Confused about SRAM crankset replacement options.

[Sham69]

I've had an SRAM iMotion9 hybrid bike for about 12 years and would appreciate advice on how to select a suitable replacement crankset.

After cycling the SRAM iMotion9 bike, I now often suffer from knee ache (started to happen over the last few years). Depending on several factors, knee ache can occur after just a few miles of cycling. Very frustrating and very limiting. I also have a GT Tequesta ATB which doesn't give me knee problems (although, these days, I rarely cycle more than 25 miles in one stint). Over the months, I've tried to asess why the iMotion9 should give me this problem whereas the GT ATB doesn't. My theory is it's the Q factor. The iMotion9 seems to have a very high Q factor whereas the GT ATB Q factor is siginificantly lower so I wish to replace the original SRAM crankset with a lower Q factor version.

Having looked into this today, I can see it's a potential minefield and would appreciate any advice or guidance on how best to go about this. Seems to me that most online sellers don't mention the Q factor of the cranksets they sell. I wish to spend as little as possible on any replacement. So far, I've identified 6 parameters that need to be met:

The replacement must have:
1 Low Q value. Unsure exactly what? Will try to measure GT ATB Q factor.
2 Crank length 170mm
3 Fitment to suit SRAM "splined" bottom bracket crankshaft (or maybe change BB for more conventional "square peg" style fitment - is that even possible?)
4 42 tooth chainring (40 or 44 would probably be OK too)
5 Accomodate a 1/2” X 3/32” single-speed chain
6 And, of course, the new chainring needs to line up with the single rear sproket - how do I ensure that prior to purchase?

Have I missed anything? Grateful for any help, thanks.

 

[Cycleops]

What is the Q factor?
Knee pain is most often attributed to incorrect saddle height.

 

[andy63]

id not heard the expression q factor before but a quick google shows it to be based on the width of your bottom bracket and there isn't much that you can do to change that from design surely...
strictly speaking its the width between pedal attachment points so the profile of the crank arms may have an effect but with wider bottom brackets the bike frame and chain stays are likely to be wider as well limiting any inward profiling of the crank arms..
just my initial thoughts..
Andy

 

[Sham69]

What is the Q factor?

FYI: https://www.cyclingnews.com/features/what-is-q-factor-and-does-it-matter-to-you/

I wouldn't rule out saddle height being a factor but I've varied saddle height on both bikes numerous times over the years and, whilst it can make cycling uncomfortable, it doesn't seem to affect my knee pain. The knee pain is the result of an old ligament injury that can be aggravated by certain activities - most of which I manage to avoid.

...a quick google shows it to be based on the width of your bottom bracket and there isn't much that you can do to change that from design surely...

The width of the bottom bracket certainly has a bearing (ouch!) but in this case, it's not the major factor. I haven't carried out detailed measurements yet (will try tomorrow) but visually, the bottom bracket of the GT ATB and the SRAM iMotion9 look to be a very similar width - doubt there's more than a few mm in it, if anything.

the profile of the crank arms may have an effect

You've hit the nail on the head!

The main design aspect that gives the SRAM iMotion9 bike such a high Q factor is the shape of the crank arms (Truvative Isoflow 1.0G). This looks similar to what I have:

https://www.getmefixed.co.uk/wp-content/uploads/2018/02/IMG_9668.jpg

Actually, the bend shown on the crank arm in the photo doesn't look quite as severe as the bend I see on my SRAM iMotion crank arms - photos can be deceptive though.

I've inspected the frame and can see no reason why the Truvative crank arms have such an extreme "outward bend" shape, other than a fitted bike stand (original) that, even when retracted, protrudes to one side. And for balance, the other side crank arm also has this "outward bend", of course. I'll happily remove the bike stand if I can source straighter crank arms (or even crank arms with a mild "inward bend" such as those fitted to my GT ATB).

 

[Jameshow]

How much chsinstay clearance do you have?

That I think will be your limiting factor. Less than 5mm don't bother more than 10mm make a change.. 5-10mm???

 

[Sham69]

How much chsinstay clearance do you have?

The crank arms are well clear of the chainstay (chain and chainring and cycle frame). Haven't measured it yet but it's more than 10mm, probably >20mm. I'll measure it.

I've done some measurements of Q factors and, despite appearances, there's not as much difference as I thought. Turns out the GT Tequesta has a Q factor of 159mm and the SRAM iMotion9 hybrid has a Q factor of 171mm, so "only" 12 mm in it (i.e. 6mm per side). I now doubt that Q factor difference is the issue here. Back to the drawing board.

I also measured the width of both bikes' bottom brackets and they are indeed the same, at 68mm.

Thanks for the advice and suggestions.

 

[cougie uk]

Different gears? Are you pedalling a bigger gear on one bike?

 

[Sham69]

Different gears? Are you pedalling a bigger gear on one bike?

Don't think so. Difficult to compare internal hub to derailleur gears but I tend to turn pedals at about the same rpm, whatever the gear I'm in - and on any bike. Guess I need to pay attention to my cycling style on both bikes for a while to see if I can work out what's going on.