Bottom Brackets......... A dummy's guide.

BB30 vs English 68mm or perhaps BB90?????

I'm sure the bike companies keep it as complicated as possible to keep us from truly understanding whats going on.  So this is my simpler version.  Detailed enough so you can blag it down the pub, but not be so boring people call you Alan Cronin.

The fundamentals
The bottom bracket is the mechanism which sits in your frame and connects your crank arms together. It is made up of a spindle which connects your crank arms and a bearing at each of it to allow it to spin freely in your frame.  Every bike has one and in general the stiffer and lighter the better.

English 68mm

The original bottom bracket before it all got silly.  It was called 68mm as that's the width of the bracket that sat in the frame.  A couple of design points on the  picture above to note are the square spindle and the small diameter bearings which screwed and sat internally in the frame. The whole thing sat in the frame with just the square spindle poking out.

English 68mm - V2

This is where Shimano did their stuff (as well as others).  3 big steps forward were made.

1. The spindle moved from a square taper to Shimano's Octalink system which used splines creating a closer more secure fit. Shimano patented this system but other manufacturers got around this by creating their own shared version called Isis.
2. They moved the bearings to sit outside of the frame.  This allowed larger diameter bearings and lengthened the width of the bracket to over 90mm (but its still called 68mm as it fits a 68mm hole)  This all made for a stiffer bracket
3. By moving the bearings out of the frame, the spindle diameter could increase to 24mm.  Because of this increase the spindle became a hollow tube and thus lighter.

BB30

Originally a Cannondale design the BB30 refers a 30mm diameter crank and thus larger bearings. If you remember from above Shimano's Hollotech 2 crank is only 24mm. 
The big differences are;

1. The larger crank and bearings are stiffer (better power transfer through the pedals).
2. The frame's bottom bracket has a wider diameter to allow the over size bearings be pressed directly inside the frame and thus be narrower.

BB90
The latest innovation by Trek  So if your still with me, you would make the common sense assumption that if a BB30 refers to a 30mm diameter spindle then a BB90 will mean a 90mm diameter spindle?  Of course not!  The 90 now refers to the width of the frame. Trek has simply taken a 68mm bracket with the extra 11mm external bearings on each end and widened the frame width from 68mm to 90mm to swallow up the bearings.  (68mm + 11mm + 11mm = 90mm).

The theory is that by beefing up and widening the frame around the bracket from a 68mm width to 90mm they can attain a new level of stiffness.  The bearings are again just pressed into the frame.

So that's it!  Hope it was useful.

Dambuster Duathlon

Last weekend saw me packing up my kit and heading over to Rutland for the British Age Group Duathlon Champs.  A friend Kerry kindly offered me accommodation with her boyfriend Will the night before, which was perfect as they only live a few miles from the start. 

So race day arrived seeing 600 athletes racing over 10k run, 40k bike and a 5k run.

Firstly my learning's (excuses) from this race

1. If a race has British Champs in the title then it will be competitive.
2. My 10 hours of weekly swimming over winter didn't help my biking.
3. Duathlon is nasty and hard.
4. Who is race ready at the begining of March?

The first run around the lake was crazy, I ran a 33.43 10k and only logged the 20th fastest run, over a minute slower than the fastest. 

Then came the bike where I acted as a mobile chicane for the rest of the field......... it was pretty depressing stuff as slower runners just kept on filing past.  Although from this vantage point I got a great view of all the drafting being done!  Why do people cheat?  Very annoying.  I clocked a 1h 11m bike averaging 21mph.

By the final run I was out of the top 40 and had lost a little heart.  Still I saw it as a good training session and headed out to get my moneys worth (cheap skate!).  Back running I cheered up a bit and clocked the 11th fastest run completing the 4.6k (it was short) in 16.18 to come in 34th overall.

So although not the race I hoped for, it was a good stake in the ground and really bought it home that I have a lot of work to do on the bike before the tri season starts next month with Chirk.

Tomorrow is a monster brick session with a cycle on the TT bike to Liverpool to race Liverpool half marathon.  I'm starting my build period in my run up to peak in June so lots of tough days like this to come.

When Les (my old house mate) came to stay this weekend I was nicely suprised he had bought me Chrissie Wellington's new book as a present.

Even nicer (if you click on the photo) was the signed message to me from Chrissie...................

Unbelievable.  Got to admire the effort though!