Ride UK Tech Columns

#77 Bottom Bracket Compendium

(This article first appeared in Ride UK issue No. 77 and is reproduced here by kind permission of Ride UK.)

You may remember that 2 years ago in issue 58 I wrote about the new Euro Bottom brackets that were starting to pop up on some frames. I also predicted that there would be some other new standards to look forward to. Well now they seem to be starting to come through so I thought now would be a good time to try to review the whole lot… Hold on to your hats, it’s going to be dull…

USA Bottom bracket.
The big fucker. These have been with us as long as the sport itself. Originally just a piece of 2,1/8” tube with some spacers turned up to fill the gap between the shell and the bearings. Hammer it together and forget it. Over the years as frames have become more sophisticated the shells have become carefully machined with reinforcing rings and nice parallel faces. But why? Who actually broke a USA bottom bracket shell anyway? Even after years of abuse you could always get the bearings in and out. My current US bottom bracket shell was scrounged of an old Hutch Trick Star and is just 1.6mm thick! It’s a loose-ish fit on most cups and while the rest of the Trick Star was pretty laced, the shell is perfect even after another 4 years in my current frame!
But there is no question that people do struggle with these bottom bracket shells. They inevitably distort as you weld the rest of the frame to them, and with no clear engineering tolerances to make them to, the sizes are all over the shop. My current method for fitting US bottom brackets is to measure the shell then machine the cups back to fit. This gives a perfect fit but not everyone has a lathe apparently?!? 
If you just force the cups in then the pressures can be quite incredible. The cartridge bearing can get significantly compressed by these pressures and may not spin as well as it should.
Luckily with such huge bearings this rarely causes them to die and can be an advantage in preventing the cranks spinning while your feet are elsewhere.
Then there is the perception that they are heavy. A big tube with big bearings in is always going to LOOK heavier than a smaller one and in many case it is. With some manufacturers using BB shells that are 3mm thick (twice what mine is) that’s a lot of extra metal to lug around with you. And on a ¾” axled crank like Profiles you have a big gap to fill with cups and bearings so while they are tough the weight can add up a bit.
… so what else is there out there?

Euro Bottom Bracket
Despite its relatively recent rise to prominence in BMX the Euro has been around for even longer than the USA bottom bracket. Originally made in French, Italian and English versions with slightly different thread forms and widths there is now pretty much just the one size. Based on a tube about 1.5” in diameter but with a major difference to the US one. It is THREADED.
It was originally made threaded so that the cups could be adjusted. In the days before cartridge bearings when everything used a “cup” and “cone” you could either adjust the cone, as on an unsealed hub or pedal, or adjust the cup.
The first (fixed) cup was screwed into the right hand side of the frame, then the axle fitted, the adjustable left-hand cup set, and locked off with the locking ring.
Over the years for some bizarre reason this is the size that most normal bikes have gone with. It has caused plenty of problems in its time, but being a fairly standardised thread there is a size to work to so the sizes haven’t drifted around like they have with the USA BB. As a result there is rarely a huge problem putting a good new one into a good new frame, and if there is there are taps to chase the threads out of the frame and ensure a smooth assembly.
Mountain-bikes and road-bikes have long since moved on to sealed for life units, with axle and bearings all tied up in a single package. You screw it into the frame and when something goes wrong you replace the whole damn thing. Shimano have made a fortune.
But in BMX things are a little different. We want to squeeze our big strong cranks in there and then treat it like shit. We want to do big jumps and land hard and there is no suspension to cushion the blow.
So we still have two separate cups but now holding cartridge bearings. We screw in the fixed cup the same as it ever was and then we screw in the adjustable cup and lock it off with the lock-ring. But this is where a lot of the problems start.
The cartridge bearings will spin smoothly over a large range of adjustments of that cup. But if you screw it in too tight you will be pre-loading the bearings something rotten (see issue 75 for stuff about spacer tubes). Add after a few side-load-impacts, from 360s and tailwhips, the small bearings can soon blow-up.
There is a simple and elegant solution to this particular problem. The Gusset cranks that come with a euro BB also come with an extra set of spacer tubes to space both the inner and outer bearing races. As long as both inner and outer spacers are the same overall length you should be able to crank the adjustable cup up as tight as you like without ever pre-loading the bearing.
The remaining problems with euro BBs are mainly to do with neglect and abuse. If you always use the right tools and do everything by the book they should be fine. But let the cups corrode into the frame, or put the wrong side cup in the wrong side of the frame and you are in for some major headaches.
Put a big dent in the bottom bracket shell and unscrewing the cup may become close to impossible.
With smaller bearings that do break much more frequently than their USA alternatives this is likely to be a more common occurrence too, so there are more opportunities to get it wrong.
There is a small but significant weight saving to be made but we close the door on many future weight saving possibilities. Even a 22mm spindle struggles a lot to fit in there, and the bearings become ludicrously small, there would be literally no chance of moving up to a bigger, hollow-er, lighter crank axle.

Spanish Bottom Bracket
A lot of thought has gone into the new Spanish bottom bracket and still it isn’t ready for release but we are told it has settled on its final size. Originally shown as an oversized euro with threads and cups it has now evolved to a cup-less design with a cartridge bearing pressing straight into the frame.
The loss of the threads can only be seen as a good thing. The problems of pre-loading and thread damage are immediately gone and it is likely to be cheaper to produce and saves shops having to buy expensive taps for cleaning a new size of thread.
The logic of putting a bare bearing straight into the frame is simple. Bearing units are already made to astonishingly tight tolerances so there is no issue over size. The 37mm bearing will be so close to exactly 37mm that it would require some fancy measuring equipment to determine just how far off it is.
With the size issue settled, they only need to make the seat in the bottom bracket shell to a hair under 37mm (say 36.95mm) and the bearing will tap in with just a gentle resistance.
Perfect…? Well maybe not. As with any other bottom bracket shell the Spanish version will be welded into the frame, there is therefore inevitably the possibility of the shell distorting under welding. A shell machined to 36.95mm might move slightly out of round as the down-tube, seat-tube and stays are welded on and the rest of the frame tries to pull it in different directions. So they will probably need to be welded on then reamed and faced back to size. This is no big deal as long as it is done, but may add some cost.
The other issue is the size itself. 37mm is a little smaller than I would have liked to see them use. You wont find bearings to suit ¾” or 7/8”cranks like Profiles on any bearing stockists shelf. Nor ones to fit 22mm Primo cranks.
Fly are confident that the bearings will become common place and fairly cheap with time but they are unlikely ever to be made in the kind of numbers bearing manufacturers consider good so there will always be a slight premium to pay. There is again little scope for crank innovation. You can get a 37mm bearing to take up to a 25mm axle but the bearings are getting close to their limit and the strength wont be fantastic. My other worry is that bearing modified to fit will be a little thin in places and may shatter unexpectedly.
I would have much preferred to see them go with a slightly larger 42 or 47mm bearing, so that stock bearings in a lot of axle sizes could be slid straight in there and others accommodated with a small cup… But I guess that puts us almost back to the US bottom bracket and leaves the door open for the same old problems of fit.

Megatech Bottom Bracket
You may not have heard of this one. Similar to the Spanish bottom bracket, it has been around a long time but hasn’t really caught on yet. Using a 47mm bearing with a 27mm crank axle it ties the frame to a limited range of cranks. Not seen any sign of it on BMX yet but if you do, now you know what it is.

Truvativ Bottom Bracket
Truvativ make a thing called an ISIS overdrive bottom bracket. Using the ISIS standard which has become fairly popular with mountainbikers and some flatlanders and can also be used with a euro bottom bracket system.
The overdrive is threaded like a euro but at around 48mm compared to the euro’s 35mm they can fit some bigger bearings in there.
The problems of thread damage are still there but because this is a sealed unit there are no problems with bearing pre-load.
The ISIS system means you aren’t going to be fitting your existing Profile and Primo cranks straight in there unless they bring out some other versions of the bottom bracket. Haro seem to be heading in this direction but I don’t have any details yet.

George’s Bottom Bracket
Finally. Its all very well for me to sit here picking holes in all these systems but can I do any better? Maybe I should shut the fuck up until I have an alternative to offer? Well as it happens I do. How about we all stick with the USA bottom bracket, make the shell thinner to save weight and agree on some tolerances for the bearing cups and shell!?!
In the absence of this common sense approach there is a simple cheap alternative, which I have done in the past and works really well. Just make the cups out of plastic. Being significantly softer than the aluminium we have now it can accommodate a much larger range of sizes. The steel bearing and shell can squeeze the cup to relieve some of the pressure on the bearing and they slide in and out very easily. Distorted bottom bracket shells make no difference, plus they are less than half the weight. I first made a set of these for Ben White about 4 years ago and they worked a treat….

If you have waded through all this twaddle then well done and I hope you get over your insomnia soon…

Suggestions and requests for future tech columns gratefully received…





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