Thorn Cycles Forum
Technical => Wheels, Tyres and Brakes => Topic started by: Peejay on January 05, 2014, 04:49:30 pm
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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.
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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.
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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.
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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?
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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.
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Nasty. Worth contacting Rigida (or whatever they are called now) and SJSC to let them know and get their view.
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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.
I'm not in any way saying this was the cause of Peejay's failure, just a republishing of Andy Blance's general cautions right here, mickeg: http://www.sjscycles.com/thornpdf/ThornRavenNomadBroHiRes.pdf , pg.6.
Tyre pressures…
...PLEASE READ THIS!
Tyres have a maximum and a minimum recommended pressure; you should consider the implications of different pressures.
Please look at our tyre pressure matrix on the left. Pressure is in pounds per square inch and there are lots of square inches in a fat carcass!
The REAR pressures are the maximum pressures that you should ever put into various width tyres. I’ve found that my bikes handle best, when the front tyre is at a slightly lower pressure than the rear.
PLEASE NOTE: For reasons, known only to themselves, tyre manufacturers often quote higher maximum pressures, for fat tyres, than any rim can withstand.
SUCH PRESSURES WILL CAUSE THE RIM TO FAIL!
YOU’VE BEEN WARNED!
At the above maximum pressure, the tyre is less able to squirm around on the rim and consequently quicker changes in direction can be made and big, out of the saddle efforts, result in more immediate forward propulsion.
At lower pressures, tyres roll more easily on uneven surfaces, this fact comes as a shock to many cyclists...perhaps the more uncomfortable the ride, the faster they think that they are going? Tyres running lower pressures are usually much more comfortable to ride.
TYRE SIZE MAX PSI
FRONT REAR
1.35"...........75.......85
1.50"...........70.......77
1.60"...........66.......73
1.75"...........61.......68
2.0" ...........53.......60
2.10"...........52.......57
2.25"...........49.......54
2.35" ..........46.......52
[Andy Blance's brochure content quoted intact in its entirety, but with some formatting changes to fit here as plain text for reader convenience]
Best,
Dan.
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Thanks Dan - I have always run with Marathon Plus tyres at 70 psi - maximum recommended by Schwalbe due to me being a big guy - 6 ft 5in and just short of 18st but regular cyclist and not a lardy!
I've never had this problem with wheels on any of my other bikes.
I'm going to follow the advice from Andy B and lower the pressures on my Sherpa which has the Grizzlies fitted.
I also have a new Raven but with Andra CSS rims fitted, again with 1.75 in Marathon Plus tyres - any thoughts on the strength of Andra rims compared to Grizzlies? Should I also run these at 61 psi front and 68 psi rear?
Pete.
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Hi Pete!
So sorry you've had this problem, and it would be great to get a definitive answer as to how it happened so it could be avoided again for you if at all possible....any thoughts on the strength of Andra rims compared to Grizzlies? Should I also run these at 61 psi front and 68 psi rear?
Though we differ in height and weight, this might be of help for comparison's sake....
This last summer when my plans for a big desert tour were derailed by unusually high temperatures and subsequent forest fires that sharply reduced air quality, I viewed it as an opportunity to take the Nomad and my Extrawheel trailer into the cool, misty heights of Oregon's Coast Range and on over to the ocean so I could test how the bike might work and handle with the equivalent of a full load for a sustained, self-supported desert tour. I considered it a shakedown run and opportunity to learn how the Nomad might work at the upper limits of my use (it did really well, not a single complaint!).
Here's how the weights came out...
With extra food, 26.5l of water, and my full touring kit (way overkill for the ride I was on, but mocked up for part of the tour I would have taken), the loaded bike weighed in at 70kg (69.8kg)/154lb (the dry unloaded bike weighs 20kg/45lb). The trailer weighed 20kg/45lb, but only a fraction of that was carried on the rear axle of the bike as tongue weight so I won't count that here 'cos I can't find my notes on it. My own body mass added another 78kg/172lb. So, total on the wheels for the loaded bike and myself was 148kg/326lb plus whatever fraction of the trailer. This was a huge load I would in no way carry regularly, but typical for the shorter-term weights I have carried and likely will again for some desert crossings where I am away from resupply and any potable water in very high temperatures and and so have to carry the lot with me.
I used 26x2.0 Schwalbe Duremes pumped to Andy Blance's maximum F/R pressure of 53/60psi on extremely rough logging roads and found the ride comfortable and rolling resistance acceptable at those pressures for that weight. I incurred no rim damage or pinch flats. The Nomad is a very sturdy bike, and the ride smooths out nicely under the expedition loads for which it was designed. Understandably, I have found the unladen ride to be a bit harsh on very rough surfaces, but that problem has now been fully addressed by a Thudbuster LT seatpost and the bike is now a wonderful, heavy all-'rounder that rides comfortably now regardless of load. I am coming to believe the sus-post may spare the rear rim some peak loads it would otherwise endure, or at least spread the amplitude of an impact over a greater amount of time and therefore result in less outright or abrupt shock to rims, drivetrain, and frame.
By the way, I'm 180cm/5'11"tall and ride a size 590M Nomad with compact drop handlebars and a short 60mm stem resulting in a 45° back angle while riding the hoods. Though we differ, overall loads on the tires should prove helpful for comparison if you go touring. In other words, I found Andy Blance's recommended maximum tire pressures to be fine on very rough logging roads at the loads I was carrying and even ended up reducing pressures by 2psi for each tire with no problems resulting. Thinking comparably, you should be okay at your weight riding a loaded bicycle that weighs 33.5kg/74lb all-up if you also used Rigida Andras shod with 2.0 tires at the same pressures. There's a small fudge-factor beyond this for the tongue weight of the loaded trailer, added to the rear wheel.
At more reasonable touring loads, I've been very happy with maximum F/R pressures of ~40/45psi.
For comparison's sake, I run the unladen Nomad with F/R pressures of 29/34psi and have found this to be the sweet spot for comfort, handling, and low rolling resistance at my total combined weight (me and the bicycle together, from completely unladen to carrying as much as 11kg/25lb between water bottles and my rack pack and/or handlebar bag).
Please keep in mind -- just as Andy's chart indicates -- if you run narrower tires, they will require higher pressures to make up for the reduced air volume so you won't incur pinch flats and so handling and rolling resistance will remain reasonable. The higher pressures of narrower tires are still "safe" for your rims because the narrower tire cross section has less of a lever effect on the rim sidewalls, and higher pressures will be okay so long as the maximums are observed for each diameter/section width.
The question of tire pressure arises from time to time on the Forum and remains a fascinating, evergreen topic for me. If you want to chase some of the past threads on this topic, a quick search using the string "tire pressure" (no quotes) either alone or as an Advanced Search combining "tire pressure" with username Danneaux will bring up the majority of related threads. I've written a little tutorial on how to easily search the Forum and posted it here: http://www.thorncycles.co.uk/forums/index.php?topic=4390.0
I really do think keeping within Andy Blance's upper limits is a Really Good Idea that will help stack the cards in favor of avoiding rim cracks, though any number of other factors could have contributed to your crack initiation and propagation.
Best,
Dan.
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Worth contacting Rigida (or whatever they are called now) and SJSC to let them know and get their view.
Pete (Peejay), as a followup to Rual's suggestion, it might be worth dropping a note to Ryde (formerly Rigida) to seek their advice, though SJSCycles/Thorn would be my first choice for practical, field- and shop-based advice and experience on the causes of such failures with similar rims.
Contact information for Ryde ( http://www.ryde.nl/en/ ) is here:
Ryde International B.V.
Netherlands
T + 31 485 561 787
/or/ + 31 485 56 17 89
E info@ryde.nl
For the story on how Rigida became Ryde, go here: http://www.thorncycles.co.uk/forums/index.php?topic=3571.0
Best,
Dan.
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Dan, thanks for finding the numbers. I knew I had seen them somewhere but had not remembered where. And I knew I had exceeded them.
I have regularly exceed Andy's recommendations for rear tire pressure with 37 or 40mm width tires, but have not exceeded with 50 or 57mm tires.
My Mavic A719 rims (700c) that I use 37mm width tires on are rated for 88 psig with that tire width. My Salsa Gordo rims on my Sherpa have no pressure rating that I know of and I have used Marathon (with GreenGuard) 40mm tires at up to about 85 psig on the Gordo rims. My Andra 30 rims on my Nomad have no rating but I have consistently been below Andy's max pressure with those rims, have only used 50 or 57mm tires on those rims.
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That is one scary photo, Pete! It sure does look like the sort of thing that could happen from too much pressure in the tires, but 70 psi doesn't seem so horribly high. My little chart: http://interdependentscience.blogspot.com/2013/06/bicycle-tire-pressure.html would suggest trying something like 55 psi rear and 48 psi front for person your size... still under Mr. Blance's suggested limits.
Someday I expect my rims will wear out. I am thinking about something like a Velocity Cliffhanger, which should have an inner well width around 22 mm though I haven't found the exact number. A wider rim can handle a wider tire, or will see less stress that a narrower rim for the same tire width. I usually ride 50 mm tires but I wouldn't mind going a bit wider. But with the 19mm Andra rims I don't want to push my luck.
Thanks for sharing that photo though. Wow.
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70 psi doesn't seem so horribly high.
A few years ago I destroyed a new rim (can't remember the make) by pumping a 2.0" tyre to about 80 psi.
My silly idea was to overinflate the tyre to seat it evenly.
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My silly idea was to overinflate the tyre to seat it evenly.
Not a silly idea at all, Martin, provided the tire had not been so wide; the procedure works pretty well on narrow road tires when other more conservative methods fail. I'm sorry it had a sad outcome.
You've given a kind and generous warning for others; much appreciated.
Best,
Dan.
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Thank you so much for sharing your experience here Peejay, invaluable information for the likes of me who has never experienced brakes snatching. Now I will know there could be something seriously wrong and will do something about it.
Since the discussion of fat tyres and low pressures thread started by Andre, and then Andy Blance's pressure limits, I have been running my RST with 26" x 1.75" Paselas at 47/50 psi or my 26" x 1.6" Supremes at 52/55 psi which I have found to be optimal for me.
Jim
(I have always ridden road bikes with 700c 25s at 97/100 which happens to be within Andy's limits.)
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Thanks for all the advice....
I'll send Ryde the photo this week and see what they come back with - thanks Dan for their details.
In the meantime I'm going to reduce the tyre pressures on both bikes and use 48 front and 55 rear as Jim has recommended - thanks Jim.
I'll let you all know what Ryde come back with.
Time now to ride with the new tyre pressures. Nice clear frosty morning in Cheshire so I'm off out for a 50 miler on my Whitchurch loop.
Pete.
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That is one scary photo, Pete! It sure does look like the sort of thing that could happen from too much pressure in the tires, but 70 psi doesn't seem so horribly high. My little chart: http://interdependentscience.blogspot.com/2013/06/bicycle-tire-pressure.html would suggest trying something like 55 psi rear and 48 psi front for person your size... still under Mr. Blance's suggested limits.
JimK,
Is your chart for 26" or 700C tyres?
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Is your chart for 26" or 700C tyres?
The table is for 700C tyres. Increase about 5% for 26" tyres.
Of course there is no real precision possible with these sorts of formulas. It depends especially on road conditions but also rim width and tire construction and anybody's guess what other factors! But this table should give a reasonable starting point.
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Interesting comments, and hope you get sorted out OK, peejay.
I run my tyres at around 50 psi on Grizzly 26" rims on poor surfaces, planning to increase to around 60 when I do longer road-oriented rides. Still within recommendations I reckon.
As a matter of interest, Relayer, how do you find the 1.75 Pasela Tourguards in comparison with the 1.6 Supremes?? I cannot recall hearing from anyone with experience of both so far.
Lewis
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As a matter of interest, Relayer, how do you find the 1.75 Pasela Tourguards in comparison with the 1.6 Supremes?? I cannot recall hearing from anyone with experience of both so far.
Hi Lewis
When I used to run my tyres at the maximum pressure on the sidewalls the Supremes were 1 mph faster on average, but since I have dropped pressure on both types of tyres there is no difference whatsoever in average speed i.e. the Supremes have 'slowed down' by 1 mph while the Paselas haven't changed.
The [folding] Paselas are definitely more comfortable than the Supremes, this is largely due to the supple sidewalls on the Paselas, but the large sipes on the Supremes also cause some vibration at higher speeds. I would add that I also had some hand/wrist discomfort with the Supremes at full pressure (i.e. 85 psi) this is no longer an issue at lower pressures.
I have switched between these tyres so many times: I tended to imagine that the firmer and narrower Supremes had to be faster than the cushy Paselas, but in practice at lower/practical pressures this proved not to be the case. I also read about people getting lots of punctures on Paselas and trouble with the sidewalls being susceptible to damage, but in practice I have never had a puncture or other problem with either of these tyres; although I did have to pick a couple of pieces of glass out of the tread of the Supremes once. Maybe the back roads I ride in the countryside have a lot less glass than urban roads.
The bottom line is that the received wisdom states that the Supreme is a superior tyre; however I think the Pasela gets some undeserved bad press (except from Andy Blance!); in any case because they have served me well enough and the comfort advantage my personal preference is for the Paselas.
Jim
P.S. I have a set of 26 x 2.0 Duremes which I used to have on my MTB, if I could get these onto my RST I would bin the others. Maybe one day I'll get a new Raven frame, and the Duremes will be there. :)
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Thanks Jim . . . .
You have confirmed what I suspected, and indeed, was hoping - what I didn't say in my post was that I had already fitted Paselas and hoped that was the right choice!!
I have been using Duremes on my Ripio - they have been excellent for mixed riding, but as I am doing some longer sponsored rides in late May, weight and speed will count for quite a bit, distances will be longer than I have been used to and a lot of the others will be on 'roadier' bikes.
First impressions of the Paselas is that they are quicker, livelier and seem faster than the Duremes - seem is perhaps the operative word, as these things are very subjective unless you test them properly, and the smaller tyres are almost bound to feel livelier!! But I have only ridden very short distances after changing.
All in all, I reckon I have made the right choice as wrist comfort etc is important.
lewis
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Hmmm, interesting post Peejay.
I'm also using Schwalbe Marathon Plus 1.50 on Ridgida Grizzly CSS rims, I'm currently running them on Andy Blances recommendations of 66 front, 73 rear, I may have to rethink these pressures in light of your experience.
It'll be interesting what feedback SJS and Ridgida/Rhyde give you, it looks like a manufacturing fault to me, hopefully an isolated one at that.
Good luck with your new wheel anyway,
Neil.