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GXP bottom brackets and bearings
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Tri3
Apr 29, 16 5:33
Post #1 of 12 (1870 views)
I ordered a BBright to GXP threaded bottom bracket, advertised as angular contact bearings, for my NP3 build. The BB arrived with ABEC-5 AC bearings on the drive side and ABEC-3 on the ND side. When I asked about this, the manufacturer basically said "We don't like to ship AC for GXP ND side. They don't get preloaded properly because the ND side cannot be side loaded."

I get that AC bearings require more preload to ensure full contact with the inner race. I don't understand the GXP side of the equation. I know that Shimano and GXP systems load differently. What I don't understand is why the GXP BB cannot be properly preloaded for AC bearings on the ND side.

Is it a matter of "can't" or is it matter of "most people don't?" If it is a matter of "can't," why do other companies manufacture GXP BBs with AC bearings--on both sides?

My choices are to go with what was sent (ABEC-3 ND side/ABEC-5 AC drive side); get something with ABEC-5 AC for both the drive and ND side; or upgrade to zero ceramic bearings on both sides. What's recommended?

In case anyone is wondering why I am converting from BBright to GXP, I will be installing a SRAM Apex crankset with the crank arms shortened to 150mm. I have 150mm BMX cranks and a square taper BB on my current bike. Love it, but there is no way to adapt it to the NP3.

Thanks!
No coasting in running and no crying in baseball
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Re: GXP bottom brackets and bearings [Tri3] [ In reply to ]
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loosegroove
Apr 29, 16 7:29
Post #2 of 12 (1823 views)
Have you looked into bbinfinite? I have no idea on their bearings, but the guy I ride with has on his p5 with his GXP quarq .

I think it's the following
http://www.bbinfinite.com/...bright-directfit-gxp

No idea on long term quality because he just installed it
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Re: GXP bottom brackets and bearings [loosegroove] [ In reply to ]
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Tri3
Apr 29, 16 7:36
Post #3 of 12 (1814 views)
Yes, I did look at that one. I like the installation of the threaded sleeves better than the one piece friction fit of the BBInfinite.
No coasting in running and no crying in baseball
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Re: GXP bottom brackets and bearings [Tri3] [ In reply to ]
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chaparral
Apr 29, 16 7:37
Post #4 of 12 (1811 views)
This is the whole design of GXP and in my opinion a very good design. The one side takes all the side loads of the system and the other does not have to deal with it. This is why the drive side axle of the crank is a smaller diameter than the non-drive side. Having angular contact bearings on both sides would be tricky to set up, since the cranks do not have a way to preload both sides. The whole system is designed to be easy, since it only needs to preload the one side.
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Re: GXP bottom brackets and bearings [chaparral] [ In reply to ]
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Tri3
Apr 29, 16 7:40
Post #5 of 12 (1808 views)
So would you go with ABEC-5 AC drive side/ABEC-3 ND, or the Zeroceramic on both sides? The square taper BB I have now is really smooth and practically maintenance free. Which of those choices would get me closest to that?
No coasting in running and no crying in baseball
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Re: GXP bottom brackets and bearings [Tri3] [ In reply to ]
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nchristi
Apr 29, 16 7:48
Post #6 of 12 (1800 views)
The part where they say the drive-side is side loaded but the non-drive side isn't doesn't make sense to me. No mater what type of BB cups you're using, the "side load" on the bearing will be exactly the same on both sides. That's basics physics, all side forces must cancel out.

Now it's true that GXP doesn't have preload, so the side load on the bearing will be lower than is it with regular BB cups. But still, it will be the same on both sides.
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Re: GXP bottom brackets and bearings [nchristi] [ In reply to ]
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Warbird
Apr 29, 16 9:34
Post #7 of 12 (1750 views)
nchristi wrote:
The part where they say the drive-side is side loaded but the non-drive side isn't doesn't make sense to me. No mater what type of BB cups you're using, the "side load" on the bearing will be exactly the same on both sides. That's basics physics, all side forces must cancel out.

Now it's true that GXP doesn't have preload, so the side load on the bearing will be lower than is it with regular BB cups. But still, it will be the same on both sides.

Unlike most other outboard bearing cranks, GXP does not "sandwich" the bearings between the 2 crankarms. The left arm is anchored to the left bearing by a step in the spindle, and the drive side actually has a small gap between the bearing and crankarm. The only side loads the drive side bearing will see is due to friction on the inner race from the spindle, as there is nothing actually seated against the bearing...
"I'm thinking of a number between 1 and 10, and I don't know why!"
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Re: GXP bottom brackets and bearings [Warbird] [ In reply to ]
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nchristi
Apr 29, 16 11:02
Post #8 of 12 (1712 views)
Yes I know, but that doesn't change the fact that in absence of any external force applied to cranks, any side load applied by the right crank arm on the right bearing must be matched by an equal net force applied to the left bearing. Side load comes from "sandwiching" the bottom bracket between the two crank arms. You can't have a net side load on one bearing without an equal (in module) and opposite (in direction) side load on the other bearing. If that were the case then there would be a net force to one side, that would accelerate the spindle in this direction and eject it from the BB. Obviously that is not the case.

When there is no side load on the right bearing (long spindle), that means the force A applied by the spindle on the internal side of the left bearing is matched exactly by the force B applied by the left crank arm on the external side of the left bearing, and there is no side load on the right bearing. If the spindle is shorter then there might be a side force C on the right bearing. A will still be the same, but now B will be greater, such that there is a net force C = B - A on the right bearing.

The manufacturer basically said that the right bearing is preloaded and that the left bearing isn't. That is just not true, and that can't be the reason for having different bearing types.

Now under load, it's a different story. The left bearing will see all kinds of side loads, that the right bearing won't see (in the "long spindle" case).
Last edited by: nchristi: Apr 29, 16 11:10
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Re: GXP bottom brackets and bearings [nchristi] [ In reply to ]
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Warbird
Apr 29, 16 11:34
Post #9 of 12 (1693 views)
nchristi wrote:
Yes I know, but that doesn't change the fact that in absence of any external force applied to cranks, any side load applied by the right crank arm on the right bearing must be matched by an equal net force applied to the left bearing. Side load comes from "sandwiching" the bottom bracket between the two crank arms. You can't have a net side load on one bearing without an equal (in module) and opposite (in direction) side load on the other bearing. If that were the case then there would be a net force to one side, that would accelerate the spindle in this direction and eject it from the BB. Obviously that is not the case.

When there is no side load on the right bearing (long spindle), that means the force A applied by the spindle on the internal side of the left bearing is matched exactly by the force B applied by the left crank arm on the external side of the left bearing, and there is no side load on the right bearing. If the spindle is shorter then there might be a side force C on the right bearing. A will still be the same, but now B will be greater, such that there is a net force C = B - A on the right bearing.

The manufacturer basically said that the right bearing is preloaded and that the left bearing isn't. That is just not true, and that can't be the reason for having different bearing types.

Now under load, it's a different story. The left bearing will see all kinds of side loads, that the right bearing won't see (in the "long spindle" case).

You're making the mistake of assuming that the crank sandwiches both bearings, as on almost every other 2 piece crank. It doesn't. I'm guessing that you haven't actually seen a GXP crank, or installed one. There is a step on the spindle that acts as a bearing seat for the left bearing, and the bearing is sandwiched between this step and the left arm. This is where all the preload on that bearing comes from. And the only thing holding the crank in place is that bearing. The right bearing is not sandwiched between anything. On the drive side, there is no bearing seat, the spindle just slides through. And there is a few mm gap between the bearing and the right arm on the outside of the bearing, and nothing but empty space until the left bearing on the inside. If the left bearing does become loose, then the spindle slides back and forth through the right bearing, and if it goes far enough to eliminate that gap, then there will be a side load on the right bearing (but at this point your BB has failed). The right bearing only has to act as radial support...
"I'm thinking of a number between 1 and 10, and I don't know why!"
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Re: GXP bottom brackets and bearings [Warbird] [ In reply to ]
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chaparral
Apr 29, 16 11:46
Post #10 of 12 (1682 views)
Warbird wrote:
nchristi wrote:
Yes I know, but that doesn't change the fact that in absence of any external force applied to cranks, any side load applied by the right crank arm on the right bearing must be matched by an equal net force applied to the left bearing. Side load comes from "sandwiching" the bottom bracket between the two crank arms. You can't have a net side load on one bearing without an equal (in module) and opposite (in direction) side load on the other bearing. If that were the case then there would be a net force to one side, that would accelerate the spindle in this direction and eject it from the BB. Obviously that is not the case.

When there is no side load on the right bearing (long spindle), that means the force A applied by the spindle on the internal side of the left bearing is matched exactly by the force B applied by the left crank arm on the external side of the left bearing, and there is no side load on the right bearing. If the spindle is shorter then there might be a side force C on the right bearing. A will still be the same, but now B will be greater, such that there is a net force C = B - A on the right bearing.

The manufacturer basically said that the right bearing is preloaded and that the left bearing isn't. That is just not true, and that can't be the reason for having different bearing types.

Now under load, it's a different story. The left bearing will see all kinds of side loads, that the right bearing won't see (in the "long spindle" case).


You're making the mistake of assuming that the crank sandwiches both bearings, as on almost every other 2 piece crank. It doesn't. I'm guessing that you haven't actually seen a GXP crank, or installed one. There is a step on the spindle that acts as a bearing seat for the left bearing, and the bearing is sandwiched between this step and the left arm. This is where all the preload on that bearing comes from. And the only thing holding the crank in place is that bearing. The right bearing is not sandwiched between anything. On the drive side, there is no bearing seat, the spindle just slides through. And there is a few mm gap between the bearing and the right arm on the outside of the bearing, and nothing but empty space until the left bearing on the inside. If the left bearing does become loose, then the spindle slides back and forth through the right bearing, and if it goes far enough to eliminate that gap, then there will be a side load on the right bearing (but at this point your BB has failed). The right bearing only has to act as radial support...

Exactly. It is the way I have designed shafts and bearing before, it makes life much easier and lets you optimize the bearings for their load. The only reason I would not do it this way is if I needed the shaft to have constant diameter.
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Re: GXP bottom brackets and bearings [Warbird] [ In reply to ]
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nchristi
Apr 29, 16 11:58
Post #11 of 12 (1672 views)
Warbird wrote:
nchristi wrote:
Yes I know, but that doesn't change the fact that in absence of any external force applied to cranks, any side load applied by the right crank arm on the right bearing must be matched by an equal net force applied to the left bearing. Side load comes from "sandwiching" the bottom bracket between the two crank arms. You can't have a net side load on one bearing without an equal (in module) and opposite (in direction) side load on the other bearing. If that were the case then there would be a net force to one side, that would accelerate the spindle in this direction and eject it from the BB. Obviously that is not the case.

When there is no side load on the right bearing (long spindle), that means the force A applied by the spindle on the internal side of the left bearing is matched exactly by the force B applied by the left crank arm on the external side of the left bearing, and there is no side load on the right bearing. If the spindle is shorter then there might be a side force C on the right bearing. A will still be the same, but now B will be greater, such that there is a net force C = B - A on the right bearing.

The manufacturer basically said that the right bearing is preloaded and that the left bearing isn't. That is just not true, and that can't be the reason for having different bearing types.

Now under load, it's a different story. The left bearing will see all kinds of side loads, that the right bearing won't see (in the "long spindle" case).


You're making the mistake of assuming that the crank sandwiches both bearings, as on almost every other 2 piece crank. It doesn't. I'm guessing that you haven't actually seen a GXP crank, or installed one. There is a step on the spindle that acts as a bearing seat for the left bearing, and the bearing is sandwiched between this step and the left arm. This is where all the preload on that bearing comes from. And the only thing holding the crank in place is that bearing. The right bearing is not sandwiched between anything. On the drive side, there is no bearing seat, the spindle just slides through. And there is a few mm gap between the bearing and the right arm on the outside of the bearing, and nothing but empty space until the left bearing on the inside. If the left bearing does become loose, then the spindle slides back and forth through the right bearing, and if it goes far enough to eliminate that gap, then there will be a side load on the right bearing (but at this point your BB has failed). The right bearing only has to act as radial support...
I know exactly how GXP bearing works, I've had a SRAM Force in the past. I understand everything you say. You're saying that there is no side load on the right bearing. Reread what I wrote, I'm not arguing against that. And in that case, there is also no side load on the left bearing. The left bearing is "pressed" between the left crank arm and the step on the spindle. Again, at rest the side load will be equal on both bearings. In that case zero.
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Re: GXP bottom brackets and bearings [chaparral] [ In reply to ]
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Tri3
Apr 29, 16 12:05
Post #12 of 12 (1666 views)
To bring this all back to the original post.... Would you go with ABEC-5 AC drive side/ABEC-3 ND, or upgrade to the Zeroceramic on both sides?
No coasting in running and no crying in baseball
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