Fork Judder Sidebar. Two ways to look at it. Latest issue discussion (Summer 2014)

[Bubba]

Spoiler alert.  Hopefully I won't give anything away that will ruin the enjoyment of another nice issue of BQ.  Anyone who hasn't gotten the issue already knows there's a road test of a Ritchey Swiss Cross, equipped with a 1x11 drivetrain and Compass 700x38 EL tires.  I won't give away any details about the road test.  

One issue with that bike was significant fork judder.  This is the phenomenon when under hard braking the front end vibrates front and back severely, seemingly caused by the front brake grabbing and releasing, grabbing and releasing at somewhere in the 5 - 10 Hz range.  Fork judder is not to be confused with shimmy, nor is it to be confused with brake squeal.  

Anyway, Jan went ahead and devoted a sidebar to fork judder to collect his thoughts on it.  First off, there's a typo, the Ritchey article claims the sidebar is on page 40, but it's not.  That's just a typo.  As with shimmy, Jan does not claim to have any absolute conclusions on what causes judder or avoids it, but listed his thoughts and hypotheses.  His hypothesis was an interesting variant on the theory I attribute to Leonard Zinn.  That Zinn theory is that hard braking causes parts of the bike to flex, starting a feedback loop with the front brake.  Zinn's description has you applying the front brake hard.  Your weight wanting to continue forward flexes the fork backwards.  If the fork blades are stiff, then a significant part of this flex is in the steerer.  If the steerer flexes backwards, that effectively tightens the brake cable, and now the brake is grabbing tighter.  This feeds back on itself until the front wheel lets go from the ground, like a mini skid or maybe even a hop.  When the front wheel lets go from the ground, the steerer springs back and the brake lets go, allowing the front wheel to re-establish contact with the ground, starting the cycle again.  This cycle happens several times per second, causing the frightening vibration feeling of for judder.  This is the Zinn explanation.  

Jan's sidebar caught my attention because he described the same theory, but happening in exactly the opposite way.  Jan posited that you apply the front brake hard, which pushes extra weight down onto the front end of the bicycle.  This weight on the front end flexes the steerer forward, not back.  This flex loosens the front brake.  The loosening front brake unweights the front end, which allows the steerer to flex back, which tightens the brake again, restarting the cycle.  

Both Jan's theory and Zinn's agree that it is steerer flex that causes fork judder.  I think both agree that having a steerer that is significantly stiffer than the fork blades should be less likely to judder.  I think both agree that sidepull brakes and disk brakes can't judder in this way because steerer flex has no effect on cable tension and brake actuation.  So, they agree on the big picture aspects.  I just found it interesting that they had completely opposite hypotheses on the original cause.  Jan thinks it's the fork flexing forward, and Zinn thinks it's flexing back.  I certainly don't know who's right between the two, and it probably doesn't matter.  There are definitely other opinions about what contributes to fork judder, since the "fix" for judder on the Ritchey was Paul brakes, with some suggestion that the amount of play of a brake on it's post has some effect.  I know others use different brake pads to combat it.  Regardless of the techniques they use, I just know that anyone who has experienced fork judder wants it to go away.  Jan's sidebar seemed to invite the discussion, so I thought I'd start the discussion here.  Thoughts?

It's a very good BQ issue.  

[Eric Daume]

My old Salsa Fargo (disc braked) shudders, so it's not caused solely by the brake cable length change. 

My CrossCheck shudders quite a bit as well. It's worse with a dubo hub. I think the extra weight down low imparts a bigger load onto the fork blades. 

Eric Daume

Dublin OH

--
You received this message because you are subscribed to the Google Groups "Bicycle Quarterly Readers' Review" group.
To unsubscribe from this group and stop receiving emails from it, send an email to bqrr+uns...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.