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How to fit a SRAM Red crank into a Trek Madone frame

By Lennard Zinn

SRAM's red crank.

SRAM’s red crank.

Photo: Courtesy

Well, the manuscript of the third edition of Zinn and the Art of Road Bike Maintenance is due today, and I’ve been burning the midnight and daybreak oil for some time now, especially the last week, to get it done. Still not certain I’ll make it.

But somehow, despite not riding or answering the phone these days, I got sucked into answering this question in some serious detail while writing Chapter 8 on cranksets. Back to the book now.

Red crank in a Madone?

Dear Lennard,
I understand, more or less, the difference between current “standard’ English bottom brackets and oversized bb30 bottom brackets and believe I get the gist of the whole BB90 craze (explained last week).

What I am not clear on is why my Red Crank (currently on a TT bike) and my Dura-Ace 7800 crank (on the road bike) are not interchangeable (both 10 speed double cranks).

Is it a difference in spacing in the bottom bracket, or in the crank spindle length, or both (because the diameter of both is the same, right?)?

My road bike, a Madone 6.5, has the BB90 press-in bearing bottom bracket but if I want to put my Red crank on it (which I do) Trek says I need a different bottom bracket bearing kit. The kit itself is only $20 but it hasn’t been available for months now. Wondering if I should just go for it or am I going to run into major problems?

Dear Spence,
You will run into major problems if you just stick that SRAM crank in there as is. You can make it work if you really can’t get the SRAM GXP (Giga X-Pipe) kit.

The problem is quite basic, and you actually only need a different bearing on the left side (or really, just a shim sleeve) and a washer to take up some space. Both of your cranksets have a 24mm spindle diameter and approximately the same spindle length (maybe exactly the same, but that’s not really relevant). The main issue for you is that the Shimano (and FSA, Easton, etc.) spindles have a 24mm diameter over the entire length, whereas the SRAM and other TruVativ GXP-compatible cranks, including my new crank, step down on the non-drive end to a diameter of 22mm.

The reason for this is that the GXP system does not allow the end user to set the bearing side load like many other systems do. When you put your SRAM crank on, you just reef on that 8mm hex key until it’s tight, right? The crank cannot move side-to-side because the left bearing’s inner race is trapped between the shoulder on the spindle and the left crankarm.

Your Dura-Ace left crankarm, on the other hand, is split at the head and has two pinch bolts to secure it to the spindle. I’m sure you’re well aware that if you were to only slide it onto the end of the spindle halfway and tighten it down, the entire crankset could slide side-to-side. Instead, you have to leave the pinch bolts loose, put in the threaded cap, and tighten it to push the left crankarm against the left bearing to eliminate side play.

The tool you tighten that cap with is a little disc you can only turn with your fingers, rather than a star driver on the end of a long wrench, because you are side-loading the bearings when doing this, and you could easily ruin them by tightening that cap too tightly.

On your Madone, however, this system provides a useful benefit. Since your bearings are only lightly pressed in (you can put them in and take them out with your fingers), the Shimano system snugs everything up laterally and holds the bearings in their seats in the frame. So the Madone bearing kit for Shimano consists of only two bearings and two cover seals; any slack in the system you take up when you tighten that left arm’s cap.

Anyway, while the drive-side 37mm OD X 24mm ID bearing of a SRAM GXP will accept a Shimano, FSA, Easton, etc. spindle, you won’t be able to get any of those spindles through the non-drive side bearing. Similarly, if you were to stick your SRAM crank spindle through your Shimano bearings in your Madone, the left end would wallow around in the bearing.

Now, if I were in your boat and really could not get the bearings, I might be tempted to rig something up to make your Shimano bearings work with your SRAM crank. Here’s the reason you could: SRAM uses the same 37mm OD X 24mm ID bearing on both sides in many of its bottom brackets; there is simply a 1mm-thick sleeve inside the left bearing that has a thicker, 3.5mm plug section behind the bearing. The total sleeve length is 11.5mm—the exact distance, when added to the 1mm-thick metal cover seal, that the spindle shoulder needs to end up away from the left crankarm.

But, stop before you head out the door to push the sleeve out of an old GXP bearing set (which you can do — almost any SRAM or TruVativ road or mountain-bike GXP crank will have one; I’ve only recently found 37 X 22 TruVativ bearings where the inner race is already the correct size for the crank, as well as sticking out 3.5mm further on one side to equal 11.5mm total length). There are a couple of other complications to consider.

The first is that SRAM’s bearings are 8mm thick — 1mm thicker than Shimano’s bearings. That’s probably why Trek is out of stock in them, because they are a proprietary bearing size. You can go to a bearing house and buy the Shimano bearing, but not the TruVativ one. Perhaps you saw my VeloNews 2008 issue #9 article on switching out your bearings in external-bearing cups with ceramic ones? The upgrade bearings you get for all of these external-bearing cranks come in a stock 37mm OD X 24mm ID X 7mm thick size. This will work with Shimano, but it won’t work with SRAM without a 1mm-thick flat washer against the left bearing under the 1mm-thick cover seal.

Despite swapping in the thinner bearing, you don’t need the spacer on the right on an external-bearing GXP crank, though. That’s because the lateral position of the GXP spindle is established by clamping the left inner race between the left crankarm and the spindle shoulder, so it doesn’t matter if the right crank is 1mm further away from the new, thinner bearing.

However, with your Madone, since the bearing is not fixed into a cup and threaded into the frame, you do need to take up that space somehow.

Even though the left bearing is locked between the spindle shoulder and the left arm, the bearing itself is not locked in place and can move laterally. You need the same thing (except smaller diameter) that a Campagnolo Ultra-Torque crank has to take up lateral space: a wavy washer. So, the Trek GXP kit consists of the following things: a right 37mm OD X 24mm ID X 7mm thick bearing, a left 37mm OD X 22mm ID X 8mm thick left bearing with an 11.5mm-deep inner race, a right rubbery cover seal, a left dish-shaped, 1mm thick, steel bearing cover, and a 24mm ID wavy washer for the right side that fits on the spindle between the right arm and the bearing cover seal.

So, three issues are solved if you want to use your current bottom bracket: you need a GXP inner-race sleeve, you need a 1mm-thick washer that has a 24mm hole through it, and you need a 24mm ID wavy washer, but one issue may still remain.

That final issue is that Shimano does not actually use 37mm OD X 24mm ID bearings in its Hollowtech II external-bearing cups. Shimano does use a stock size, but it is a 37mm OD X 25mm ID (7mm thick) bearing! It fits on a 24mm spindle because the outer bearing cover seal is not just a flat disc like it is on a SRAM crank; rather, it is shaped like a top hat without a top, and the sides of the top are 7mm tall and 0.5mm thick (24mm ID, 25mm OD). FSA does the same thing, whereas Easton uses a stock 37 X 24 size.

Now, when you buy, say, an Enduro or Wheels Mfg. ceramic bearing kit for a Shimano or FSA crank, you get stock 37mm OD X 24mm ID X 7mm thick bearings, and you just use a flat cover seal against the bearing like on a SRAM GXP crank.

Madone bearing kits that I have seen do have that same bearing – a 37mm OD X 24mm ID bearing, but I can’t guarantee that it has always been that way. You could have a 37mm OD X 25mm ID bearing with a top-hat cover seal like is in a Shimano bottom bracket. And while it would not matter on the drive side, I would be loathe to push a GXP left inner race sleeve into one of those plastic top-hat-shaped cover seals if you have a 37 X 25 ID bearing. So, check to make sure that when you pull out your Dura-Ace crank that you find a steel surface inside the bearing; if it is a plastic surface connected to the outer cover seal, you have a 37 X 25 bearing, and I would not use that on the left side.

Of course, even if your Shimano bearing kit in your Madone has 37 X 25 bearings, you could always just get your own 37 X 24 bearing for the left side (even a ceramic one if you want to go 0.001mph faster), and push in a sleeve you’ve swiped out of the left bearing of your buddy’s TruVativ mountain bike crank.

If you can get your mitts on an Enduro or Phil Wood puller for removing and installing the bearing in an external-bearing cup (I described how to use these in that VeloNews 2008 issue #9 article on ceramic bearing upgrade), then you can just pull the left bearing out of a TruVativ cup and use it, along with the cover seal it had. You can just leave the right bearing from your Dura-Ace crank in the frame, but you need to get a 24mm ID wavy washer or come up with some other ingenious way to take up a little lateral slop without putting lateral stress on your drive-side bearing.

Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”

Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn. Zinn’s column appears here each Tuesday.