Totally in the weeds bike geo question

[shoota]

Say I have a CX/gravel bike with a slack front end. It wants to go straight. The initial steering input takes a bit to get it to move, then it wants to flop pretty quickly. That’s fine, I can adjust, it’s not a super-precise TDF road bike. But I live in a windy state and wonder how that will affect the handling. Today, with 12-15mph crosswinds I was surprised at how much the wind affected my very shallow alloy wheels and road tires. I really want deep carbon wheels but I’m curious how this will affect the handling, especially in the crosswinds?

[Peter P.]

My experience is, a bike with a higher trail figure/less fork rake is less disturbed by crosswinds. With a neutral trail front end, I find I'm constantly correcting for those blustery spring winds.

However I ride box-section rims so there's not a great lever for the wind to act against. Once you add a sail of deep-section rims you're asking for even more trail. But building a front end around a particular rim shape is really restrictive.

There are trade-offs. Many riders would choose to leave the deep section rims at home on windy days. Having wheels that can only be used in calm conditions seems expensive to me.

I say ride what you got and skip the deep section rims. You could also get a fork with more rake to tame that wheel flop although it won't do much for your spring winds problem.

[madsciencenow]

My experience has been that deeper section wheels are more problematic with a steeper HT angle. Put another way, if the bike feels twitchy to begin with or is on the sensitive side with how much I need to pay attention to slight variations in steering input, this will be magnified in wind and deeper wheels. I've not had issue with this with a HT angle that's more relaxed (less angle).

[Nomadmax]

What's the trail number (or there about) where the front feels floppy from side to side, especially when standing? I have a gravel bike with 72mm of trail and it's steady in crosswinds but the front wheel wants to squirt the bars out of my hands when standing to climb or sprint. My road bikes are Merckx SLs/SLXs and display none of that, yet, with 50mm rims can be a handful in crosswinds.

[shoota]

Thanks guys!

It looks like the trail of my bike is 61mm. (71 degree HTA, 55mm fork offset, 700c wheel)

It's funny you mention the out of saddle climbing, I noticed it getting what I called "randomly twitchy" yesterday. I've never experienced that before.

(This is a 56cm 2019 CAADX btw, https://www.rei.com/product/145820/c...disc-bike-2019)

[Nomadmax]

Just found this:

https://www.chriskvalecycles.com/new-page-2

[NHAero]

I would think that the answer to whether low or high trail geos are more susceptible to sidewind loading is known. I don't myself know the answer, but the variables are limited, yes?

[prototoast]

I think higher and lower trail w/ deep wheels both have their problems.

In general, lower trail makes the bike deviate in its path more when the wheel angle is disturbed. So if hit with a large gust of wind, the bike will deviate more. But the flip side is the handlebars will feel more stable.

Conversely, with higher trail bikes, the bike moves less when the wheel angle is disturbed. This can make the bike deviate less, but the handlebars more. So in this case, the path of the bike is more stable, but the steering can sometimes feel more "wobbly" and less stable.

Depending on your preference, you may prefer one form of instability over another. In practice, as with all questions about trail, most people prefer a balance, though not everyone prefers the same balance. Personally, tend to prefer trail a little on the high side at baseline, but with deeper wheels, prefer trail a little lower.

[NHAero]

Originally Posted by prototoast

I think higher and lower trail w/ deep wheels both have their problems.

In general, lower trail makes the bike deviate in its path more when the wheel angle is disturbed. So if hit with a large gust of wind, the bike will deviate more. But the flip side is the handlebars will feel more stable.

Conversely, with higher trail bikes, the bike moves less when the wheel angle is disturbed. This can make the bike deviate less, but the handlebars more. So in this case, the path of the bike is more stable, but the steering can sometimes feel more "wobbly" and less stable.

Depending on your preference, you may prefer one form of instability over another. In practice, as with all questions about trail, most people prefer a balance, though not everyone prefers the same balance. Personally, tend to prefer trail a little on the high side at baseline, but with deeper wheels, prefer trail a little lower.

Isn't another aspect of this how much force the wind applies to the wheel? To take it to the extreme, a bike with zero fork offset has an equal area of wheel in front of and behind the steering axis. As trail is reduced, more of the wheel is in front of the steering axis than behind. Isn't the effect a combination of the actual forcing function, and how the bike reacts to a given force?

[prototoast]

Originally Posted by NHAero

Isn't another aspect of this how much force the wind applies to the wheel? To take it to the extreme, a bike with zero fork offset has an equal area of wheel in front of and behind the steering axis. As trail is reduced, more of the wheel is in front of the steering axis than behind. Isn't the effect a combination of the actual forcing function, and how the bike reacts to a given force?

If I understand what you're saying, I think I would say yes, but also I don't know if it makes a difference given that realistically most bikes fall within a pretty narrow trail range.

[Mark McM]

[QUOTE=NHAero;3224737]Isn't another aspect of this how much force the wind applies to the wheel? To take it to the extreme, a bike with zero fork offset has an equal area of wheel in front of and behind the steering axis. As trail is reduced, more of the wheel is in front of the steering axis than behind. Isn't the effect a combination of the actual forcing function, and how the bike reacts to a given force?

A bike with zero trail would only have equal portions of its side area ahead and behind the axis of rotation if the head angle was 90 degree (vertical). As mentioned in a different thread, the wheel rotates about the ground contact point. on an axis parallel to the steerer tube. So for typical head tube angles, the axis of rotation will be a line that passes through the ground contact point and is angled up and back. Which puts most of the side area of the wheel ahead of the axis of rotation.

The next question is, what is the correct reaction for stabiility to a side wind perturbation? Normally, balance is maintained by steering into the direction of lean, and in fact that is what traditional bicycle steering geometry helps to do. If someone to the left of a cyclist gave the cyclist a push to the right, the bike would start to fall to the right. To regain balance, the rider would initially steer to the right (direction of lean) to get the wheel's ground contact points back under their center of gravity, and then steer back to the left to return to their initial direction of travel. The same would be true if the rider was hit by a gust of wind from the side - they would first have to steer away from the wind to regain balance, and then back into the wind to return to their original direction of travel. So is it better if the wind helped to steer away from the wind, to regain balance, or if the wind helped to steer into the wind, to return to the original direction of travel? Or maybe its neither, and the best reaction is for the wind to provide not steering torque to the front wheel at all, and let the front end geometry react normally to the lean of the bike?

[Peter P.]

Originally Posted by Nomadmax

Just found this:

https://www.chriskvalecycles.com/new-page-2

Thanks for the link. That was interesting and informative.