[Closed] SRAM Crankset hitting chainstay in granny gear

Not that familiar with SRAM cranksets so there may be something else going on but it's weird that it's only happening in one gear.  If you were missing a spacer or something then it would be happening all the time surely.?  Is it a full sus bike.?  Is there play in the pivots.?

That's the only thing i can think of that could cause the cranks to hit the chainstay sometimes but not always.!?

Smallest gear = more torque through frame = more flex in frame = crank hits frame?

Xx1 chainset were available in two different Q factors. Basically there’s a short axle and a long axle   , well at least there with the 30mm axle one.

Im not so sure about  gxp ones but easy to check on the sram site.

Some frames need the long axle one and some don’t

What’s odd is it’s only happening sometimes

I'm running a boost crankset on a boost frame.  I think something is flexing as it only happens when I come to a techy bit and you need to spin it up.

Orangeboy : Xx1 chainset were available in two different Q factors. Basically there’s a short axle and a long axle   , well at least there with the 30mm axle one.

The Q Factor of cranks is determined by the shape of the arms, not the length of the axle (for the same BB).

OP: how much clearance is there between the arms and chain/seat stays at the narrowest point? Is it the same each side? What is the bike - offset rear triangle? Definitely not your heel?

Thanks fogstomp. It’s good to lean stuff.

Id just assumed the two axle lengths were the cause of different Q factor. But seems there’s more to it.

http://forums.mtbr.com/drivetrain-shifters-derailleurs-cranks/sram-xx1-bb30-q-factor-168-short-spindle-long-spindle-903770.html

Goes some way to explaine about the two axle lengths and how to tell if you have sort or long spindle by measuring the drive side spacer

Q factor is the total width across the pedal facing bits of the cranks. So it is a property of the whole system, not just the crank arms.

So it is a property of the whole system, not just the crank arms.

Agreed, but for the same BB (as mentioned) it is the crank arms.

That is true for square taper, ISIS etc, where the axle comes with the BB*.  Not for modern systems where the axle is built in to one of the cranks**.

* though with these it is the axle length that affects Q factor, not the BB shell width the BB is designed for.

** Though obviously the BB shell/ BB combination has to be capable of fitting the axle.

Check the spacing side to side and if there's any spacers used make sure that the arms are sitting central to the frame. As has been said above there's no reason for things to be hitting in one gear only and you've probably just fitted it slightly squiffy.

With the longer axle version of the xx1 you have a larger spacer on the drive side end of the axle , this axle is part of the chainset and not the B.B.

but from other threads its seem sram have changed the arm shape as well as the axle length but struggling to find a definitive answer

I had this with some cheap sram s1000 cranks, they would flex enough that the 5mm or so of clearance would dissapear & Id clip the stays - if i pushed hard enough I could get them to bend enough(couldve been the bb I suppose) , on a different bike it was fine as the rear end was narrower

I'm saying broken frame.

Low clearance or a crack in the frame.

If the clearance between the crank and frame is the same both sides, I'm going with a cracked frame.

It's fairly surprising initially how much things will flex under power.

I'm probably being dense, but I assumed that the Granny Gear referred to the lowest gear (biggest cog) rather than the highest gear (smallest cog) which would be the one generating the most power; are we definitely talking about the highest gear?

I’m probably being dense, but I assumed that the Granny Gear referred to the lowest gear (biggest cog) rather than the highest gear (smallest cog) which would be the one generating the most power; are we definitely talking about the highest gear?

Granny = smallest ring on a triple, traditionally. Now smallest on MTB double, only applies to a road bike if you have a triple though. So smallest gear, but at the other end.

Fair enough. I'm aware of the granny ring on a triple (or double) but as this is the lowest gearing option I'd assumed that a granny gear was the lowest gear (biggest cassette cog on a single ring setup like the XX1).

One internet definition I've come across advises that it is indeed the lowest gear whilst another advises that it's the smallest cog (highest gear) due to it having fewest teeth (like a granny).

lowest gear whilst another advises that it’s the smallest cog (highest gear) due to it having fewest teeth (like a granny).

I assume they were just getting confused by backwards explanation of the term. I certainly always understood it to be the gear that would only be required by one's granny.

Not the same but I occasionally get tyre rub on the front mech when in the granny (smallest front ring) - usually when pushing hard on bumpy terrain. I'd just thought it was frame/wheel flex in that gear and not to worry about it cos it was fine for 99% of the time 🙂

which would be the one generating the most power

Not sure how you got to that. Power is force at the pedals x speed of the crank. So you can generate the same power in any gear. Torque is all over the place though.

And granny gear has been the lowest available ratio for decades (at least 3). No idea where the less teeth idea came from.

>Low clearance or a crack in the frame.

>If the clearance between the crank and frame is the same both sides, I’m going with a cracked frame.

>It’s fairly surprising initially how much things will flex under power.

The fact that the chain is on the right side of the bike affects how each side of the bike acts under load. When you're applying load with the left crank the chain tension is also attempting to twist the bottom bracket clockwise about the seat tube, compressing and bending the drive side chain stay, which can lead to the drive side crank arm being twisted into the chain stay, especially if the rider is pulling up a bit on the right hand pedal.

When pressure is applied with the right hand crank, the force vectors of pedalling resist the forces of the chain and result in less BB twisting.

So, the amount the BB will twist is dependant on the tension in the chain, which will be highest in the lowest gear, under hard climbing, and the maximum deflection will be when the left crank arm is at 2-3 o clock, and will tend to drag the drive side crank arm into the chain stay.

Clear and concise explanation there, thanks @brant.