[hookem34]

I recently listened to the Kendal vs. Kendal podcast with a VP from Rockshox. They talked a little about how over forking a bike actually serves as a nice balance. The example they used was a 160mm front fork paired with a 140mm rear shock. The front fork is actually only 144mm of vertical travel when you factor in the head angle (they used 66.5 for this example).

Some sort of trigonometry is needed to calculate this and I would love to know the formula. Alas, they did not disclose.

Can anyone provide this?

 

[One Pivot]

Normal trig/algebra. Sohcahtoa. https://www.mathsisfun.com/algebra/sohcahtoa.html

However, they're assuming a flat drop to flat ground, which is sort of silly. I think the whole idea of "balance" doesnt make sense either. A 160mm fork on a 100mm rear travel bike built to support that makes perfect sense and works great.

Think about how the suspension reacts when you go charging into a rock garden at speed. To an extent, your frame isnt actually moving all that much. Its jiggling, but your suspension is absorbing the hits to keep you on balance and flat. The fork is compressing UP, instead of their example where the bike is flatly compressing straight down towards the ground.

 

[BmanInTheD]

I mentioned that this might be why a lot, if not most, bikes now have more travel in the front than rear in a thread a while back and got laughed out by the know-it-alls. IDK if a formula works to compare front and rear cuz the rear wheel doesn't travel straight up in a vertical path, either. Not to mention that the hypotenuse changes as the fork compresses. But I DO think there is some merit in that idea. There is no real reason that front/rear have to have the same travel.

 

[DHRracer]

Does the industry have a standard for how rear wheel travel is measured? One brand may say rear wheel travel is 140mm. Is that actuall wheel travel or vertical travel? On the rear the two may not have the same discrepancy as the front does. I think for me what is more important is when the bike is weighted in a comfortable attach position does the bike feel balanced. Regardless of travel I don't want impacts whith the front wheel to have a noticable effect on the rear and vise versa. I want my center of mass to be the fulcrum point.

 

[Lone Rager]

Fork travel x sin HTA = vertical travel. e.g. 160 x sin (66.5) = 147 mm

Presumably we're talking about hitting object in forward motion. Since your CG is high behind the fork and in front of the rear wheel the front and rear dynamics are very different. Your weight and momentum push the fork down into the obstacle. They pull the rear wheel up over the obstacle. The front end feels much bigger dynamic forces than the rear end. This is why hardtails work.

 

[One Pivot]

Its entirely possible to have negative vertical travel (frame moves UP) while having your fork compress. Like hitting a sizable rock at speed.

Last year I rode a bike with 23mm more rear travel than front. It wasnt unbalanced or weird or anything.

Simply for the sake of proving an idea, I think someone could make a 150mm rear travel xc bike that fit 100mm forks, and it would work pretty awesomely. Dial in the pivots to provide significant anti squat and it would pedal like a conventional XC bike. Going down, you could pump the bike, slack out the head tube and gobble rocks. It would be a very strong descender and climb normally. We've already done the opposite with ~100mm frames that fit 160mm forks.

Or hardtails with any length fork. Same idea. Theres no reason for balance.

 

[294037]

Does the industry have a standard for how rear wheel travel is measured? One brand may say rear wheel travel is 140mm. Is that actuall wheel travel or vertical travel? On the rear the two may not have the same discrepancy as the front does. I think for me what is more important is when the bike is weighted in a comfortable attach position does the bike feel balanced. Regardless of travel I don't want impacts whith the front wheel to have a noticable effect on the rear and vise versa. I want my center of mass to be the fulcrum point.

There's no standards or accepted tolerance at all, many frames aren't close to the stated travel no matter what way you measure it.

Same goes for any bike metric, you're best to just work on purely whether a bike is riding well or not rather than figuring out ideal numbers unless you are very accurately measuring them yourself

Vertical fork travel is meaningless, it doesn't take a rocket surgeon to figure out that a 65* head angle bike will ride better than a 70* bike with the same fork travel. The slacker the angle the longer the fork has to absorb a bump so you have lower shaft speeds and any acceleration at the handlebar will be longer too.

Rear travel is generally closer to vertical anyway so by that logic you're better off with more rear travel from purely a bump perspective, but we all know that when it comes to riding there's a lot more to it than that.