Grizzly CSS rims snatching.....

[Peejay]

Help - this is driving me mad!

Thorn Sherpa with Grizzly CSS rims, fitted with XTR V brakes and blue Swiss stop brake blocks. The bike has run with this set up from new, no problems.

Now - the rear brake is snatching real violently just in one place on the rim. This has started over the last 200 miles and is getting worse

This is what I've checked, but with no cure:-
1. Rim is true, within 0.5mm laterally
2. No rim flats or any damage
3. Brake blocks are in good condition and clean
4. Rim has been cleaned with Isopropyyl Alcohol wipes
5. Join in rim has no raised areas, but that's not the place where it's snatching anyway
6. Just tried lightly cleaning the braking surface with 600 grade wet and dry, still no good
7. I have checked the rim closely with a magnifying glass all the way round on both braking surfaces and cannot see anything that's different

Any help or experience with this problem guys?

Thanks, Pete.

[Danneaux]

Hi Pete!

I will check later today to see if I can find it, but I recall a similar dilemma mentioned in the past, now in the Forum archives. As I recall, it had to do with contamination. I don't recall the particulars, but I do remember it was very resistant to cleaning/removal and it wasn't so much a spot of higher friction as it was most of the rim except one spot having lower friction; the result being a "snatch".

Hope this helps.

Best,

Dan.

[Danneaux]

Hi Pete!

Here we go...three potentially relevant posts:

http://www.thorncycles.co.uk/forums/index.php?topic=3563.msg15520#msg15520
http://www.thorncycles.co.uk/forums/index.php?topic=5233.msg27539#msg27539
http://www.thorncycles.co.uk/forums/index.php?topic=4803.msg24147#msg24147

...and I recall a couple more that were in related threads.  Framesaver contamination has been identified as one possible contributor by Hamish, as I recall. I also remember a similar dilemma on BikeForums or the CTC forum where a waxy substance caused the problem.

I hope something in the above will prove helpful.  Does it seem worse in the wet or dry, or about the same? Very nice job on the things already checked!

Best,

Dan.

[Peejay]

Hi Dan,
           Thanks for the quick reply!

I'm going to take the tyre off and carefully measure the width of the rim on the braking surface with a micrometer at each spoke to check if there are any fine differences in width. I'm sure it will only take the rim to be wider by a few thou to cause the snatching.
If so, I don't know what will have caused this as the bike has not been off road or hit any pot holes.

Contamination is interesting, but I can't see any change in the brake surface where it is snatching, and certainly the rim hasn't been sprayed with any grease or oil.

I'll keep you posted.

Pete.

[il padrone]

How many kilometres has your rim done?

Because one symptom of a rim worn to the point of near failure is a grabbing brake.... as the rim wall begins to bulge outwards.

[Peejay]

Hi,
    The rim has done approximately 6,300km.
It certainly doesn't look worn and there's no change or deterioration in the CSS surface. I have another bike with Andra CSS rims on and when comparing the rims the braking surfaces look exactly the same.
I agree though - it must be a slight bulge causing the brake to grab. I'll check this week with the micrometer.

Thanks,

Pete.

[il padrone]

OK, yeah, CSS rims will last much more than that. My Andras are up to 19,000kms and very little signs of any wear - the CSS surface is still there.

[onrbikes]

Maybe its not the rim, but the brakes.

Make sure the the mounting pivot is working correctly.
Maybe something stuck in the cable mechanism?

If you replaced the brake pads with some other ones, and it still happens, it would verify its not the rims but the brake mechanism.

[triaesthete]

If you have later  xtr brakes with the "parallel push" parallelogram type pad mounts, check the pads/holders for free play relative to the brake arms. Once there is play in the linkage the brake has pretty much had it.

Can be rectified using the cheaper, simpler and equally powerful Deore  V brake that does without all the moving (wearing  ) parts.

Good luck
Ian

[Peejay]

Update for my fellow Thorn pilots - I have conceded defeat.

I have checked the rim with a micrometer and virtually no change in rim width all the way round - not that.
I have dressed the rim with a carborundum stone in the area where it snatches - made no difference.
Thoroughly checked the XTR brake mechanism - all spot on with virtually no play in any component.
Thoroughly cleaned the brake blocks with Scotchbrite and brake cleaner - made no difference.

But, but....

When I moved the brake blocks vertically, one up and the other down slightly - to move slightly away from their normal running position, still overlapping a bit, but not in a "new" position on the rim - surprise, surprise things improved significantly. The snatching was barely detectable when braking, you could "just" feel it.

But can't leave the blocks in this position as they are over the edge of the rim - so a new rim has arrived today from SJS and we're wheelbuilding this weekend.

A big thanks for all your help and advice.

My conclusion is the braking surface must have got contaminated at the snatch point with something to upset the friction coefficient.

Pete.

[mickeg]

If you reuse the spokes, do you know the trick of taping the two rims together and then moving the spokes from one rim to the other, one at a time?  Loosen all spoke nipples a couple turns at first.  Then move one over at a time.  That is the easiest way I know of to switch rims.

[Peejay]

Hi fellow Thorn pilots - well I've found why the brake was snatching....

Check the photo out!

When I was checking the rim earlier this week in the truing stand I didn't take the rim tape off - foolish now I know.

As we all know a Grizzly rim is a double wall, single eyelet rim. Where the brake was snatching the rim was cracked between around 6 spoke holes on the outer wall. With the tyre back on and in the bike, the force from the tyre pressure (Marathon Plus at 70 psi) was enough just to distort the rim - but you couldn't see it.

New rim fitted today and all built back up and the brake works fine.

Pete.

[Danneaux]

Whew! So glad you found this, and "another reason" to put in the causes file for rim-snatching.

Thanks so much for the result and photo.

'Glad the rebuild went smoothly and swiftly for you.

Best,

Dan.

[geocycle]

Glad you found the problem and thanks for posting it. Something to check next time I have a tyre off. I wonder why it happened as your mileage was low. A random failure?

[mickeg]

Maybe I will have to rethink the Thorn recommendations for maximum pressure due to rim strength.  I do not recall what the recommendation was, but I know there was a recommendation.