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Q Factor???
Hello all
I am confused as hell on what SRAM's Q factor actually is.....
I have an SB95 and i already have a set of XX cranks that are 156 q factor. When the bottom bracket is installed properly i am unable to get the drive side crank to go all the way through the bottom bracket..... if i unscrew the non drive side bottom bracket cup and move it out about 8-10mm it allows me to push the drive side crank arm all the way into the bottom bracket but as soon as i screw the non drive side arm onto the spindle it pushes the drive side arm out of the bottom bracket. I was going to be dodgy and put a spacer on the non drive side cup but they chain doesnt shift very well at all on the front chain rings, i am assuming the incorrect Q factor has something to do with it
Now this is where i am confused.
Everything i can find on Q factor says the lower the number the narrower the cranks. When you look on the Yeti website it says that if you are using XX cranks on an SB95 they have to be a Q factor of 166.....
Correct me if i am wrong but based on the lower number being narrower, wouldnt i need to try and find cranks narrower than 156? pretty sure 154 is the narrowest.
The Big Top i stripped ran fine with 156 Q factor and 42/29 rings and i was told that they would work by the LBS, well they dont, so i bought another pair with 156 Q factor and 39/26 chain rings, well they dont work either and i dont want to try and source another XX set if i am going to end up stuck with them as well 
The last question i have is does this Q factor apply to other cranks in the SRAM/Truvativ range as i cant really find anything that says it does, which leaves me really confused yet again. XX cranks are worth a small fortune and i was thinking of getting something a little lower specced
Any help would be greatly appreciated
Brad
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Sram uses the Mega Exo BB, which has a smaller non drive side bearing. This is to stop side load on the bearings like Shimano does. (cleaver idea, just not as good as Shimano for some reason)
It just squishes the non drive side bearing between a shoulder and the crank arm.
Sram Q factor is set by where they machine in the shoulder, your will be 156 you just don't have enough spacers in to bring the cups out to the ends of the crank shaft.
So you push your cranks through till the shoulder rests up against the smaller bearing.
Check your chainline and adjust spacers on ether the drive side or the non drive side till you are happy. Just make sure the drive side is not pressing up against the bearing as this will cause pre-mature wear. leave a millimeter or so gap if you can.
BTW you can use a shimano BB on the drive side when the shitty Sram bearing dies.
Hope this helps.
thanks for the reply Crispy
I somehow think that adding spacers will be the dodgy way to fix it though. I have tried to bodgy up the positioning of the cranks by leaving the bottom bracket cups loose and playing with the spacing and it just doesnt seem to shift right. I cant find that happy spot that lets the gears shift effortlessly like they used to on the Big Top
It also states on the Yeti website that i need 166 Q factor, and i know i shouldnt doubt them as they designed the bike, but i struggle to see how that can be the case when the lower the number is the narrower the cranks are
From what I understand, the Q factor doesn't have anything to do with the bottom bracket width, it's the distance between the ends of the cranks and their ability to clear the chain stays. The problem is the bearings you're using, you need to use the correct ones for the cranks. I have Sram XX cranks with the 156 Q factor and I had to use the Correct Sram bottom bracket bearings for it all to work properly. Don't use other brands, and there should be a star spacer that keeps the drive side against the bearing. If you want I should be able to find the details of the bearings I used,
Hi Zoom
Its got the right bottom bracket. I bought the whole groupset together and it came with the SRAM GXP bottom bracket
I guess i will annoy Paul Rowney again with this question. Given he is the Yeti Oz distributor he should be able to clear it up straight away.
I am sure Paul has cleared this up, but to confirm - Q-Factor has nothing to do with BB width. Its only about the distance between the pedals and the 'virtual' centerline of the BB. How die your feet are apart essentially.
This is decided by the amount of 'flare' (for want of a better word) of the cranks from the BB axle.
All BB axles from SRAM are the same length.
The SRAM 156 Q-factor is currently the narrowest 'non-custom' set-up avaiable. People like it narrow as it replicates their foot position on their roadie, which will have a Q-factor way lower.
Wider Q-factors are needed to allow for cranks to clear beefy rear-ends - swing arms, pivots and short chainstays on hardtails with any mud clearance mean that narrow cranks can contact the frame. A stroy for another day.
The good news is that it means the issue you have is only down to spacing of the BB shell / cups, which will be easily resolved by Paul.
What this means I think is that you will need to adjust your chainline by changing the number/width of spacers between the BB shell and the *non drive side* BB cup. Then pack out the drive side to fill the gap and support the crank axle (optional). It's the NDS BB cup position that sets the chain line with Mega Exo cranks.
Guess i give it a go then hey. Will have to go to some bike shops tomorrow and see if they have some
Well done sir.
I think you'll be fine. Most English BB mtbs have 2 spacers on the drive side anyway, and if memory serves correctly the thread length is no different on the non-drive side. Not sure I'd have used loctite, I prefer grease and the right torque (Zinn's is a pretty comprehensive source of info) but a little bit of 243 can't hurt I suppose.
I've got BB30 on the new chariot and I've got NFI on how any of that goes together. Now I'm going to have to read up and tool up for a different standard. Ah, change, The only constant.
Sheldon Brown is a fountain of information on everything bike. Take a look at the link below to see what Q factor is actually talking about. Hope it helps post fixing of bike.
http://www.sheldonbrown.com/gloss_q.html
I once got an email from Sheldon, back in the days when the internet was very young.
RIP Sheldon.