Why don't bicycles have the rear sprocket OUTSIDE of the frame spacing? (Single speed)

because having to use a tool seems pretty dumb, while it could simple to put the sprockets outside, I know it would put a little more weight on sides but it doesn't seem particularly significant, maybe the sprocket could hit something, but well so does the whole bicycle.

It does not protect against, rain, dust, or anything else. There seems no reason, a freehub could not protrude from the bases, or a freewheel.

The front sprocket is already outside, and it is fine on bicycles.

Perhaps because it is impractical to have different parts and standards just for single speed bikes. Today single speed bikes have a lot in common with derailleur bikes. Same frames, same axle spacing, same axles, similar/same freehubs.

If on single speed bikes the sprocket would be moved outside the frame that would require a redesign of at least the axle and frame dropouts, perhaps freehub too.

It is important to acknowledge that design you have described is not impossible. It is just impractical for most chain-drive bikes. Having that said, I have seen (pictures of) belt-driven bikes with a sproket outside the frame.

On belt driven bikes this design makes a lot of sense because it removes the need to have a gap in the rear triangle to install the belt.



Simplicity. In the simplest wheel hub design the wheel axle is just a piece of threaded steel, the bearings and locknuts that hold the wheel are threaded on the axle, and finally the same threads are used to clamp the wheel in place. A quick release skewer or modern thru-axle design also work well because non-rotating axle can be clamped to the frame. Quick release skewers are backwards compatible with threaded axle and nuts, and thru-axle is close enough that some hubs can be converted to old axle type.

Putting the cog outside frame would require a spinning axle to transmit power, and that would require a new and probably more complex way to fix the bearings to frame. As shown in the other answer, it could be still be a good tradeoff to avoid the gap that belt drive otherwise requires.

Bottom brackets do have a spinning axle, and they are much, much more difficult to disassemble than removing a wheel is.

It has been done. Probably the best-known example is the Windcheetah trike (although even this is obscure); I saw another prototype, many years ago.



I agree with OJS' point about complexity. Removing the rear wheel to fix a puncture would require a completely different engagement mechanism. This would require a very different dropout design (so it wouldn't work with existing frames, and framebuilders would need to get on board) that would likely increase weight.

Cinelli developed something related with their Bivalent hub, where the freewheel was attached to the frame, and stayed there when you removed the rear wheel, although in this case, the freewheel was inside the triangle.

The two main reasons why this is not done are:

(i) because the rear axle design would be far more complicated, as it would have to spin to transmit the drive to the wheels; removing the rear wheel would also be a much more complex task, and rear wheel removal is far more common than chain removal, which would become simpler

(ii) the sprocket(s) would be too far away from the centre-line of the bike, so the chainline (already marginal on the smallest sprocket of a cassette) would be very bad unless the front chainring was also moved further out

If the front chainring(s) are moved outwards to keep a decent chainline, then that in turn causes the problem of increased 'Q-factor', that is the horizontal distance between the feet. Anytime you move the sprocket outwards to (x)mm from the centre-line of a bike, that means the rider's feet are (2x)mm horizontally apart. For efficient riding and to prevent injury, x should be as little as possible, and the non-drive-side pedal should be as close as possible to x(mm) horizontally from the centre-line of the bike.

If one pedal is (x)mm horizontal distance from the centre-line, and the other pedal is a different horizontal distance (y)mm from the centre-line, then the rider is sitting asymmetrically on the bike with one leg spread further out than the other as they pedal.

The advantage of having the sprocket outside the frame is that the chain can be removed without breaking the chain. But the chain would still need to be breakable, because chains cannot be supplied new and fitted as a complete loop. Bicycles need varying lengths of chain according to the bike size, design and gearing. Therefore chains are sold as a length, not a loop, and fitted to a bike with any excess links removed.

Even if there was a popular length of chain, sold already looped, on geared bikes the chain would still need to be breakable, so that it could thread through the front mech and jockey wheels of the rear mech.

If you look at the bike from the top you can see the pedal axis is much shorter than the rear wheel axis.

You may recognize that the rear wheel is wired assymetrically with the chain ring(s) "buried" inside.

The reason is simple, the closer the angle between the chain and the axes is closer to 90 degrees the more efficient the transmission is.

With the chainring outside the frame you either force the chain not to be perpendicular to the axes or you force the wheel hub being narrower. Or the pedal hub being wider.

On the other hand if you let the caing in the frame you can have broad rear hub and chain perpendicular to the exes.

There is also a simple mechanical advantage to have the dropouts as far apart as possible, creating a wider triangle with more acute angles between the seat stays/chain stays and the wheel axle, when looked at longitudinally. A wider triangle with more acute angles can withstand lateral forces better.

An asymmetrical frame would be less stable than a symmetrical one, in the same way that back wheels which typically have shorter spokes with a less acute angle on the drive side are less stable than a symmetrical wheel would be.