Am I being Thick please HELP

[Chappy]

Right as the title says I may be being thick here but he we go.

The dimensions you need to measure on any Bottom Bracket to replace one is the width across the cone and the axle length. From what I have seen there is 2 common widths for a BB,

There's 68 & 78

So why when I measure the gap do I only get 38

AAAAAAAAAAAARRRRRRRRRGGGGGGGGGGGHHHHHHHHHHHHHHH

Can anyone explain it to the big thicko

 

[RedBike]

No, it's the width of the frame. 68 or 73mm

The axle should be something like 110/113mm

 

[Chappy]

Do you mean the BB cone that it all sits in

 

[yello]

You're measuring in inches (and tenths)? i.e. 3.8 inches? That's about 100mm.

 

[rustychisel]

No, it's as redbike said, the measurement is across the bottom bracket shell (ie the frame). For English BB, the most common there is, the width is 68mm.

 

[yello]

So back to the question....

So why when I measure the gap do I only get 38

 

[Chappy]

Thanks for the help guys I think I was having a thick moment. I have measured it again correctly and it is 68mm Huzar


Cheers Guys

 

[Chappy]

So back to the question....

I was being stupid and measuring the diameter of the BB shell not the width of the frame.

 

[yello]

Ah ha! Top marks for owning up Chappy!

We all do things that seem perfectly sensible at the time and only afterwards wonder what on earth we were thinking! I'm absent minded on occasions and end up doing all manner of daft stuff. I don't learn either!

 

[Mr Pig]

The best thing to do is take the old one out and take it with you to the shop. I've been given the wrong one before.

 

[Over The Hill]

Why is it the BB on a bike lasts a few thousand miles while the wheel bearing on my car has done 122,000 miles with about 100 times the forces going through it.

Basically they are the same thing but why is the one on the bike giving up so soon?

 

[mickle]

Why is it the BB on a bike lasts a few thousand miles while the wheel bearing on my car has done 122,000 miles with about 100 times the forces going through it.

Basically they are the same thing but why is the one on the bike giving up so soon?

It's the trade off between durability and light weight. And the fact that many of the standards for bikes were established a hundred years ago. I have a bike which was built in the 1880s which would accept, if I wanted to, a brand new XTR BB. 'English' threads, 68mm shell.

A friend of mine made the same comment about cars during one of those 'where did all the Cortinas go conversations, 'look at a Taxi' he said, 'They'll do a million miles. Why can't all cars be built like that?' Well because they're essentially just small trucks, they sacrifice handling, comfort and speed for durability.

You could build a BB bearing to last 200,000 miles but you'd have to change the dimensions of the BB shell, it would weigh a ton and slow you down. Once you decide to fit an engine to a vehicle rather than pedals, issues of weight and drag become insignificant. Some of our 'mega bikes' use motor-bike discs which are an absolute pain to pedal because of the constant drag. Horses for courses innit.