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I'm looking to swap out shorter cranks on my bike. It's this bike, but of note has a SRAM PF30 bottom bracket and SRAM Force eTap AXS, 48/35 chainset.

I'm trying to work out if these will be compatible. They look the same, and I can see DUB on my bike, but I'm really struggling to find a definitive source that says they are compatible. Can anyone answer or at least say what needs to be matched?

My second question is does anyone know how involved switching these will be? All LBSs are booked for months so was planning to attempt myself but is still advisable for a relative novice? As far as I can see I'll need a torque wrench up to 50nm(I've already got a smaller one) but again, I'm struggling to find material on how to go about this from start to finish. Any advice would be appreciated - the best I could do was how to install a spider power meter which does involve taking the crank off the spider.

Thanks in advance!

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They look to be compatible. The website says they'll work with any road DUB BB, which you have. Changing the spider shouldn't be too difficult as long as you follow the video. You can also find more info on the SRAM Service website. For example, here's the spider and chainring installation guide.

Here's the DUB BB compatibility manual: DUB™ MTB and Road Cranksets and Bottom Brackets

As for the big 50Nm bolt, either take it to a bike shop or even a car garage and ask to borrow their torque wrench ("It's just one bolt, can't I just use it for a quick second!"), or just guesstimate the torque, perhaps with the help of a little math to find the amount of force needed with your own wrench. As long as it's on pretty dang tight, it shouldn't fall off.

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  • Thanks very much. I think part of my confusion is as to whether my "SRAM PF30" s a DUB BB or not. As the other answer says, they sell a 30mm one and the specs for my bike don't mention DUB. – Ben Jun 23 '20 at 8:02
  • @Ben If you remove your current cranks, you can measure the spindle diameter to know for sure. DUB BB’s also have the letters “DUB” molded on the outside as well. Although the fact that your cranks say DUB on them is pretty telling of what system they use. – MaplePanda Jun 23 '20 at 22:29
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Here is a different way of thinking of things. All bottom brackets are designed for a certain type of BB cup. Cannondale has chosen the PF30 standard for this frame, and it requires the BB to accept cups that are 46mm in diameter, with a BB width of 68mm. There are, as you know, other specifications.

Mostly independent of the BB cup specs, crank spindles come in various diameters. One set of them is around 24-25mm (e.g. Shimano, SRAM GXP, Campagnolo Power- and Ultra-Torque). Another set is around 30mm (e.g. Easton, Praxis). SRAM DUB spindles are (I believe) a uniform 28.99mm in diameter. Thus, they are in the "around 30mm" group in my head.

If you have a SRAM DUB BB, it was sized so as to take a 28.99mm DUB axle. Thus, all you need to look for is another crank with a DUB axle. If you had got a frame or bike without a BB, you would need to look for a BB that is compatible with PF30 shells and DUB axles. You will need to parse the BB's name and its spec sheet. For example, SRAM's website lists DUB BBs for the British thread standard (i.e. older style threaded shells), for PF30 shells, for BB30 shells, and for regular Press Fit shells.

SRAM's page also has a PF30 and BB30 BB, but those are for 30mm spindles, and they won't fit a DUB crank without an adapter, if one exists. I haven't looked, but one may not exist yet. If there's enough demand and there are no engineering issues that actually prevent compatibility, then someone will make one if there's enough demand.

You might also come across PF30 BBs that claim compatibility with Shimano, GXP, or Campagnolo axles. Don't use those! Those are for spindles narrower than the DUB or 30mm spindles, and you'll never be able to make them compatible. The trade off is that the manufacturers should be putting larger bearings in those BBs, which should increase bearing durability (all else equal).

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  • Thank you, that's really helpful. Part of my confusion was as to whether my BB was DUB or not. As you say, there's a 30mm PF30 and the specs for my bike don't mention DUB. I'm thinking the fact my chainset says DUB on it means this is probably a mistake. – Ben Jun 23 '20 at 8:10

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