This article will cover the technical and engineering aspects of SRAM GXP. Sram GXP is a very popular crankset standard used by SRAM. It offers an industrial based bearing arrangement and was touted as having the lowest weight and rolling resistance of any crankset bottom bracket standard. The engineering will be analysed in detail. Currently the standard has been superceded in the retail market, but a large base of existing users and OEM customers such as TREK continue to use GXP.
This guide will explain some of the quirks with the GXP standard and why fitting it can be problematic.
SRAM GXP was originally a design from the company Truvativ. GXP standing for Giga X Pipe. SRAM bought Truvativ in the early 2000’s and since then it was used as a drop in rival to the advancing Shimano Hollowtech 2 standard. At the time, threaded bottom brackets were common place and both the Shimano and the SRAM standard moved the bearings outwards beyond the extremities of the frame thus increasing stiffness and reducing the amount of cantilevered torque transfer.
From an engineering perspective it is obvious that GXP was designed to skirt around Shimano patents related to Hollowtech 2 whilst maintaining close compatibility to frames. It appears to have been designed so a modification (additional spacer) to a Shimano bottom bracket could be made to allow it to make it GXP compatible.
- GXP axles are steel
- The section towards the drive end is 24mm
- The section towards the non drive crank arm is 22mm (stepped)
- The crankset split is on the non drive side and uses a tapered fit (this is problematic, see further on)
- The non drive side crank arm attaches to the axle using a very shallow taper.
GXP was one of the few crank standards that used a stepped shaft and subsequently a fixed/floating bearing arrangement. The other notable exception was Praxis with their M30 standard, this came after GXP and shared many common technical traits albeit in a larger diameter (30mm and 28mm step).
Asymmetrical bearing loading
The action of pedaling causes two forces to be generated. The forward rotation is the most obvious and easily accounted for, this is called a radial bearing load. A less well known force is an axial force which is caused by the Q factor of the pedal force. This axial force places a load on the bearings that tries to push the axle out of the bike frame.
On most cranksets (Shimano, SRAM DUB, 30mm, Campagnolo), the load is distributed to both bearings within the bottom bracket. In the case of SRAM GXP the axial forces are taken up solely by the bearing on the non drive side.
This causes problems as this bearing is heavily loaded and comparatively small in comparison to the potential loads that could go through it. Users often report the non drive side bearing fails quickly and this is the reason for this phenomenon.
The fixed-floating bearing setup that GXP uses is common in the industrial world, many axial fans, pumps and compressors use this setup but those systems are running with constant torque. Pedaling is a notoriously peaky torque application with the majority of torque occurring between 1 and 5 o’clock of the pedal rotation per side.
Many bicycle teams that ran GXP often used to remove the inner clamp ring and emulate a Shimano setup with two floating bearings that were preloaded. From a maintenance, longevity and frictional perspective, this was advantageous.
Poor manufacturing Tolerances
A lot of people are totally unaware of the poor manufacturing tolerances associated with SRAM products. GXP is no exception. There is an issue around the taper of the non drive side. The taper locks the non drive side bearing up. The poor tolerances associated with GXP mean that the length of the 22mm section can vary significantly despite the same bolt torque being applied.
SRAM are obviously aware of this, their solution involves some frankly ropey method of regreasing the bolt and “trying” a few times to induce some surface wear until it clamps up. This is clearly stated in their installation instructions, which can be downloaded below. The appropriate paragraph has been extracted and pasted for convenience.
The nominal length of the 22mm section is 12mm but ranges from 10.5 up to 14.5mm, this range is excessive and indicative of a poorly thought-out system or insufficient manufacturing accuracy to ensure repeatability.
The photographs below show how to determine the Distance Between Crank Arms (A) and the length of the 22mm section (B). Both dimensions are required for accurate setup of GXP.
The procedure to work out the A and B dimensions are as follows
- Bolt the non drive side Crank arm on to the axle.
- Mark the point where the axle enters into the crank arm
- Remove the axle and measure from the drive side to the edge of the mark. In this case the DBCA (A) is 93mm, the length of the 22mm Section (B) is 11mm. The length of the 24mm section is by calculation 82mm (93 – 11 = 82).
SRAM’s non drive side bearing for GXP has equally poor engineering. The bearing has a 22.2mm (approximately 3/4″), this is to interface onto an axle of only 22mm. The resulting 0.2mm clearance fit is frankly inexcusable.
Hambini GXP Bottom Brackets
it is impossible to create a bottom bracket to work with an axial range of 4mm. Hambini bottom brackets are made with a clamp axial spacing of 12mm, if there is any play after the adequate amount of torque has been applied to the non drive side crank arm. The remainder should be taken up by the addition of additional spacers.
The clamping section on Hambini bottom brackets are 22mm with a H7 fit onto the axle. Hambini GXP Bottom Brackets are available in BB86 GXP and BB30 GXP as off the shelf units. Other frame fits can be made to order