Different Q Factors on Drive/NDS? BB30 SRAM Red

[proletariandan]

Hey y'all,

I just noticed that on my Santa Cruz Stigmata with a BB30 and Red Crankset that the drive side crankarm sits about 1/8" farther out than the non-drive side, measured against the downtube and seattube. I assume that is a problem - right? My road bike with a GXP crankset has the same measurements on both sides...

Thanks!
Dan

[Mark McM]

Firstly: Are you sure the downtube and seat tube on your frame are symmetrical (at the bottom bracket)? For strength and stiffness, tubes are often made as fat as practical, but because there needs to be clearance on the driveside for the chainrings, the tubes are assymmetric at the bottom bracket (the downtube and seat tube may project outboard further on the non-driveside).

Next: If the pedal offsets (Q-factors) of the crank really are asymetric, you may want to if the spindle spacers are installed correctly. Depending on the width of the BB shell, SRAM BB30 cranks use different width spacers to make up the width difference. You can find instructions on SRAM's website on how to install the cranks with different BB shells (BB30, BB386, etc.)

[kingpin75s]

Some bikes are sold with offset crank setups.

I have seen as much as 8mm offset on a stock bike. Any time you are balancing chainring clearance against tire clearance, this can be a thing. You do not see it much, if at all, in true road bikes for obvious reasons.

Only matters if you notice. Some people are more Q and asymmetrical offset sensitive than others.

I will take a little offset to clear my ring and maintain a low Q, but for me maybe 3-4mm at most. I seem to be more Q sensitive then asymmetrical offset sensitive.

[proletariandan]

Good point about the downtube (which is massive on the Stigmata) possibly being offset. I'll try measuring against the BB shell width (which I assume would be centered) and divide that in half!

[Road Fan]

With square-taper installations, here's how i see it and what I've done. Sometimes a chainset may be designed for a little offset, but the spec that the owner (that's us) is given only says "113 mm JIS." Owner goes out and buys 113 JIS and the package does not talk about offset, so the bike is built or repaired with a BB carttidge that does not really match the chainset, but is not far enough off to be a problem or to be irritating.

If you can't feel the offset, that's good. If neither crank arm is hitting the chainstay and if the chainring or 5-pin bolts are not hitting the chainstay, again not a problem. If the knowledge that your carefully-blueprinted frameset with chainset is not perfect is in itself irritating (this is me!), then it's not a real problem, but you might want to keep your eye open for a support group or a source of BBs that are well-described. If the non-problem is an obsession, seek professional help, but also get a Sutherlands' 5th and study JIS spindles and cupsets, so you can buy in the exact spindle offset needed to satisfy your obsession and restore balance to your universe. I found this last option to work and to be satisfying.

I also realized that three items of perfection can be achieved in this process, at least with square-taper systems - balanced Q, correct chainline, and minimum Q.

[oldpotatoe]

Originally Posted by Road Fan

With square-taper installations, here's how i see it and what I've done. Sometimes a chainset may be designed for a little offset, but the spec that the owner (that's us) is given only says "113 mm JIS." Owner goes out and buys 113 JIS and the package does not talk about offset, so the bike is built or repaired with a BB carttidge that does not really match the chainset, but is not far enough off to be a problem or to be irritating.

If you can't feel the offset, that's good. If neither crank arm is hitting the chainstay and if the chainring or 5-pin bolts are not hitting the chainstay, again not a problem. If the knowledge that your carefully-blueprinted frameset with chainset is not perfect is in itself irritating (this is me!), then it's not a real problem, but you might want to keep your eye open for a support group or a source of BBs that are well-described. If the non-problem is an obsession, seek professional help, but also get a Sutherlands' 5th and study JIS spindles and cupsets, so you can buy in the exact spindle offset needed to satisfy your obsession and restore balance to your universe. I found this last option to work and to be satisfying.

I also realized that three items of perfection can be achieved in this process, at least with square-taper systems - balanced Q, correct chainline, and minimum Q.

Ahhh, a mention of Sutherland’s 5th edition, the best....