Chain rub against rear tyre

trevorh

I have a 27.5+ bike. Sram GX setup with 10-42 on the rear. In the 42 cog the lower chain run is rubbing against the tyre. I have never notice this before but came up whilst cleaning. From above the upper and lower chain runs do not appear parallel with the upper run easily clearing the tyre. The tyre widths are 2.8" which were specced with the bike. Any ideas on why this is happening and how to rectify?

cooldad

Unless the mech is badly bent it shouldn't be possible. Pictures might help.

robertpb

Check for a bent mech hanger, also check to see if the wheel is running true in the frame.

02gf74

I have 2.2 tyre on 29er and there is a finger width clearance so 15 mm.

Check the chainring is not too far in, can be fixed by less offset.

Have any parts changed from factory specification

02gf74

Also check wheel is centred n\a for thru axle and dished correctly.

cougie

Bet the wheel isn't in straight.

trevorh

I had to replace the original WTB trailblazer rear tyre with another (Schwalbe Rocket Ron). Turns out the RR tyre is slightly wider albeit still a 2.8" plus tyre. Swapped the RR to the front and the front Trailblazer to the rear and the chain is about 5mm away from the tyre in the 42 sprocket so this has solved the issue. The trailblazer tubeless tyres are super tight to fit and in the process I pinched the tube so now have to try and replace that tonight!!! :oops: :oops:

larkim

But surely as the chain is supposed to run along the same lines top and bottom (i.e. to the crank and to the cassette) it shouldn't have been rubbing only at the top and not at the bottom. Are you sure the mech / hangar aren't bent at all, and that the wheel is in straight?

02gf74

larkim wrote:

But surely as the chain is supposed to run along the same lines top and bottom ?

You'd think so but does it?

Checked my cannondale with XT 11 speed and on 42t lower run is 3-5mm closer.

I don't believe there is wear or anything bent.

trevorh

Spoke to the bike manufacturer and the lower chain run does run slightly closer to the wheel normally.

larkim

Well I suppose thinking about it the wheel should / must be parallel to the centreline of the frame. Otherwise you'd be riding in circles ;-)

The chainline will be at an angle to that, depending on which gear you're in. Little chain ring / largest cassette ring should probably be at a slight angle with the chain at the crank slightly further out compared to the chain at the back (let's say gears 1-8 are the appropriate range from the smallers chain ring, is it fair to say that the chain should be perfectly straight from smallest chainring to say gear 3 or 4 at the back?)

So the chain gets progressively nearer to the centreline of the bike the further back along the bike it gets (in the easiest gear / gear 1)

Given that the chain runs out to the cassette lower down compared to the axel height of the wheel, the point at which the chain will pass the wheel will be further back along the bike than where it passes as it comes back from the cassette to the chain ring. So if it is going to be closer, it will be marginally closer presumably between the chain ring and the lowest jockey wheel than between the chain ring and the cassette, which is as described by the OP.

Interesting. Just wanted to write that down so I could grasp it!

The Rookie

As the mech is pulled forward so that the run goes to align with the sprocket further forward where the lower pulley is, it's pretty obvious the lower run will be closer to the tyre in the lowest gear.......if people actually thought about it!