geo questions

[kgreene10]

Every once in a while, a used frame really catches my eye. I know enough to understand the effects of ST, TT, and HT length as well as ST angle, but I don't know how to interpret HT angle and front-center.

For instance, a local guy has a nice frame that has a 73.5* ST angle and a 72.5* HT angle on a nominally size 58 bike. The ST will require sliding the saddle further back. But I have never seen such a slack HT angle. What does that do to the steering and the rider's weight distribution?

I get the sense that two bikes with different HT angles could end up with the same front-center if you use different fork rakes, but does that make them handle the same?

Illumination please.

[Ken Robb]

This question is beyond my pay grade. Did that make me sound presidential?

[vqdriver]

good framebuilders can wrap their heads around these numbers and build a frame to a desire ride characteristic. for (most) us consumers, it seems fruitless to try to translate frame geometry into tangible ride qualities. tho we can get general ideas and have educated people try to explain it to us.

since the guy's local, i'd just offer to swap bikes and go for a ride with him.

[David Kirk]

Prepare for a response - not so much an answer.

No matter what the bikes will feel different. Even two identical bikes will have slight differences let alone two different bikes with different geometry/fit/tubes.....etc. It's like comparing apples to oranges in a way.

The question might be better phrased - 'will the bike ride and handle well?' Even that question is hard to give a good answer to. I can tell you that if the numbers were really out of wack that it won't handle well but as long as the trail and weight distribution are in the ballpark it should function fine.

Will you like it is the real question. You are the only one that can answer this one. Can you test ride it?

dave

[eddief]

as I recall you may be, like me, the owner of a Fisher Cronus. My Cronus is a 58. I also own 3 other bikes. Those three all have more bb drop than the Cronus and they all have more relaxed HT angles. I seem to actually prefer more bb drop and slightly more relaxed HT angles. I am a bit more comfy on my other bikes, but the Cronus feels more race-y and quicker in the steering. A fine alternative to change things up after riding the others. (it weighs 5 lbs less too!) I am not a racer, so the less race-y approach is probably my preference.

I have a Gunnar Sport and that bike sounds very similar in geo to the one you are considering. Love my Sport and you can see the geo chart here. I asked the same sort of geo questions when I bought the Cronus as I was concerned about the differences between it and what I was used to.

http://gunnarbikes.com/site/bikes/sport/

[ultraman6970]

What do you call front center??? Frame reach?? Since you use the word fork rake you are not talking about that. Trail??

Pretty much the head angle says how fast the frame can steer but at the same time u have the fork trail also. It is a combination of things.

A Shallow head tube angle will make the bike to be more stable at fastest speeds but that doesn't mean will steer faster.

A steeper angle will make the bike crispier when steering, if you need to move the bike fast like for example when u are going to charge in a track sprint match, u need to get the bike out of the line fast.

U can fix a bike with poor steering changing the fork as u said also but in general also the head tube angle and the fork offset and rake work together.

Hope this helps.

[cmg]

what are the dimensions/geometry you're acustomed to seeing on a 58cm frame?

[1centaur]

If you squinch your eyes half shut and blend what I say with DK's answer you'll probably be in the ballpark.

I have a 72.5 HT angle on my last 2 customs, and have had 72.x on some Colnagos and I think my Merckx. My TT is typically around 57 so not far from a nominal 58 frame no matter how that's measured. I've used on-line rake and trail calculators to combine HT angle with fork rake to get a trail number, and I've compared that number to the trail on my various other bikes to get a sense of relative handling effect. I find 72.x to be more relaxing going down hills than 73 or higher; I'm not twitching in a pack or accelerating through a corner in a crit. I think of 72.x as classic Euro road race geometry in my frame size.

I don't know how it interacts with front center, though my FCs are pretty close to each other and experience has shown that the calculated effect is the actual effect. Handling is also about weight balance, which is part of what I think DK was almost saying. If the frame fits you well (and shoving the saddle back on a 73.5 makes me wonder a bit about weight between the wheels), then IMO 72.5 is normal/desirable if you want a relaxed handling bike and the fork has a rake typical for a stock fork. Good enough for Ernesto and Eddy, good enough for me. But I'll close by saying the difference between 73 (supposedly "neutral") and 72.5 is not eyepoppingly different; it's subtle but there when you focus on it.

[Peter P.]

Geez; what a load of great, insightful replies from such a wide variety of forumites! They're all loaded with good opinions formulated from a lot of first hand experience.

I personally ride much smaller frames than you (52-53cm) so you have to take my views with a grain of salt, I guess.

I don't get this "front center" thing. I understand the measurement, but I don't understand why builders are insistent on jacking up head angles in an attempt to keep the wheelbase within a certain dimension. I feel you can make a quality, fine riding tall racing frame even with slack head angles. Heck; a 'cross frame in your size might just have a 72.5 head angle.

As far as weight distribution goes, I've experimented with seat fore/aft from mid-rail to full rearward and don't see any disconcerting change in how my bike handles. Regarding the slacker head angle; my racing bike has a 73 head angle and combined with a ubiquitous 45mm raked fork, I've always felt the steering was too light for my taste, and I raced this bike for 11 years. My commuter bike has a 72.5 head angle with 47mm of rake and I could ride that thing all day, fast or slow. It's more calm than the 73 without being slow or lazy. There are fast riders out there with slower reflexes that would prefer a slacker head angle.

If you found the bike's steering was too slow, you could always sort of regain a little liveliness by mating it to a fork with more rake.

Dave Kirk's answer is spot on; "Will you like it?

[eddief]

may not do anything to steering handling or perceived weight dist., but does, for me, affect power to the pedals.

[kgreene10]

Thanks for the really helpful replies. I was kind of hoping that there would be sort of a formula for understanding how the various elements of front end geo interact, but I now realize that there are more complexities and subtleties than I initially figured.

It would be great to have one of the builders write up some thoughts on the subject. I'm sure many of us wonder how builders design so that we gawky humans can manage to interact with their precision machines.

[David Kirk]

Thanks for the really helpful replies. I was kind of hoping that there would be sort of a formula for understanding how the various elements of front end geo interact, but I now realize that there are more complexities and subtleties than I initially figured.

It would be great to have one of the builders write up some thoughts on the subject. I'm sure many of us wonder how builders design so that we gawky humans can manage to interact with their precision machines.

I'm sure that there is a good bit written on the subject and if you did a search you'd find some good stuff. That said here are a few rules of thumb -

Trail - more trail is more stable and less trail is less stable. Like everything - moderation is good and you can have a bike that is too stable. I feel that a trail of about 5.7 is good for road race/performance use and that for more casual riding 5.9 is a good place to be. I never ever go over 6.0 and never under 5.6 cm.

Front center - a longer front center will mean that the front wheel is further out from under your body and that it will carry less load.........and therefore be less stable. It's your body weight acting on the trail that makes the bike stable and gives the steering self centering. A short front center will put more weight on the front wheel and make it track better. BUT..... a short FC will also mean a shorter wheelbase and that means the bike will turn sharper and be more likely to want to turn. So you gain a bit in one direction and lose it in another.

Another thing about front center - top tube length is a big factor in setting a given front center. If you need a long top tube then you need a long top tube period. One can make the front center a bit shorter on a long top tube bike by making the head angle steeper and adjusting the rake to bring the trail in line but anyway you cut it a bike with a long TT will have a longish FC. And that is not a bad thing.

There are a lot of things to consider when designing a bike for someone once the fitting points are set in stone and a nearly infinite number of ways to get to the end. Some will feel better to you and some less so but few of the options will be wrong. I sometimes get questions like "what is a good FC?" or "what is a good head angle?" and of course there are no answers to these questions really. They are both just single numbers in a large group of numbers that will determine how the bike feels to the rider. I never feel comfortable saying that you can't isolate a given number or two or three and make any real judgements based on them but it's true. The answer in a way feels like I'm trying to make it a mystery and keep the questioner in the dark and wow them with hocus pocus. But I guess in the end it's true. You can't take any of this stuff out of context.

and at the same time one need to be careful to understand that none of these numbers are gospel. Botha 72.5° and a 73° seat angle can be great for a given rider and to dismiss a design over the 1/2° would be silly.

The only 100% sure way of knowing if you will like the bike is to ride it. The more bikes you ride the more you will understand that resonates with you and what doesn't matter. nothing I can tell you or that you can read about bike design online can change that.

That's all I gots.

dave

[kgreene10]

Thanks DK! I'm beginning to see the outlines of the complex picture that is bike design.

I study politics for a living, so I'm no stranger to trying to teach people that the puzzle has a lot of interacting pieces. My education on this particular issue will be a slow one, but I think I may have a few good years of riding left.

And how awesome is it that some guy like me bares his ignorance and gets a thoughtful response from one of the finest builders in the land? This forum is tops.

[soulspinner]

I'm sure that there is a good bit written on the subject and if you did a search you'd find some good stuff. That said here are a few rules of thumb -

Trail - more trail is more stable and less trail is less stable. Like everything - moderation is good and you can have a bike that is too stable. I feel that a trail of about 5.7 is good for road race/performance use and that for more casual riding 5.9 is a good place to be. I never ever go over 6.0 and never under 5.6 cm.

Front center - a longer front center will mean that the front wheel is further out from under your body and that it will carry less load.........and therefore be less stable. It's your body weight acting on the trail that makes the bike stable and gives the steering self centering. A short front center will put more weight on the front wheel and make it track better. BUT..... a short FC will also mean a shorter wheelbase and that means the bike will turn sharper and be more likely to want to turn. So you gain a bit in one direction and lose it in another.

Another thing about front center - top tube length is a big factor in setting a given front center. If you need a long top tube then you need a long top tube period. One can make the front center a bit shorter on a long top tube bike by making the head angle steeper and adjusting the rake to bring the trail in line but anyway you cut it a bike with a long TT will have a longish FC. And that is not a bad thing.

There are a lot of things to consider when designing a bike for someone once the fitting points are set in stone and a nearly infinite number of ways to get to the end. Some will feel better to you and some less so but few of the options will be wrong. I sometimes get questions like "what is a good FC?" or "what is a good head angle?" and of course there are no answers to these questions really. They are both just single numbers in a large group of numbers that will determine how the bike feels to the rider. I never feel comfortable saying that you can't isolate a given number or two or three and make any real judgements based on them but it's true. The answer in a way feels like I'm trying to make it a mystery and keep the questioner in the dark and wow them with hocus pocus. But I guess in the end it's true. You can't take any of this stuff out of context.

and at the same time one need to be careful to understand that none of these numbers are gospel. Botha 72.5° and a 73° seat angle can be great for a given rider and to dismiss a design over the 1/2° would be silly.

The only 100% sure way of knowing if you will like the bike is to ride it. The more bikes you ride the more you will understand that resonates with you and what doesn't matter. nothing I can tell you or that you can read about bike design online can change that.

That's all I gots.

dave

Thanks Dave for all the time you spend here helping us understand bikes (and for making killer rides).

[1centaur]

DK's answer was great. I will chip in that I have a bike with a trail slightly over 60 and I like it very much, but that 57-59 range seems down the center of the reasonable fairway of what I have ridden.

[RPS]

Front center - a longer front center will mean that the front wheel is further out from under your body and that it will carry less load.........and therefore be less stable. It's your body weight acting on the trail that makes the bike stable and gives the steering self centering. A short front center will put more weight on the front wheel and make it track better. BUT..... a short FC will also mean a shorter wheelbase and that means the bike will turn sharper and be more likely to want to turn. So you gain a bit in one direction and lose it in another.

True, but your statement is only meant as long as “everything else is equal”, right? And that’s not necessarily always the case.

A longer front center reduces front wheel load as long as the chainstay length is the same, thereby adding to wheelbase “only” in front of BBKT. This would indeed shift weight from front to rear wheel. On the other hand if the chainstay length is made longer to compensate for a longer front center the weight distribution can be kept the same, or made higher or lower.

Personally I think there are too many variables for people to design bikes in their own heads. It’s probably best if left up to experts like you. In this case the frame’s chainstay length wasn’t even brought up as far as I recall reading.