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Please help me understand how stem length affects steering

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Please help me understand how stem length affects steering

Old 06-21-21, 11:04 AM
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VicBC_Biker
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Please help me understand how stem length affects steering

I've been experimenting with differen stem lengths to try to find a good riding position on my bike.

Online, I read that shorter stems cause 'twitchy' (or even in one video 'terrifying') steering response and handling.

I haven't noticed much difference, and I don't understand why stem length should affect steering response.

Imagine bolting a piece of plywood to the top of the steerer tube.




If I'm pushing at the edge of the plywood, it doesn't matter whether I grab spot A,B, or C - the board will rotate the same number of degrees and the fork/wheel will turn the same amount.

Longer bars grabbed at the ends will result in 'less sensitive' steering, and the really short bar used on some hour record bikes will make the steering more responsive.


Similarily, my hands on the 'tops' nearer the middle make steering more 'twitchy'.

But stem length? I don't understand.

Last edited by VicBC_Biker; 06-21-21 at 11:05 AM. Reason: typos
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Old 06-21-21, 11:10 AM
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unterhausen
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It has barely any effect all the way down to zero length. I once told a 'bent rider that their idea of a zero length stem was a bad idea, and then decided to draw a free body diagram and check. It takes extreme steering angles to affect the forces on the fork.
Of course, not even mechanical engineering undergrads know how to draw or interpret a free body diagram anymore, so it's always a waste to tell people this.
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Old 06-21-21, 01:29 PM
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Originally Posted by VicBC_Biker View Post
If I'm pushing at the edge of the plywood, it doesn't matter whether I grab spot A,B, or C - the board will rotate the same number of degrees and the fork/wheel will turn the same amount.
Well, technically yes but no not really. You don't really move your hands a set number of degrees, you move them some distance. Hands in the A position are a greater radius from the stem axis, thus they need to move farther (a larger arc radius) to turn the stem a certain number of degrees.

What is super important to note is with your hands in the B position any side to side pressure from your hands will not turn the bars. As a guy who switched out his drop bars for a set of LONG bullhorns is it pretty easy to pull side to side, especially when out of the saddle and pulling on the bars. I had to practically relearn how to ride when I went to the bullhorns. I have to be super deliberate with my hands or I can easily take myself down.

So while getting your hands a greater radius from the stem axis should in theory make the bike less sensitive to sloppy hand movement, I think it is the side to side leverage from having them way out front that makes the bike feel "twitchy". When you think about it most people ride on the top corners of their drops, which is near position B on your diagram.
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Old 06-21-21, 01:43 PM
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I have to wonder these days if people have forgotten how to use google?

https://cyclingtips.com/2015/03/how-...-and-handling/

https://www.cyclist.co.uk/in-depth/4...-1-stem-length
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Old 06-21-21, 01:43 PM
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I should have drawn the sketch a bit better:


Hands in position 'A' correspond to hand position on drop bars (on the hoods/tops/drops) with a 'normal' (90-120 mm ??) length stem.

Hands in position 'B' would be a 'zero' stem.

Most of the online explanations focus on the stem length, not any curve in the bars.
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Old 06-21-21, 01:44 PM
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Originally Posted by prj71 View Post
I have to wonder these days if people have forgotten how to use google?

https://cyclingtips.com/2015/03/how-...-and-handling/

https://www.cyclist.co.uk/in-depth/4...-1-stem-length
Yes, if it is on the internet, it must be correct.
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Old 06-21-21, 01:46 PM
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I don't think stem length affects steering enough to notice much, but it does affect weight distribution, and that can make a bike feel like it's steering differently.

I'd say trail and rake have a greater effect on steering than stem length.
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Old 06-21-21, 01:49 PM
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Originally Posted by Pop N Wood View Post
So while getting your hands a greater radius from the stem axis should in theory make the bike less sensitive to sloppy hand movement, I think it is the side to side leverage from having them way out front that makes the bike feel "twitchy". When you think about it most people ride on the top corners of their drops, which is near position B on your diagram.
I didn't draw the diagram well...please see my 2nd attempt.
Riding on a standard bar setup is like position 'A' - out front.
Riding on an absolutely 'zero-length' stem would be like position 'B' - and that is supposed to be twitchy/terrifying, etc etc
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Old 06-21-21, 01:54 PM
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Originally Posted by Rolla View Post
I don't think stem length affects steering enough to notice much, but it does affect weight distribution, and that can make a bike feel like it's steering differently..
That's my impression as well - a shorter stem can move weight off my hands and make the steering feel less 'solid'.

Originally Posted by Rolla View Post
I'd say trail and rake have a greater effect on steering than stem length.
Yes. Bike geometry definitely makes a difference.
I don't have much experience with a lot of different bikes but some feel like they 'steer themselves' and tempt me to ride 'no hands' while others are warning me to pay attention.
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Old 06-21-21, 01:59 PM
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Article about zero-length stems and the general handling characteristics of various length stems and bar widths, which are related:

https://www.cyclingabout.com/steerin...teering-speed/

The short version: "If you switch from a 50mm-to-0mm stem, you’ll probably not notice any twitchiness difference using a flat or riser bar, but you will notice the extra twitchiness with narrower drop bars."
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Old 06-21-21, 02:05 PM
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Originally Posted by VicBC_Biker View Post
Yes, if it is on the internet, it must be correct.
Says the guy asking questions...on the internet.
I'm not trying to be critical here, but did you read the two pieces in the links? Do they make sense to you?
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Old 06-21-21, 02:07 PM
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Originally Posted by Rolla View Post
I don't think stem length affects steering enough to notice much, but it does affect weight distribution, and that can make a bike feel like it's steering differently.

I'd say trail and rake have a greater effect on steering than stem length.
It's actually head angle and offset (rake)...those 2 numbers result in a specific amount of trail. I'm glad everyone in this thread seems to understand what is really happening. Changing stem length doesn't change how a bike turns, that is soley determined by geometry mentioned previously (head angle and rake). Just as you say weight distribution changes how the bike feels, but not how it turns.
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Old 06-21-21, 02:07 PM
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Only hand position matters, which is determined by a combination of the bars and stem.

1. With hand position in line with the steering tube, hand movements are purely front and back. This is the position where a unit of hand movement has the greatest steering input.

2. As hand position moves in front of the steering tube, steering inputs grow more side to side, and the radius from steering tube to hand increases. Therefore, a unit of hand movement produces decreasing steering input.

3. Inputs grow side to side with hands behind the steering tube, but in the opposite direction.

For most people, 1 feels twitchy, 2 feels stable, 3 feels weird.
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Old 06-21-21, 02:11 PM
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Originally Posted by VicBC_Biker View Post
Yes, if it is on the internet, it must be correct.
Point out what's incorrect in the two articles I posted.
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Old 06-21-21, 02:24 PM
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As I've always understood it, the points raised above with regard to bike handling/stem length apply only to very low speeds. Above that speed range, you aim the bike by leaning, not steering. Twitchy/scary handling is a consequence of your front/back weight distribution being less than ideal. (Further, it's my understanding that professional bike fitters recommend stem lengths on the basis of front/back weight distribution, not handling characteristics.)
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Old 06-21-21, 02:33 PM
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Originally Posted by Rolla View Post
Article about zero-length stems and the general handling characteristics of various length stems and bar widths, which are related:

https://www.cyclingabout.com/steerin...teering-speed/
Thanks, Rolla

Here's a diagram from that article:

Notice the misrepresentations?
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Old 06-21-21, 02:46 PM
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Originally Posted by bOsscO View Post
Says the guy asking questions...on the internet.
I'm not trying to be critical here, but did you read the two pieces in the links? Do they make sense to you?
For me, there is a huge difference between personal experiences of people who respond in (good) online forums, and internet articles and videos which are primarily designed to generate clicks and ad revenue.
The latter often simply repeat material from other sites and articles, which may or may not be accurate and well thought-out. It's not unusual for 'long held' beliefs and ideas to be wrong.
I have read quite a few articles online and watched videos like the one from GCN on YouTube.
They didn't correspond with my experience in fitting a minimum length (35 mm center-to-center) stem to my road bike.
So I was wondering if other people here had had similar experiences, or could explain to me why my ideas were wrong.

I appreciate all the info I've gotten here.
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Old 06-21-21, 02:55 PM
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My bikes with swept bars have hand positions ranging from in-line with the steering axis, to about 2 inches behind the steering axis. It doesn't seem to affect handling at all.

But it can potentially affect braking, something that I'm always conscious of. During braking, more of your weight shifts to your hands, and having your hands further forward gives you better control of the bike and less chance of going over the bar.
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Old 06-21-21, 03:45 PM
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Originally Posted by Pop N Wood View Post
What is super important to note is with your hands in the B position any side to side pressure from your hands will not turn the bars. As a guy who switched out his drop bars for a set of LONG bullhorns is it pretty easy to pull side to side, especially when out of the saddle and pulling on the bars. I had to practically relearn how to ride when I went to the bullhorns. I have to be super deliberate with my hands or I can easily take myself down.

So while getting your hands a greater radius from the stem axis should in theory make the bike less sensitive to sloppy hand movement, I think it is the side to side leverage from having them way out front that makes the bike feel "twitchy". When you think about it most people ride on the top corners of their drops, which is near position B on your diagram.

An example of a configuration extremely sensitive to such side to side pressure is aero bars.
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Old 06-21-21, 03:59 PM
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As I'm sometimes math-challenged, I went down to the shop and built a (full-scale) model.



I made measurements for three different turns: 7.5 degrees, 15 degrees. and 30 degrees.
As @Trakhak mentions, the larger (degree) turns will only be made at very slow speeds.

The imagined hand position was at the hoods, and movement was measured at an imagined vertical line at the forward curve of the bars.

The model stem lengths were 25mm C-C and 100 mm C-C. 25mm is less than any real stem length could be, with normal steerer tubes and bar sizes.

It IS correct that with the longer stem, more movement at the ends of the bar is required for the same rotation of the steerer tube, compared to a short stem.
For a 7.5 degree turn, the longer stem setup will require my hands to move approximately) 40 mm (along an arc), the shorter stem 25 mm. For a 30 degree turn the longer stem will require a hand movement of 155 mm, the shorter stem 115 mm.

The longer stem will require more hand movement, and should seem 'less sensitive'.

So my initial thoughts and conceptual model were incorrect.

Either I adapt to different setups very quickly, or I'm not skilled enough to notice the difference with different stems.
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Old 06-21-21, 04:00 PM
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Originally Posted by terrymorse View Post
Only hand position matters, which is determined by a combination of the bars and stem.

1. With hand position in line with the steering tube, hand movements are purely front and back. This is the position where a unit of hand movement has the greatest steering input.

2. As hand position moves in front of the steering tube, steering inputs grow more side to side, and the radius from steering tube to hand increases. Therefore, a unit of hand movement produces decreasing steering input.
Thanks - very clear.
​​​​​​​I agree.
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Old 06-21-21, 04:26 PM
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My thought about "steering input" is that any cyclist is strong enough to produce vastly more steering input than can be used without crashing, when riding under normal pavement conditions. We can, and must, apply a limited and varying amount of torque to the steering axle, that also depends on speed and other conditions. So we're already adapting, or we'd be dead. Now the handlebar is doing other things for us. It helps us to stabilize our own body position relative to the bike, and to manage the weight distribution between the front and back wheels. Those things all have to do with control of the bike.
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Old 06-21-21, 04:34 PM
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Originally Posted by VicBC_Biker View Post
I didn't draw the diagram well...please see my 2nd attempt.
Riding on a standard bar setup is like position 'A' - out front.
Riding on an absolutely 'zero-length' stem would be like position 'B' - and that is supposed to be twitchy/terrifying, etc etc
I interpreted your drawing the way you intended.

I guess my experience is the opposite of what you are saying, i.e. getting the grips farther forward makes the bike more sensitive to improper steering input.

I put a riser bar and Jones H bar on one of my bikes to turn it into an upright cruiser. The bars are probably in line with the stem. Super stable bike.
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Old 06-21-21, 04:40 PM
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Originally Posted by csport View Post

An example of a configuration extremely sensitive to such side to side pressure is aero bars.
Exactly.

Except with the bullhorns putting the hands so far apart it must feel different. IDK, do people come out of the saddle and pump with their arms while on the aero bars?

I'm telling you the first few times I rode the bike I almost dumped it. Maybe things go non-linear once you get too far forward.

I can also tell you with a recumbent one needs to be very conscious of the "tiller effect". Takes some practice to get just the right touch. When I first learned to ride that thing I held the bars up so they rotated directly in line with the steerer tube. Made the bike much less twitchy.

Maybe I am just the outlier.

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Old 06-21-21, 05:17 PM
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Originally Posted by Pop N Wood View Post
I interpreted your drawing the way you intended.

I guess my experience is the opposite of what you are saying, i.e. getting the grips farther forward makes the bike more sensitive to improper steering input.

I put a riser bar and Jones H bar on one of my bikes to turn it into an upright cruiser. The bars are probably in line with the stem. Super stable bike.
It's complicated! (As Gresp15C points out..)
For instance...
I've read in many places that hands in the drops is the most stable and controlled position, but because I'm not in great cycling shape, I don't feel very comfortable or agile when my hands are in the drops.

A
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