Odd backpedal issue.....

Author	Message

maddog7hoo

Posted: Dec 28, 2019 at 11:03

Quote

Hey all,
This is my first post here. Hello!!
I wonder if I can run something weird by you all.

I have a Whyte 905 hardtail and decided to change the mech and shifter from the STD SRAM GX to Shimano XT M8000.

Before the change I used a mech alignment tool to check the hanger and it was spot on, no need for even the slightest tweaking.

In total I changed the mech, the cable and the shifter. I followed the Shimano setup guides to the letter (where possible!) and the drivetrain shifts beautifully up and down BUT on the 42T largest sprocket it drops gears when back pedaling.
It's bombproof on the next smallest one.

I can't be 100PC sure that I never experienced the issue previously but I ride the bike 2/3 times per week and don't remember experiencing it and noticed it before I'd even set off for the test ride after the mech/shifter change.

The reason I find this so weird is that from the top of the crank backwards to the 42T sprocket nothing at all has changed and it is at the 12 o'clock position that the chain begins to drop, so nowhere near the mech.

Before I removed the chain I even marked it so that I'd replace it the same way.
I did have a brainwave and thought I might have put it back inside out, so I removed it again and re-threaded it inversely, still no difference.

Any thoughts?

Kind regards
David.

AyJo

Posted: Dec 28, 2019 at 11:23	Quote
I'm pretty sure this is a fairly common issue with M8000. Other halfs bike used to do it and I can't remember how I cured it. Will have a think. In the meantime try googling "M8000 back pedal problem" Edit: Did you change the cassette? I think OH's bike stopped doing it when I changed the XT cassete for a Sunrace 50t and put a new non-shimano chain on.

maddog7hoo

Posted: Dec 28, 2019 at 11:50

Quote

Hey AyJo,
Thanks for the response.
No I didn't change the cassette, it's still the original SRAM one.
I followed the mech setup instructions to the letter and even tried all sorts of under and over adjustments,
I even tried putting more and less tension on the cage spring, over adjusting the limit screw, gently pressing the cage in and out while turning the cranks etc and nothing seems to have made a difference.
I only rode the bike a couple of days ago and don't recall any such issue.
As I say, I find it really odd since the actual drive train, chainline, chain etc itself is unchanged.

AyJo

Posted: Dec 28, 2019 at 13:11

Quote

I just found some old threads that I looked at when I encountered this issue and everyone said it was the chain that caused it and a few people had the same problem. They all recommended fitting a sram PC1170 chain, which is what I did and it solved the problem. It seems that the issue is caused by the chainline and many chains just dont flex sideways enough to cope with it while back pedaling.

Edit: Sorry, just seen that you didn't change the chain. In that case I don't know. Does it do it with the clutch mech engaged or disengaged?

2nd edit: another thought, is your old chain the right length for the new mech? The cage could be longer causing the chain to be too tight on the big cog.

maddog7hoo

Posted: Dec 28, 2019 at 13:20	Quote
Haha, Yup same clutch on or off. I think I was on hiding to nowt asking like. There appears to be absolutely no reason for it to have happened. The only thing I can think of is that I have not replaced the chain exactly as it was before and perhaps it has worn in some way. I dunno tbh Anyhoo, off to sleep on it Thank you for taking the time to consider AyJo Cheers D

maddog7hoo

Posted: Dec 29, 2019 at 11:20

Quote

AyJo wrote:

2nd edit: another thought, is your old chain the right length for the new mech? The cage could be longer causing the chain to be too tight on the big cog.

I compared them as I swapped them out and best I could see (So by eye!

) they looked very similar distance between the jockeys and the first jockey to the pivot, the SRAM jockeys looked a tiny bit bigger diameter perhaps but not much at all.

I'm confused like!!

AyJo

Posted: Dec 29, 2019 at 14:03	Quote
New/different split link in the chain?

wickedfatchance

Posted: Dec 29, 2019 at 14:21	Quote
B tension screw, adjust it a bit, check instructions.

Maverick-dm

Posted: Dec 29, 2019 at 23:32

Quote

wickedfatchance wrote:

B tension screw, adjust it a bit, check instructions.

What ^ he said. Set a larger gap. The instructions probably say something like 3 mm gap. Set it around 10.mm.

Aaronz25RS

Posted: Dec 30, 2019 at 9:10	Quote
Sram and shimano use slightly different spacing and chain widths, you probably need the change the chain at the very least and possibly the cassette

Ben00

Posted: Jan 7, 2020 at 1:33

Quote

The back pedal issue is present with most SLX/XT builds. Weirdly It probably means you've set it up correctly if it's doing that.

You should find that you don't notice it when riding the bike - if you are back pedalling in the lowest gear (which would be unusual) then the chain will jump straight back on when you pedal in the forward direction and you probably wouldn't even notice.

It's one of those problems that you notice in the workshop but not on the trail.

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