SRAM GXP in BB90

Discuss light weight issues concerning road bikes & parts.
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Valbrona
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Joined: Fri Oct 28, 2011 3:25 am
Location: United Kingdom of Great Britain and Northern Ireland

by Valbrona

Not done one of these before.

Do you fit an MR2437 bearing both DS and NDS, and then do you fit a shim to reduce the ID of the NDS bearing from 24 to 22?

If so, where do you get the shim from? Does it come with a 'SRAM GXP for BB90' bottom bracket, or do you make a shim from a fag packet or similar?

Thanks.

by Weenie


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Calnago
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by Calnago

Trek has “kits” that will allow you to do this. If you contact them they’ll be able to tell you the exact part number for the kit and you should be good. It’s not too expensive.
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alcatraz
Posts: 1298
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

MR2437 bearings are easy to find replacements but they aren't original GXP bearings.

You need a shimano to gxp conversion kit. Which essentially is a cylindrical spacer/sleeve for the NDS side and a wavey washer. The 2.5mm spacer sometimes supplied is for shorter shell widths and is not needed with bb90.

I have one of these conversion kits running on a bb90 domane and a sram red crank. It works fine. Depending on the thickness of your dust caps you will maybe have some difference in compression of the wavy washer. You need to observe the compression of that washer closely.

At first I thought the crank seized and that I had to change the order of the sleeve/dust caps. In the end I put it back the same way as it once appeared seized and it spun nice and free again. My tip is to make sure the bearings are pressed in fully and that the spindle is clean and goes through all the way.

Ideally the wavey washer should be nearly completely compressed. Otherwise you will have some side play which is bad for front shifting.

Make sure you have
1. bearings
2. dust caps
3. wavy washer (24mm ds)
4. reducer sleeve 24-22
5. patience

Then you should be good to go.

alcatraz
Posts: 1298
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

If you buy the bearings and conversion kit you will not have dust caps.

The crank will have more side play than with an original gxp bb, but your spindle and left bearing should last longer as they arent locked together.

/a

TobinHatesYou
Posts: 1883
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

alcatraz wrote:
Sun Aug 19, 2018 5:22 am

Ideally the wavey washer should be nearly completely compressed. Otherwise you will have some side play which is bad for front shifting.
Play is bad for more than that. With Trek BB90 being more of a slip-fit system, play would probably result in the bearings backing out of the BB shell/bores.

IMO, the best thing to do would just be to get the appropriate 22x37x"8" bearing with the slightly wider inner race.

hambini
Posts: 356
Joined: Tue Nov 15, 2016 8:13 am

by hambini

The method I have used in the past is to use a 6805 bearing on the top end of the tolerance spectrum, then machine POM for the interfaces.

SRAM manufacturing tolerances are very poor so after you have wound the taper on the non drive side crank arm in, you can have a gap that can vary by 2-3mm.

I try to avoid having a hardened steel bearing running against a relatively soft crank axle as it will eat it hence the POM

jlok
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by jlok

Giant Propel Advanced SL Disc 1 / BMC TM02 < Propel Adv < TCR Adv SL Disc < KTM Revelator Sky < CAAD 12 Disc < Domane S Disc < Alize < CAAD 10

alcatraz
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Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

TobinHatesYou wrote:
Sun Aug 19, 2018 6:38 am
alcatraz wrote:
Sun Aug 19, 2018 5:22 am

Ideally the wavey washer should be nearly completely compressed. Otherwise you will have some side play which is bad for front shifting.
Play is bad for more than that. With Trek BB90 being more of a slip-fit system, play would probably result in the bearings backing out of the BB shell/bores.

IMO, the best thing to do would just be to get the appropriate 22x37x"8" bearing with the slightly wider inner race.
I'd also recommend to just buy a gxp bb for bb90.

It just struck me though that what you said about conversion kit not being good because of the bearings backing out can't be any better with a gxp bb. The NDS bearing must be much easier to back out when the system relies solely on the one bearing holding the crank in place sideways.

If that doesn't happen I wouldn't worry about it happening with the conversion kit. I'd worry more about dropping chains. Luckily that hasn't been a problem after tuning the fd carefully.

Besides we always have a wide range of retaining compounds to use for bearings that won't stay put. Is the rider putting out 1500w sprints then it would probably require a different approach.

/a

Valbrona
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Joined: Fri Oct 28, 2011 3:25 am
Location: United Kingdom of Great Britain and Northern Ireland

by Valbrona

Thanks folks. Great advice.

TobinHatesYou
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Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

alcatraz wrote:
Sun Aug 19, 2018 11:40 am

It just struck me though that what you said about conversion kit not being good because of the bearings backing out can't be any better with a gxp bb. The NDS bearing must be much easier to back out when the system relies solely on the one bearing holding the crank in place sideways.
Essentially correct. The preload from the wave washer helps sandwich everything in place with press-fit GXP BBs...conversion or not. The only BBs that don't need the wave washer are external threaded BBs and conversion BBs which hold the bearings captive.

hambini
Posts: 356
Joined: Tue Nov 15, 2016 8:13 am

by hambini

In my opinion GXP is the worst bottom bracket standard out there. Made to dodge some Shimano patents and they made a hash of it.

Valbrona
Posts: 1504
Joined: Fri Oct 28, 2011 3:25 am
Location: United Kingdom of Great Britain and Northern Ireland

by Valbrona

hambini wrote:
Sun Aug 19, 2018 4:40 pm
In my opinion GXP is the worst bottom bracket standard out there. Made to dodge some Shimano patents and they made a hash of it.
Not as bad a Stronglight ActivLink.

by Weenie


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