SRAM Apex bottom bracket

[guzzimag]

Hello there,

I have a Spa Audax Ti which I love, but it's developed a clicking bottom bracket bearing. It's only done about 500 miles, so taking it back to Spa this week for them to have a look and do it's first service.

My question is: what alternative bottom bracket bearings will work with the SRAM Apex chainset? I'm not sure I want to replace the Apex bottom bracket with another if it's only going to last 500 miles! Something of a better quality would be nice with a max price of say £50 (so not the top end SRAM ceramic bearings)

Any suggestions?

[Brucey]

one like this?



if so I understand the magic words you need to look for are '24mm stepped axle' and/or 'GXP' in the specs. The same system is used by Truvativ and Bontrager as well as SRAM.

if you see here;

http://chrisking.com/files/pdfs/GeneralBBManual5-10-A%20Web.pdf

you can see how GXP system system cranks can be fitted to a Shimano-compatible CK BB by using special spacers. I believe the required spacers are available separately; it may be possible to convert other shimano-style BB units using the same spacers, but I have not BTDT.

[edit; Hope do H-II converters also, e.g. http://www.bikeoutlet.co.uk/products/Hope_Truvativ_GXP_Bottom_Bracket_Adaptors_Convertor_Kit-761-34.html fair bit cheaper too...]

If in any doubt re. compatibility, it is not a bad idea check with the vendor before purchase.

hth

cheers

[ChrisButch]

I have a Hope BB with the Truvativ/SRAM adapters, which I'm using with a GXP chainset. The BB is fine, but what I don't like is the way the GXP locking system forces the outer seal on the BB to rotate with the crank, which means it doesn't spin freely. I imagine the theory is that the added friction is negligible - but I'd rather have the satisfaction of a smooth, free-spinning unit.

[Brucey]

isn't the seal designed to rotate with the crank?

cheers

[ChrisButch]

Yes, it clearly is - but that doesn't mean it doesn't add friction! Some time since I read the manufacturers' blurb on the Truvativ/Sram GPX design - but as far as I remember they argue that their system, in which the non-drive side crank clamps hard against the face of the BB, reduces the lateral stresses on the bearings.

[Brucey]

my understanding is that the bearing preload is (in Campag UT, PT, and these GXP designs) controlled by a wave washer (or similar).

In the GXP design, unless there are too many spindle spacers, no amount of crank tightening can overload the bearing preload.

BTW seal drag can feel worse than it really is; unlike several other forms of friction and drag, seal drag is fixed and independent of load. Thus it can feel quite bad when just turning the cranks, yet be a tiny proportion of the transmission losses when under real load.

cheers

[ChrisButch]

My memory of the blurb is hazy, but I'm pretty sure that they argue their design means the bearings aren't preloaded. For anybody not familiar with the system, the Truvativ/GXP fixing method is not unlike a square-taper BB on the non-drive side - a captive bolt in the centre of the crankset screws directly into to tapped end of the BB spindle, the circumference of which is splined. The recommended torque as very high. I'm sure what you say about the drag is right - but there's something very satisfying in spinning you cranks with the chain off and trying to beat your mate's maximum revolutions before standstill.

[Brucey]

Such bearings normally need a small, controlled preload on them; without this preload the bearing separator piece (that spaces the balls apart in the groove) will start to wear because the balls won't be rolling round, they will be pushed round by the separator piece instead.

Here;

http://chrisking.com/files/pdfs/GeneralBBManual5-10-A%20Web.pdf

part 'H' for the GXP version is a disc spring that preloads the bearing. I don't know this for sure (I've not taken every version apart) but I understood this type of spring (or equivalent) to be fitted to every GXP BB. (It certainly exists in the Campag ones also; although these are differnt the bearing is preloaded in the same way).

cheers

[ChrisButch]

I've tracked down the following on the Sram/Truvativ website:

"GXP is different — and better. Other systems rely on side-loaded bearings to eliminate play, causing bearing wear. GXP captures the left-side bearing between the spindle and the left crankarm and allows the drive-side bearing to float axially on the spindle. There's no need to side-load bearings during adjustment. No excess wear. And a much simpler installation"

http://www.sram.com/truvativ/technologi ... om-bracket

[Brucey]

Hrmph... that page might just as well say stuff like 'our bearings are lovely, please buy our product' for all the hard technical facts in it.... there is (if you look hard...) some further info on the SRAM website.

See the 2010 (latest one available via their website BTW) technical manual p.5 (edit; p10 actually) here;

http://cdn.sram.com/cdn/farfuture/aV8_nnEJibsHn-JnuB_6DiWm0ljCDDYGHKNr4BdgeRQ/mtime:1299190122/sites/default/files/techdocs/my10-truvativ-tech-manual-rev_1.pdf

the GXP BB assy contains wave washers that I can see.

I suspect that 'floating' may imply to some 'complete absence of preload'. However as I have noted previously designers do not tend to do this willingly for good reason. I suspect that in this case 'floating' may be taken as 'controlled spring preload'.

cheers

[guzzimag]

Brucey et al - that's really useful information thanks will have a look through it all. Bike is booked into Spa for it's first service and a look at the ill BB so see what they say

[ChrisButch]

Pass!

[guzzimag]

To finish up the tale....the bike came back from Spa a bit back, with a stripped, cleaned and rebuilt BB. It lasted another couple of weeks before it went south, looked like the chainring was warped and no amount of adjusting the front derailleur would stop the chain rub.

Back to Spa...bottom bracket was knackered, not the chainwheel so I now have a new BB (Sram again). Can't fault the service at Spa, although was slightly put out when it was initially suggested I'd buggered the chainring by shifting under load! Harumph. I like to think I'm not a total butcher with the gearchanges. Anyway the replacement BB works, might be my paranoia but it just doesn't feel as solid as the Shimano Hollowtech II on my Hybrid and still makes some noise.

If it goes again I'm going to stick a Hope one in. Maybe the SRAM just can't take the grunt I'm putting out when cranking up the hills in a big gear

[guzzimag]

A further update for anyone interested. The replacement SRAM GXP bottom bracket is knackered, AGAIN.

Not impressed, this one lasted 400 miles before the crunching and grinding started again. I've had enough: fitting a Hope BB next week. Still love the Spa though but the SRAM running gear is not great, wish I'd specced 105 now but you live and learn!

[guzzimag]

I've now Installed a Hope BB with GPX converter shims, done 300 ish miles on it and it's a syrupy smooth thing of wonder and has transformed the bike. I love my Spa again all is forgiven