Yet another stupid fork rake question.

[akelman]

So, I still don't get it (this is true of so many things, actually). Leaving aside my generally muddled state, let's focus on the issue at hand: if one wants a relatively neutral handling bike, and said bike has a steeper-than-average headtube angle, one would want a fork with more rake, right? Say, closer to 45 rather than to 40? Or is that wrong?

[dave thompson]

The other way; with a steeper head tube angle, you'd want a fork with less rake. An example was my Spectrum, it had a head tube angle of 74 degrees and a fork with a rake of 40MM. My touring bike had a head tube angle of 72.5 degrees and a fork rake of 50MM.

[rice rocket]

http://upload.wikimedia.org/wikipedia/en/thumb/c/c7/TrailDIAG2.jpg/458px-TrailDIAG2.jpg

[akelman]

So, based on what you're saying, dave, and what rice rocket's diagram is showing, a steep head tube angle + a 45 mm rake fork = very little trail. Which means that the bike will be jittery? Why is this so hard for me? It's not like I'm asking about the meaning of life. Ugh.

[rice rocket]

I don't know how you define neutral, but more trail = more self steering forces generated from the road/tire interface.


Edit: I don't know if one would define it as jittery. Think of it as driving a car with steering that won't self center. Ever driven a Subaru? It's like that.

[akelman]

I don't know how you define neutral, but more trail = more self steering forces generated from the road/tire interface.

Meaning, less jittery, correct? And by neutral I mean not sluggish and not jittery, somewhere in between, just right, like the Goldilocks story.

[rice rocket]

Maybe find a bike you like the handling on, and measure the trail...and mimic that on this frame in question. Remember that there's other characteristics that affect the handling (wheelbase, cg height, etc), but the front half of the bike will react similarly.

http://upload.wikimedia.org/math/6/2/5/6252cb832f48f294cfc5cd0a1276a3d2.png

Where Rw = tire radius, Ah = head tube angle, and Of = rake (offset).

[dave thompson]

I copied the below from somewhere and it may help you visualize the effect of rake and trail and handling in a general sense. Like rice rocket inferred, the front end is only part of the whole handling package, everything affects everything else.

just in case someone missed the first semester of frame geo 101...

"Trail" is the blue patch along the ground in this diagram.
What it "is" is the distance away from the tire patch on the ground
that the steering axis is pointed at. The further away the steering axis
is pointed, the slower the bike steers (or more stable it feels, if you prefer)
The closer the steering axis is to the contact patch, the more nimble
the front end feels.

Generally speaking, trail falls between 5 and 6 cm's... but it's shockingly
common for small bikes to have shallow head angles and "stock" forks
with 43mm of rake... which produces 63 to 66 mm's of trail...
and very slow steering.

Somewhat counter-intuitive, but more fork rake produces less trail.
More rake moves the red line forward, toward the black line of the steering
axis, thus reducing trail.

[akelman]

Between rr's equation and your post, dave, I actually think I've got it. Thanks so much to both of you.

[dave thompson]

Between rr's equation and your post, dave, I actually think I've got it. Thanks so much to both of you.
That's 20 Pesos please.

[bmeryman]

I've always found that with concepts such as this it's best to extrapolate based on the given information and think about what the extreme cases would be.

I think I've got a handle on this, though it is late so correct me if I'm wrong. If you weren't moving and you were on a bike with no trail, the wheel would just turn as you dictated. If the bike had lots and lots of trail (think chopper), just a little nudge on the bars could cause the wheel to flop over seemingly of its own volition. The key really (as stated earlier) is to find your 'sweet spot' and stick with that.

Sorry if this way of thinking about it doesn't work for other people; I've got a strange imagination sometimes.

[akelman]

That's 20 Pesos please.

We usually drive through Spokane every summer on the way to Glacier. I'll put a check in your box this June, some time around the 10th.

[ultraman6970]

Agree with bmaryman.

The handling of the bike is given by many factors but the fork trail can tell you how it will ride. Also u can make a bike to handle better chaging the fork, tihng im sure u know already.

Good luck.

[Kontact]

Meaning, less jittery, correct? And by neutral I mean not sluggish and not jittery, somewhere in between, just right, like the Goldilocks story.
Trail does not produce one result, it produces multiple effects depending on speed and lean angle.

A low trail bike will actually not feel jittery at low speeds - it will want to go straight and be hard to hold in a turn. At high speeds it will feel weird, but not jittery.


IMO, there is little reason to depart very much from neutral trail, since the results are not even across the speed range. There are other ways of making a bike handle slower or faster that don't produce uneven results.

[khjr]

Can anyone say (Dave?) what sort of trail you'd want if you were building a bike to carrry a front load, in a big bar bag or fork rack?

I'm thinking that there, you'd want lower trail than otherwise...

Anyone?

[khjr]

.

[roydyates]

Can anyone say (Dave?) what sort of trail you'd want if you were building a bike to carrry a front load, in a big bar bag or fork rack?

I'm thinking that there, you'd want lower trail than otherwise...

Anyone?
Yup, or at least that's the opinion (http://janheine.wordpress.com/2011/02/10/a-journey-of-discovery-part-4-front-end-geometry/) of Jan Heine and the Bike Quarterly randonneuring set.

[palincss]

A low trail bike will actually not feel jittery at low speeds - it will want to go straight and be hard to hold in a turn. At high speeds it will feel weird, but not jittery.


That's an unwarranted over-generalization. I have two low trail bikes that don't handle anything like what you're describing.

Trail is one of several parameters that affect front end handling. Tire width and front end load play a role as well. The front end geometry needs to be matched to the tires. As tire width increases, so too does pneumatic trail - where the tire itself provides centering force on the front end. Decrease tire width and you need to increase geometric trail to provide the same steering feel -- and you can see that vary in the geometry of bicycles from the 1950s - present. As road surfaces got better and tires got narrower, geometric trail went up.

Ride a bike that was designed to be used with a 32mm tire running around 70 psi with a narrow high pressure tire, and you'll end up with a bike that will wander all over the road by itself.

Front end load enters into the picture as well. Some weight in a handlebar bag acts a bit like a steering damper. When I picked up my Kogswell P/R from the LBS where it had been put together and rode it around the parking lot without a bag on the front, my immediate reaction was "WEIRD! What the ^%$#@ is wrong with this bike?!!" It felt extremely light, almost like the way a car feels when you're driving on ice. Adding the weight of a handlebar bag and one tube made it feel "normal."

The biggest difference I notice between my low trail and high trail bikes is in cornering: the low trail bikes do not lock in on a line in a turn. You can make a steering adjustment any time in a corner and change the line, if you want to. Now if you're used to and crave a bike that feels like it "corners on rails," not having that feeling can indeed feel strange.

There are other differences. My low trail bikes also have low wheel flop, and in general high trail bikes tend to have more wheel flop, which tends to make them react more to steering changes.

Riding very slowly up a very steep grade in a low gear my high trail bikes tend to start getting a little unstable, and steering corrections to keep the bike on a straight line can in a worst case cause the bikes to veer off in an unexpected direction. By contrast, even at very low speeds on steep grades, the low trail/low wheel flop bikes are easy to keep in a straight line.

The low trail bikes also tend to be less sensitive to side winds. I've been on rides where gusty side winds have pushed others on the ride right off the pavement, but my low trail bikes have not been so affected.

[Peter P.]

My experiences have been the opposite of palincss and more in line with Kontact's comments.

At high speeds, my low trail bike felt hard to hold a given line in a turn. In fact, the front wheel felt like it was on ice. The bike needed constant steering attention.

I don't know about this pneumatic vs. geometric trail palincss speaks of; I always calculate my trail figures with the tire I'm going to use. Sure, I guess it becomes a factor if you change tire sizes a lot and maybe that's what he's referring to, but I stick with one size although I don't do it just to keep my trail figure constant.

I've never experienced wheel flop with high trail bikes. Mountain bikes have much higher trail figures than road bikes but I never notice flop with them.

Interestingly enough, I know of two framebuilders, both of whom build road racing frames. One of them prefers designing high-trail front ends (59-60mm) and the other, low trail (50-53mm). Both are successful and enjoy a following despite their trail figures being outside the neutral range of 55-57mm.

[palincss]

My experiences have been the opposite of palincss and more in line with Kontact's comments.

At high speeds, my low trail bike felt hard to hold a given line in a turn. In fact, the front wheel felt like it was on ice. The bike needed constant steering attention.


As I said, low trail doesn't "lock in on a line" in a turn. If that's what you want, you don't want low trail.


I don't know about this pneumatic vs. geometric trail palincss speaks of; I always calculate my trail figures with the tire I'm going to use. Sure, I guess it becomes a factor if you change tire sizes a lot and maybe that's what he's referring to, but I stick with one size although I don't do it just to keep my trail figure constant.


I am not talking about using the wheel size in a formula to calculate how much trail a given amount of fork rake will produce. When I talk about "pneumatic trail" I am talking about the steering-centering affect the tire itself exerts as it resists the turning forces on it. A wider tire will have more than a narrow tire. Low trail needs to be matched with front end load and with appropriate tire width or else you get sub-optimal results.


I've never experienced wheel flop with high trail bikes. Mountain bikes have much higher trail figures than road bikes but I never notice flop with them.


Of course you have. When you turn the handlebars, the frame drops. You countersteer to initiate a turn. That's wheel flop doing that.

[khjr]

Decrease tire width and you need to increase geometric trail to provide the same steering feel -- and you can see that vary in the geometry of bicycles from the 1950s - present. As road surfaces got better and tires got narrower, geometric trail went up.

Ride a bike that was designed to be used with a 32mm tire running around 70 psi with a narrow high pressure tire, and you'll end up with a bike that will wander all over the road by itself.


That's exactly how it was with my vintage Tommasini. When I first built it, I ran 20 mm Continental Ultras and absolutely hated the way it handled. Very twitchy, hard to ride no-handed, drifty in descents. After I installed the same tire in a 25 mm width, there was a night and day difference. Very stable now, and I can even corner no-handed (broad corners, anyway...).

[Kontact]

That's an unwarranted over-generalization. I have two low trail bikes that don't handle anything like what you're describing.

Trail is one of several parameters that affect front end handling. Tire width and front end load play a role as well. The front end geometry needs to be matched to the tires. As tire width increases, so too does pneumatic trail - where the tire itself provides centering force on the front end. Decrease tire width and you need to increase geometric trail to provide the same steering feel -- and you can see that vary in the geometry of bicycles from the 1950s - present. As road surfaces got better and tires got narrower, geometric trail went up.

Ride a bike that was designed to be used with a 32mm tire running around 70 psi with a narrow high pressure tire, and you'll end up with a bike that will wander all over the road by itself.

Front end load enters into the picture as well. Some weight in a handlebar bag acts a bit like a steering damper. When I picked up my Kogswell P/R from the LBS where it had been put together and rode it around the parking lot without a bag on the front, my immediate reaction was "WEIRD! What the ^%$#@ is wrong with this bike?!!" It felt extremely light, almost like the way a car feels when you're driving on ice. Adding the weight of a handlebar bag and one tube made it feel "normal."

The biggest difference I notice between my low trail and high trail bikes is in cornering: the low trail bikes do not lock in on a line in a turn. You can make a steering adjustment any time in a corner and change the line, if you want to. Now if you're used to and crave a bike that feels like it "corners on rails," not having that feeling can indeed feel strange.

There are other differences. My low trail bikes also have low wheel flop, and in general high trail bikes tend to have more wheel flop, which tends to make them react more to steering changes.

Riding very slowly up a very steep grade in a low gear my high trail bikes tend to start getting a little unstable, and steering corrections to keep the bike on a straight line can in a worst case cause the bikes to veer off in an unexpected direction. By contrast, even at very low speeds on steep grades, the low trail/low wheel flop bikes are easy to keep in a straight line.

The low trail bikes also tend to be less sensitive to side winds. I've been on rides where gusty side winds have pushed others on the ride right off the pavement, but my low trail bikes have not been so affected.
If you had quoted me in full, then you'd have less to disagree with: "There are other ways of making a bike handle slower or faster that don't produce uneven results." You then go on to describe pretty much what I did - low trail bikes don't like to hold a line in a corner.

Using average racing size tires and pressures and not changing any other parameters, low trail will produce about the results I mention. But wheelbase, balance, stem length, tire pressure, HTA, wheel weight, etc can all moderate those effects.

Trail only has certain influence, but if you account for those other factors you will get the more similar results to what I outlined.

My real point in posting was just to illustrate that trail is not the best thing to play with in an effort to improve handling. The further trail varies from neutral, the greater the difficulty in predicting what the bike is going to do in different handling situations.

In my opinion, front center and chainstay length are the most important factors to manipulate to create fast vs. stable racing bike geometries. Trail is best left near neutral, and HTA serves fit and wheelbase concerns. Certainly, companies like Colnago greatly depart from this, but the people who are known for designing terrific handling road bikes stick closely to this principle.

[palincss]

In my opinion, front center and chainstay length are the most important factors to manipulate to create fast vs. stable racing bike geometries. Trail is best left near neutral, and HTA serves fit and wheelbase concerns. Certainly, companies like Colnago greatly depart from this, but the people who are known for designing terrific handling road bikes stick closely to this principle.

The universe of bicycles is much larger than the tiny special case of modern racing bikes. For that special case and the narrow range of modern racing tires, there's no reason to consider low trail. The geometry you're describing is optimized for those conditions.

However, the geometry that works so well for that racing bike might not work so well for a touring bike meant to carry loaded front and rear panniers, or a fat-tire gravel road randonneur meant to carry a big handle bar bag.

[Kontact]

The universe of bicycles is much larger than the tiny special case of modern racing bikes. For that special case and the narrow range of modern racing tires, there's no reason to consider low trail. The geometry you're describing is optimized for those conditions.

However, the geometry that works so well for that racing bike might not work so well for a touring bike meant to carry loaded front and rear panniers, or a fat-tire gravel road randonneur meant to carry a big handle bar bag.
I agree. That's why I specified "racing bike geometry". As someone already pointed out, loading the front of a touring bike does things to the steering that trail manipulation can fix.

Road race bikes are a special case in two ways: They are built for agility above any other concern, and they are ridden at higher speeds than just about anything else. That combination means that the geometry practiced doesn't depart much from the theoretical. Remake a bike to haul loads, work in mud, have large tires or suspension and then you need to get away from certain ideals.