KH Moment Cranks, Q, and body twist

[pkittle]

Hi all--

I posted a thread back in May about problems with body twisting on my 29er. I did a 7 mile ride yesterday up a pretty big hill (3.5 miles all up, and then back down again) on the 29er, and noticed the problem even more than I had in recent rides. Granted, I was on a paved surface (unusual for me), and it was a steady and sometimes steep climb, so those are slightly out of my ordinary riding. Anyway, as I was riding, I played a lot with foot position to try to rectify things, but really couldn't figure out a solution.

So today I got out my digital calipers and measured the distance from my crank ends to my frame, when each crank was in the 12:00 position (the "Q" factor, though I know I'm using that term loosely here). What I found was that my right crank is a hair more than 2 mm closer to the frame than the left.

That's not a lot of difference, I know, but I'm wondering if it's within the specs of the cranks, or if one of them is bent slightly. Would it be enough to be noticeable? Would it be possible (or smart) to try to straighten it out and make them both about the same, in terms of the Q factor?

Any insights would be welcome!

[Borgschulze]

They're aluminum, they will snap before it bends.

Did you check your frame isn't slightly bent?

[skrobo]

it isn't the cranks, it is the frame. both of my torker frames had this problem(one leg of the frame is slightly longer, only a little more severe. i found that shimming it is the only way to make it better. my 05 dx frame had about 1mm clearance on each side so when it was off at all it would hit the frame. this also happened quite commonly when i took the uni off big drops and it was under a lot of stress.

the easy solution is this: get rubber strips and place them between the frame and the bearings. i found that "livestrong" type bracelets work quite well. Al is also a common fix for this problem, i believe thats what terry did. My DX came with some thin peices of plastic(i'm assuming) for this purpose.

[pkittle]

Hmmm. I'll check the frame, then. But the wheel is true and centered in the top of the frame, so it seems a bit odd. But I'll give the shimming a try and see what the results are.

Thanks for the tips!

[skrobo]

oh, that is weird. are you sure your cranks are on perfectly straight? and tight on the spacers all the way around?
i don't know what else it could be except mechanical failure... but then again, 2 mm(about the difference it would make) is not much at the top of the frame.

[pkittle]

Okay, so I was thinking the wheel was centered at the top of the frame because I have brakes and the wheel was spinning straight through them (like I was using the pads for checking how true they were). But when I went back out with my calipers and measured rim-to-frame distance, they were off by a little more than 3 mm. So I think it is a case of the frame being slightly short in one leg, and I'll give the shimming treatment a try and see what that does.

Thanks again for the tip--I wouldn't have thought to measure there, since it looked like it was already centered. The brakes, it turns out, kind of hindered my ability to observe, since I was focused on the relationship of the rim to the pads, not to the side of the frame.

[saam]

If your crank was bent inward by 2mm then the outside of the pedal would wobble in a ~2.8mm diameter circle. I'm not sure how noticeable this would be. Do your pedals feel straight?

I don't think this 2mm has anything to do with your twisting because your shoes/boots probably arn't in the exact same position on each pedal, and if they were, i doubt that your feet would be in the same spot inside your shoes/boots.

Personaly i would not bother trying to shim your frame and realigning your brakes. I know it is probably fustrating but try to ignore it and relax.

[skrobo]

uhm
you shouldn't have to realign your brakes... and besides if it isn't right, it needs to be fixed.

[Borgschulze]

Originally Posted by skrobo

uhm
you shouldn't have to realign your brakes...

Why not?

If he shimmed the frame leg, it would effectively move the brakes to one side probably three times or more as much as the thickness of the shim, based on my geometry.

This means he would have to move the brakes in the same direction, so they are equally close to the rim.