Eccentric Bottom Brackets

[weebryan]

I was looking at single speed dirt jumping bike the other day and noticed it had no chain tensioner it used a eccentric bottom bracket is this the way forward?

[Matt_Tupman]

It's a nice design and could be easily built into a trials frame.

I prefere snail cams for simplicity though.

[dave85]

Mr Leeson has got it sorted.

[craigjames]

theres also the issue as the bb is moved round in the cups the effective geometery changes, so its a great way of tensioning the chain but not so great for trials for that reason.

[Token]

Plus you'd only be able to use it on 135mm stocks. Tensioning the BB, would pull the chain tight, but unless you have some snail cams on the back on your mod to stop the wheel moving, then the tense chain will pull the wheel over to one side which would be useless for setting your brake up.

[Biff...]

mods would have to have vertical dropouts then tht would salve it

[poopipe]

mods (and all other sensible frames) have horizontal dropouts so there's no point

[ManxTrialSpaz]

Tensioning the BB, would pull the chain tight, but unless you have some snail cams on the back on your mod to stop the wheel moving, then the tense chain will pull the wheel over to one side which would be useless for setting your brake up.

You could just cut your chain and wiggle the bb thing so the wheel was all the way forward in the dropout, so when the chain gets slack, you could pull the chain back with the wheel into the back of the dropout.

[Christophe']

for custom build its reet!

[Token]

for custom build its reet!

But Snail Cams are 'reet' aswell. They are extremely cheap to produce and are kept out of the way.

Eccentric Bottom Brackets just seem like abit of a fad. They'll realise soon enough there are cheaper more effective ways of tensioning a chain.

[dave85]

Cheap and effective isnt the reason for an eccentric BB. Its almost one of those things that only gets done because it can be done. It's a very elegant solution to the problem, but isnt really for the masses to my mind.

[King C]

ermmm I think the 360 dropouts that Mr. Leesons got couples well with the eccentric BB.

[python_man]

has anyone ever heard of that saying

" if it aint broke , dont fix it."

??????????

[dave85]

Yep, thought up by a neandathal who would be happy for us all to live in mud huts eating seeds and berries and die young

[python_man]

Yep, thought up by a neandathal who would be happy for us all to live in mud huts eating seeds and berries and die young

fair doos

shuts me up

[ScotchDave]

theres also the issue as the bb is moved round in the cups the effective geometery changes, so its a great way of tensioning the chain but not so great for trials for that reason.

Right, once you've got the tension in the chain right, even with chain stretch the geometry of the bike will change VERY little. We're talking, say the chain stretches a centimeter, and the radius of the axis of the outer shell to that of the bb is 5 centimeters then using S=r@ @=angle moved through, S is the chain stretch and r is the radius, I know that's an approximation but it will do, @=S/r=1/5 radian. A radian is approximately 60 degrees, so it will move through an angle of 12 degrees, feck all really...

[weebryan]

i am not a big fan of horizontal dropouts cause the

1 the force on the chain moves the wheel causing the chain to be slack and the brake rubbing

2 snail cams are not strong enough always let the wheel move

3 running discs would be problem with chain stretch

so a fixed drop out (normal) would solve this and use a eccentric bb could solve the problem and have the axles all standard sizes and also there would be no movement of the wheel that was my thinking or go down mr leesons road

[Broomer]

You would have to have a bmx bb shell to allow enough space for the normal sized bb to move.

[poopipe]

You would have to have a bmx bb shell to allow enough space for the normal sized bb to move.

hmm, that would mean increased stiffness and strength at the BB area which could mean that trials frames might not snap every 6 months and daddy koxx would go out of business.

can't see that getting popular

[RobinJI]

Great idea for single speed riders who want a rear disc. Other than that, whats wring with horizontals?

1 the force on the chain moves the wheel causing the chain to be slack and the brake rubbing - Not it you run tensioners/snail cams

2 snail cams are not strong enough always let the wheel move - I use a DMR tensioner on the drive side and nothing on the non-drive-side, wheel never moves, I've only ever had to adjust it for chain stretch, Never had problems with snail cams on my Koxx

3 running discs would be problem with chain stretch - Yep

And even with discs, I know countless people including myself who have/do run disc breaks on horizontal dropout bikes and who don't have a problem with it.

If you want to see any frames with eccentric BBs then check out the on-one inbred ti single speed disc, or pretty much any tandem in the world. And as said above there's also that Leeson.

[paolo]

I personally am all for the idea of putting BMX bb shells in trials bikes. But I would prefer horz drop-out and running a bolt axles and maybe chain-tugs AND having a BMX bb, because let's face it, the tiny weeny bearings in an MTB type shell don't last very long. Anyhow, I hope both options will be available one day. On another note, you might want to check the White Industries eccentric ENO hub, even better, it allows you to run the ENO freewheels at the back! : This would be the link, right here.