agree on much, so it’s time for an afternoon ride. :beer:

[Ken Robb]

is this how engineers' bar fight? :)

[bshell]

3-4 months ago I had the opportunity to ride Look's new 695 super duper w/the one piece carbon crank (standard & compact in one!) and the adjustable carbon stem.

I haven't been on tons of bikes in my time (previously on a Ti Legend for @5 years, I've been on a Look 585 for the last 5), but this was hands down the most miserable ride I've ever been on and I was kinda shocked(our roads are not great). Tire pressure/wheels/etc. were all normal for me. I can't imagine riding this thing unless my county were paved in sheet glass. Oh, the suck!!!!

I traded out for @10 minutes with a Specialized SL2 (also carbon wheels, and tire pressure in the ball park) and it was night and day more pleasurable. I've had an unfounded aversion to the red S for some time now but it was a great frame and I'd take that thing over the 695 in a heartbeat.

All this just to unscientifically say that there is something dramatically different in these two pricey carbon machines w/ all carbon bits.

Give me some compliance, yo!

[MadRocketSci]

i've just kinda glanced through this, as I'd rather talk about the nuances of pizza (:)), but I just wanted to throw out a distinction.

Each individual tube will have a harmonic natural frequency, which is what it rings at when you flick it with something like your finger or hammer. This will be pretty low amplitude and high frequency.

The bike structure itself, consisting of the tubes joined together, will have bending modes, corresponding to each degree of freedom in the system. One example of this may be sinusoidal side to side motion of the headtube. These will be at lower frequencies and higher amplitudes.

The "buzzy" type vibration is probably the individual tubes ringing, if the road disturbance has decent content (fourier decomposition) at the tubes' natural frequencies. The oscillations in the frame that are larger amplitude, like a shimmy type shaking, probably involve bending mode frequencies of the frame.

[auto_rock]

IMO vibration theories should hold fine as long as applied correctly.

Absolutely.

is this how engineers' bar fight? :)

Absolutely.

[RPS]

is this how engineers' bar fight? :)
Great post. We should all be so civilized while fighting. ;)

I’ve been thinking about the issue of relative stiffness and what we can detect for a long time. The recent wheel thread reminded me of the similar presumption that because running shoes are far more cushioned than either concrete or asphalt surfaces that it shouldn’t make much difference which surface we run on. Granted that wearing shoes versus running barefoot makes more difference and is most important for me (equivalent to soft tires are the first line of defense), but I think many runners will agree that running on concrete feels much different on the legs than running on asphalt even though common sense from a purely analytical standpoint may question why.

[RPS]

Here is an example that should be easy to visualize without an engineering or science background.

My large van develops a slight annoying vibration at 72 to 73 MPH when the tires are not perfectly balanced (pretend balancing tires is not a permanent solution :rolleyes: ). I happen to drive in the 70 to 75 MPH range a lot, so I encounter this problem regularly.

If I stiffen the front suspension by replacing the springs I could move that critical speed to something like 80 MPH where I would not encounter the problem. At lower speeds the ride would likely feel a little harsher but while cruising for hours at a time down the interstate this annoying vibration wouldn’t become an issue.

I’m aware that from a technical standpoint I might be able to do the same with lighter tires and wheels but suspect the amount of possible change is probably less. In this case a little stiffer ride may be better.


Bikes are not sensitive to speed like cars because lack of balance isn’t an issue. However, because of their inherent stiffness compared to cars slight variations in road conditions can affect them a lot more.

[AndrewS]

Here is an example that should be easy to visualize without an engineering or science background.

My large van develops a slight annoying vibration at 72 to 73 MPH when the tires are not perfectly balanced (pretend balancing tires is not a permanent solution :rolleyes: ). I happen to drive in the 70 to 75 MPH range a lot, so I encounter this problem regularly.

If I stiffen the front suspension by replacing the springs I could move that critical speed to something like 80 MPH where I would not encounter the problem. At lower speeds the ride would likely feel a little harsher but while cruising for hours at a time down the interstate this annoying vibration wouldn’t become an issue.

I’m aware that from a technical standpoint I might be able to do the same with lighter tires and wheels but suspect the amount of possible change is probably less. In this case a little stiffer ride may be better.


Bikes are not sensitive to speed like cars because lack of balance isn’t an issue. However, because of their inherent stiffness compared to cars slight variations in road conditions can affect them a lot more.

Good example. How about this solution:

1. Remove you van's wheels and replace them with 5 foot diameter spoked wheels.
2. Remove the body from the van, reducing the suspended rigid weight of the van by 2000 pounds.
3. Stand 20 people in/on the van so you now have 3500 lbs of flesh suspended on muscle.

You have now duplicated a bicycle's multilayered suspension/dampening system. Every one of those parts - tires, spokes, frame, suspension and people each have a different harmonic, travel and dampening rate. The net effect is that you will not see resonance effects like your largely rigid, single suspension van used to. You will still feel road chatter and big bumps, but none of them will have a chance to amplify themselves throughout such a complex system.

Bicycles are rigid in the sense that they are not able to absorb much amplitude, but they have the ability to absorb a wider range of frequencies than a vehicle with only a single kind of suspension. That makes resonance difficult to start and propogate.

[RPS]

You have now duplicated a bicycle's multilayered suspension/dampening system. Every one of those parts - tires, spokes, frame, suspension and people each have a different harmonic, travel and dampening rate. The net effect is that you will not see resonance effects like your largely rigid, single suspension van used to. You will still feel road chatter and big bumps, but none of them will have a chance to amplify themselves throughout such a complex system.
I don’t dispute that bicycles are different than cars. However, what I don’t understand aside from your sarcasm is what makes you think that other than differences in numerical values that a car’s “tires, wheels, frame, suspension and people don’t each have a different harmonic” that are analytically comparable to that of a bicycle? The numbers are different, but so what? How does that make it impossible for magnification not to occur under the right conditions?

There is no doubt that systems with multiple degrees of freedom are more complex to analyze and to visualize how the pieces interact, but the thing to consider is that when there are numerous degrees of freedom there are also more natural frequencies. And any one of these semi-independent components can reach resonance at its given frequency.

In the case of a cargo panel van the body can resonate at a high frequency making noise that is disturbing. My front wheels can vibrate at 13 to 14 cycles per second which is much higher than the suspension’s 1 to 2 Hz. The frame vibrates at its own frequency. The live rear axle vibrates differently than the front, etc…. A car is not one simple unit either; it’s not that dissimilar from a bike in that respect.

Maybe the issue is one of semantics in that you use “magnification” differently than I.

[AndrewS]

There was no sarcasm intended.

I keep trying to make the point that this is both an issue of complexity and scale.

Scale - bicycle wheels don't need to be balanced. The velocity and stored energy involved in the dynamic portions of bicycles are below the thresholds inducing oscillations. Bicycle wheels are not actually all that well balanced, but they don't turn fast enough to produce the vibrations we associate with car wheels. We wouldn't balance car tires either, if the car stayed below some critical speed.

Complexity - external forces have to contend with layered damping in the case of bicycles that is not comparable to simpler systems used in something like a car. Even if a particular road treatment was regular enough to produce a vibration that might be harmonic to one part of the bicycle, it would be dampened by the differing harmonics of the rest of the system. This might not be true if the vibrations had sufficient amplitude to make it through all those layers, but that gets us back to a discussion of scale.

An example of the complexity part of this would be a bent cog tooth and stiff link. The chain will fall off if the stiff link runs over the bent tooth, but difference in link number and cog teeth means that the occurance is actually rare. Having a layers of suspension with different harmonics makes it unlikely for system resonance to occur, because getting the vibrations to line up is so unlikely.

If you changed the scale of any part by an order of magnitude, of course we'd start seeing resonance effects that would net increase perceived vibration. With a 500 pound rider or a bike going 120 mph, some of this stuff would become a factor. We could also got some effects if the wheels were solid, for instance. But at our end of the scale, looking for resonance is a bit like worrying about time dilation on an airplane.