inspclouseau

Any ideas on how to diagnose and fix chain slap?

Googled around and seeing most caused by a dirty derailleur. This is certainly not the case.

Bike is about a month old and meticulously cleaned and recently tuned. Also it doesn’t slap when hitting bumps as many googled articles return.

it’s reproducible while pedaling and suddenly stopping. Annoying yes. Became dangerous yesterday while sprinting. I was full gas so don’t remember the specifics. But I believe I may have paused to stand. The chain slapped and felt like it dropped but it didn’t. Sent the bike into a shimmy, I quickly recovered and hit it again.

I demonstrated to a mech on the ride and he had no idea. I was big ring up front little cog in the rear so no cross chaining.

thanks.

base2 

You don't say what kind of bike. But on some level it doesn't matter. My guess is you have one of those ginormous 11-50 cassettes &/or a reasonably long cage derailleur.

What makes me think this is the sudden stop of drivetrain input after a hard sprint exposes the rotational inertia of the giant cogs & the decreased leverage the long lever (derailleur cage) has on the system.

If this is true, then your derailleur may have a clutch mechanism to help the weak lever component of your problem. Engage it.

The other option is to get a short cage derailleur to fight a high friction drivetrain like would be found on an IGH hub.

Another option is to not sprint, spin up all the heavy giant cogs, then suddenly stop the input.

noodle soup

What rear hub do you have, and can you reproduce the chain slap on a workstand?

eduskator

A month old? So you bought it brand new? What did your LBS say?

How stiff is your rear derailleur spring? Might be a manufacturing defect.

Sapperc

Yes to all the above. Also, check your “B” screw adjustment. Insure chain wrap around your small cog is not binding or too loose. Lastly, check your chain length for proper length. Might be a bit long.

Good luck!

noodle soup

+1

I've seen massive cassettes cause this issue, especially when combined with new hubs that have slightly draggy freehub body seals.

inspclouseau

Thanks all for the replies. Answers to the above questions:

Not sure why the bike matters but its a Look 795 Blade RS.
I did get it brand new, but the shop I bought from is 2 hours away. I will be bringing to the local shop but would like an idea of what it is first.

I believe it is a long cage derailleur, the thing is huge, the cassette is 11-32, and its a full r8000 Ultegra groupset.

The wheels are Shimano RS 370. My regular wheels with my preferred cassette are at the manufacturer for a repair.

No workstand at home, but I can easily reproduce by picking the bike up, pedal forward a time or two and suddenly stop pedaling. Just tried this and it was on the middle cog.

To be more exact, i think its the backward motion when suddenly stopping to pedal that causes it.

woodcraft

Chain too long?

inspclouseau

I am pretty sure before every sprint on a ride I tell myself I am not going to sprint, but it just happens. So I gladly accept this advice, but say I wont sprint anymore until I fgure this out but doubt that will hold true.

inspclouseau

Possible? Hard to believe since the bike was not a custom build, but I dont really know.

noodle soup

This was part of the cause for the case that I saw. The owner had a very jerky stop to his sprint, and it caused the rear axle to flex slightly(and bind the freehub body). When I tested the bike with a smooth sprint, I didn't have the issue, but when I stopped sprinting the way the owner did, the chain would slap. He tried another wheelset, and the issue was gone.

When you lift the rear wheel, and slowly pedal backwards, does the wheel spin backwards?

seypat

OP better get a chainstay protector on there if it doesn't have one.

inspclouseau

it does not. Also, discs so thru-axles.

when this happened on the sprint it was mid sprint. The sudden movement was me looking for more watts and I think standing, not when I first took off for the sprint.

wphamilton

I don't see how inertia of the cog could have any effect on the chain slap. I would pause, or disregard, that avenue of investigation and look at the other suggestions: the chain, hub behavior, loose cogs and the DR arm action.

Shimagnolo

The one time I ever had this issue it was caused by a draggy freehub that was not disengaging promptly when the rider abruptly stopped pedaling.

noodle soup

You don't see how a cassette that continues to spin for a moment(for whatever cause), after the crank stops turning, could cause chain slap?

noodle soup

This is the most likely cause, especially because it's a new bike. Seals in the freehub body are probably a little sticky.

inspclouseau

Thanks. So just let it break in a bit?

noodle soup

I'd probably try a different wheel from your LBS(preferably one that's broken in), and see if the problem continues. If their wheel works fine, look into why the freehub body is sticking.

wphamilton

Wrong question. Does having a larger cog installed make that severe chain slap happen is the question.

Rotational inertia is from all of the cogs together, and in my estimation the difference in total inertia from having a larger cog installed won't be enough to cause this by itself. Slapping in the small cog, you could probably take the big cog completely off and it will still slap.

inspclouseau

Thanks! I really don’t like those wheels. It would have been nice as backups, but my regular wheels were sent to the manufacturer on Monday to be relaced. Hopefully when they return all will be well. And hopefully that return will be in the next few weeks and not next few months.

Cypress

Imma need to see some pics of this bike. For...reasons.

inspclouseau

haha. Sure thing.

bampilot06

hot

Rogerogeroge

As others have said, the freehub isn't properly 'freewheeling'. Look at some YooTube vids and overhaul the freehub, it's not hard. If it's chain slapping, it's worth fixing.

Beautiful bike. I'm trying to deduct points for improper presentation, but everything appears to be in order. Valve steams vertically oriented, tire labels oriented to the valve stems, crank at 3:00 with chain in big chainring.....but I suspect your steerer needs to be cut 10 mm. Perhaps handlebars should be rotated up a smidge and the levers moved down that smidge.....and those crazy vertical slats make it harder to judge compared to a backdrop of a white garage door..... but nonetheless, a grand showing of a grand steed.