Lots of play between sprocket and spindle?

[Maker]

I've just bought a pair of BMX cranks off eBay, along with a couple of sprockets.

The hole in the sprocket is about 2mm bigger than the spindle on the cranks. Should there be a thin washer/spacer type thing between them?

They are second hand parts but there's no wear on the shaft and the inside of the sprocket is still anodized, so they're definitely made thay size.

 

[Seadog]

I'm not an expert in things push bike but surely it sits on a carrier? There's no other way to attach it to the crank otherwise. What about the witness marks on the longer holes, they seem to contact wit another part at three points?

 

[waddycall]

Surely something must take up the gap otherwise the sprocket would rotate around the fixing bolt in the crank arm and knock against the shaft?

Edit -

Looking at it should the sprocket fit further onto the crank arm so that the raised boss passes through one of the long slots? Perhaps it's a snug fit on the crank arm boss???

 

[Revs1972]

At around 30 seconds , he mentions a spacer to take out the gap between the crank and the sprocket

 

[Revs1972]

Is that spacer sold as a component of the bottom bracket assembly ?

 

[Maker]

At around 30 seconds , he mentions a spacer to take out the gap between the crank and the sprocket

So he does, time to make a spacer then.

Now I think about it a little more, there's two common sizes of bottom bracket spindle, it would make sense to make one size of sprocket and two small spacers rather than make 2 sizes of sprocket.

 

[colnerov]

Hi, Don't forget to do it on a nice white carpet.

Colin

 

[Fazerruss]

Not quite on topic but although I do a bit of cycling I can't understand why the industry calls it a bottom bracket? It's a shaft that has a crank on it so surely it's a crank shaft!!!!

 

[Revs1972]

Not quite on topic but although I do a bit of cycling I can't understand why the industry calls it a bottom bracket? It's a shaft that has a crank on it so surely it's a crank shaft!!!!

Can see what you are saying with the old type where the shaft was integral to the bottom bracket, but now the shaft is fixed to the crank then technically it is the crank shaft. Bottom bracket now being just 2 shells to hold the bearings

 

[Maker]

I replaced the spacer that came with the BB with one that has a little step on top of it to take up the slack.



I bought the cranks, BB, two sprockets and pedals as one listing, presumably he'd been using it as I got it without the spacer, which might explain why one of the sprockets (Not the one I was going to use but that's not the point) is cracked.

 

[graffian]

Can see what you are saying with the old type where the shaft was integral to the bottom bracket, but now the shaft is fixed to the crank then technically it is the crank shaft. Bottom bracket now being just 2 shells to hold the bearings

Mains?
looking at your pics I guess it has a sleeve and a bolt through the sprocket in to the arm.

 

[Gareth0123]

I can't understand why the industry calls it a bottom bracket? !!!!

Because in the good old days before tube notching and tig welding when the frames were socketed and brazed it was the frame's bottom bracket which just happened to be in a perfect location to house the crank and its bearings. Just like this one:

 

[Fazerruss]

No, I've repeatedly heard them call the shaft bottom bracket ( no bearings of frame parts attached)

 

[Gareth0123]

That is interesting because I have been purchasing "English" (1 & 3/8" 26TPI thread) bottom brackets and eccentric bottom brackets for years and what I get is a piece of threaded tube that I weld or braze to the rest of the frame

English type bottom brackets:

Eccentric Bottom Bracket.

and then when I have welded the bottom bracket into the frame, I install the crankshaft (7) and crank bearings (5, 6 & 8) and lockring (9) before fitting the crankset (1,2, 10. 11, 12, 13 & 14 etc.) and pedal crank (3) and pedals (not shown). Ignore the cotter pin (4) as I use modern square taper crankshafts and cranksets.

in other words, this lot:

 

[Maker]

What's an eccentric bottom bracket for?

It took me long enough to workout whether my frame used Spanish, Euro, American or Mid BB, why do BMXs have to be so complicated.

 

[Gareth0123]

Eccentric bottom brackets were original designed so that the captain and stoker (front) chain on a tandem could be tensioned. The rear chain on a tandem being tension by adjusting at the wheel drop outs or on modern derailleur gears by using spring tensioned idler sprockets.

I use them on the cargo bikes, trikes and quads that I make, mainly because I have a preference to use Internal Hub Gears from Sturmey Archer or Sram.
I use the eccentric bottom bracket to tension the front chain: pedal crank to jack shaft, with second chain from the jack shaft down to hub tensioned with a spring loaded idler arm.