Does Wheelbase Change as Fork Compresses?

jeffj · Jul 18, 2016

I have no idea if it's relevant or means anything if it does, mostly curious if it does so that when comparing wheelbase of a bike with 150 mm travel fork to say something with 120, 100, or rigid, if you could factor in that the wheel base would be slightly more or less when actually riding the bike :idea:

ut:

The dimestore engineer in me feels that if I drew a straight line form the rear axle to the top of the headset, and then a perpendicular line toward the ground, it would show that the fork would be at a < 90° angle to that line. I can't determine if that would mean that the wheelbase would increase or decrease, but it makes sense to me that unless it were at 90°, it would do one or the other.

If I were to hazard a guess, I would guess that if it were < 90°, wheelbase would grow, and if > 90°, wheelbase would shrink?

Probably should have asked this question first: Does it mean a darn thing?

Procter · Jul 18, 2016

Yes, breaking out a little high-school SOHCAHTOA shows that the wheelbase gets shorter by the amount

WB change = Cos(head tube angle * PI / 180) * fork travel

If fork travel is in mm, the result will be in mm.

EDIT, actually my trig isn't perfect, this is an approximation. Bonus points if anyone can spot why.

jeffj · Jul 18, 2016

Awesome! Thanks for the cocktail napkin diagram :thumbsup::thumbsup:

otb-klunker · Jul 23, 2016

Bonus points if anyone can spot why.

You didn't account for Fork rake/offset, but it should create a very minor deviation (<10%).

Procter · Jul 23, 2016

otb-klunker said:
You didn't account for Fork rake/offset, but it should create a very minor deviation (<10%).

That won't affect the change in wheelbase. Notice that the equation doesn't care what the original wheelbase was, only the HTA and fork travel.

There's a different reason why it's inaccurate. Bonus points still up for grabs.

Hint, it overstates the change in wheelbase for greater fork travel.

Shawn in da Canyon · Jul 23, 2016

Yes, it does change. So much so, it's going to **** up your entire ride. You should just stop riding before you hurt yourself.

otb-klunker · Jul 23, 2016

Hmmm, only other variable I can think of is that the head tube angle with respect to the ground changes as the fork compresses.

Mudguard · Jul 23, 2016

jeffj said:
If you could factor in that the wheel base would be slightly more or less when actually riding the bike :idea: ut:

Can shorten quite a bit. Life threatening if you're on Pinkbike and called Protour...

Procter · Jul 23, 2016

otb-klunker said:
Hmmm, only other variable I can think of is that the head tube angle with respect to the ground changes as the fork compresses.

Boom, this is it. The HTA will get steeper, and the contact point between the tire and the ground will rotate CCW in the picture. How much that affects the wheelbase is a harder trig problem.

That's assuming front compression only, or hardtail. As the above pic shows (dh guy landing), full front / rear compression adds even more variables.

Lone Rager · Jul 23, 2016

Small error in your diagram and calc. Wheel WB2 will not be parallel to WB1.

Procter · Jul 23, 2016

Yes, that's another way of putting it, this was my point about HTA in post #9.

someoldfart · Jul 23, 2016

Also important is that as the fork compresses trail decreases assuming no rear compression. So a steep head angle and long travel do not mix. In theory you could end up with less trail at the time when you need the most stability.

noapathy · Jul 24, 2016

Darn and I was just thinking it might have been tire compression, spoke flex, or fork flex. This could get really complex really fast. lol

jeffj · Aug 4, 2016

noapathy said:
Darn and I was just thinking it might have been tire compression, spoke flex, or fork flex. This could get really complex really fast. lol

Yes, it does. However, how much it matters, and when, could be a whole 'nother Oprah.

Joules · Aug 4, 2016

jeffj said:
Yes, it does. However, how much it matters, and when, could be a whole 'nother Oprah.

as for how much it matters, I'm sure you've already put more thought and discussion into it than it's worth.
So the wheelbase shortens by ~5% for a split second at bottom out... How often do you bottom out at the exact moment when a slight change in wb would affect something?

Fleas · Aug 4, 2016

Let's ponder this another way (I know I have)...

For a hardtail, I think the WB change is insignificant, but the head angle and, subsequently, the steering/fork trail changes quite a bit. You would likely notice this more if your fork is sagged too much, but it is probably common to all to some degree.

You go through a G-out or something and your bike might feel like it wants to just turn off the trail, or tuck the front under altogether (this is where dual suspension offers some forgiveness). You might also feel weird things if you are rolling over short mounds or rollers (a la pump track, but slow) and steering. The contact point of the front tire to the ground keeps moving in front of and then behind the front axle, demanding continuous steering input to keep you on course.
Lastly, if you hit a bump and compress your fork, the steering doesn't feel weird at all because the rest of the bike maintains the same angles to the ground. If you brake hard and compress the fork, the angles change and steering becomes more twitchy - not that you'd normally be doing a lot of hard braking and steering at the same time, but again, if your fork is sagged too much, the steering manners will be worse, especially under braking or on steep descents, or where you might weight the front to increase traction. Also, if your full suspension bike is unbalanced front-to-back as far as suspension settings, you could get funky steering as well when one end compresses significantly more than the other.

Comparing the WB from one bike to the next is valid, but probably not in regards to suspension travel.

-F

jeffj · Aug 10, 2016

This whole line of thought came to me while discussing wheelbase (and possible effects on cornering in tight switchbacks) with a friend, and comparing various bikes, both hardtail, and FS and in various sizes. Picking the fly poop out of the pepper so to speak. Interesting discussion, and thanks to all that have taken the time to contribute.

Does Wheelbase Change as Fork Compresses?

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