New wheels = pulsing brake

peaslaker

I've recently swapped out from the Mavic Aksiums that came with my bike to new Shimano RS81 C50 wheels and am using them with Ultegra 6800 brakes. I have a severe pulsing problem with the braking that I didn't have with the Aksiums. The braking system is otherwise in good order with good, new cables with no friction issues.

So far I have:

1. Cleaned braking track with detergent and isopropyl alcohol.
2. Aligned new pads.
3. Cleaned the pads and rim again with IPA and detergent
4. Picked out a couple of embedded splinters of aluminium brake track (presumably from the join) from the pads

All of this to no avail. The braking is still pulsing badly, particularly on the front. Compared to the Aksiums that came off the bike the braking is dreadful. At one point in a revolution I have all the power I might expect and then it fades out dramatically for the rest of the revolution.

Only remaining thoughts are:

Option A: intentionally toe in the pads; I've been aligning them flat so far; I don't have any brake squealing, just pulsing performance.

Option B: abrade the brake track

Option C: reject the wheels as faulty

Is this a common problem? Is the fault with the wheels or with the mechanic?

keezx

Obvious a manufacturing problem with the rims (variable width)
Go back to the seller.

robbo2011

Return the RS81's and go back to using the Aksiums?

ugo.santalucia

The problem is not severe and is very common in this model. It boils down to having a very light rim with only 16 spokes. The large gaps make the rim weave a bit... it's only 0.1-0.2 mm deflection, but you can feel it when you brake. I have tried to correct it in the past, but it never really goes away.
Worth checking it anyway, in case yours is particularly bad

Bobbinogs

I haven't seen you mention anywhere about checking whether the rim is actually true. This is really easy to do by just offsetting the brakes so that one block just clears the rim and then slowly rotate the wheel and see if it sticks. Then repeat on the other side. If the wheel is off-true such that it doesn't rotate on either side then take it back for a second option. The problem with low spoke count wheels is that I wouldn't recommend anyone without experience tinkering with spoke tension to retrue the wheel as it often descends into It is now worse than when I started and I wish I had just taken it back so that when the shop says "have you touched this" I could honestly answer "No" and not be undermined by the badly scuffed nipples...

drlodge

16 spokes on a wheel??? Thought I was risking it going for 20 spokes on the RR440. My current wheels have 16 spokes on each side!!

mamba80

i ve just come back from Mallorca using a hire bike with 6800 brakes and rs81 c35's .... they were terrible for pulsing front and rear, toe'd in the pads by about 1mm, problem completely went away, its not an issue i ve had before using rs80 c24's or the DA versions, using 6600 calipers.

shimano wheels are cxap for a lot of reasons but stiffness imho isnt one of them.

i dont know why but the toe in fixed it completely.

JayKosta

peaslaker wrote:

...
At one point in a revolution I have all the power I might expect and then it fades out dramatically for the rest of the revolution.
...

---

This sounds different than typical 'pulsing' where you feel the pulse in the lever and your hands.

Does it seem that the actual friction between the rim and pads is reduced for part of the wheel revolution? Are there any visual differences in how the rim braking surface appears?

Jay Kosta
Endwell NY USA

peaslaker

Thanks for all the replies. I'm glad to be hearing that others have experienced something like this with these wheels. At least I'm not going mad. I'm going to try toeing the brake pads and I'll double check with some measurements on the rim and checking the alignment.

Thanks for all the help.

peaslaker

Just been for a first ride with the brake pads toed in. Seems to have removed the worst pulsing effects even if the brake action is not as positive. This appears to be acceptable and a lot less dangerous.

redvision

I have the RS81 C24s which I bought fairly recently. So far i have only done a couple of hundred miles on them but have been extremely impressed - at risk of opening a can of worms, I much prefer them to the mavic ksyrium elites they have replaced.

What brake pads are you using?
I wonder if that could be a factor?

peaslaker

Brake pads are Ultegra 6800. Nothing that I'd expect to cause a problem.

On Friday (before I'd toed in the brake pads) I had to apply the brakes hard because of a car cutting across me. The grabbing brakes bucked the rear wheel in the air (remember, this is just over part of the wheel rotation so is pretty unnerving). I think its a lot safer with the pads toed in.

redvision

peaslaker wrote:

Brake pads are Ultegra 6800. Nothing that I'd expect to cause a problem.

On Friday (before I'd toed in the brake pads) I had to apply the brakes hard because of a car cutting across me. The grabbing brakes bucked the rear wheel in the air (remember, this is just over part of the wheel rotation so is pretty unnerving). I think its a lot safer with the pads toed in.

Something is obviously not right with them. If they are causing difficulty in keeping the bike under control whilst braking i wouldn't risk riding them and would send them back.

I will be keeping a very close eye on my RS81s, although they have been fantastic so far, so hopefully your experience is a one off and a replacement set would be better.

peaslaker

I've found the documentation which talks about the 0.5mm toe in requirement. My initial experiments with toe in suggest that this is the answer. Can't explain why this wheel set is causing the problem.

Screenshot of the Shimano documentation here:
https://www.dropbox.com/s/2p1asafobyi752i/Screen%20Shot%202015-04-14%20at%2018.32.14.png?dl=0