Brompton chain tensioner design

[Brucey]

.... The problem I see with moving the tensioner towards the midpoint of the chainstay is it's then within striking range of your feet and the cranks.....

there is no real reason why the arms that have the pulleys on them can't be on the other side of the chain if needs be.

[pjclinch]

I can see what Brucey means and I think he's right, you don't need a longer tensioner arm. The problem I see with moving the tensioner towards the midpoint of the chainstay is it's then within striking range of your feet and the cranks. If you're at all duck-footed that could be a problem. I suspect that's why the tensioner is in rear derailleur position.

I can't and I suspect he isn't, that's why I'd like a diagram or annotated photo to clarify what he means.

When unfolded the tensioner arm is lined up in-board of its mount and at that point there's no particular reason you couldn't mount the tensioner on the chainstay, all it has to do at this point is deal with a little bit of suspension bob or if it's got a derailleur-ette (as the G-electric, T line and 6 speeds have to some degree).
But when one folds the rear under the chain length is very significantly reduced so now the tensioner has a different job, and relative to the chainstays it rotates to almost the completely opposite direction, more or less parallel with the chainstays and extending well beyond them.

Here's a Brompton rear in fold position...

Bromton rear wheel in folded-under position

The chain needs to get pulled to point A in order to be taut enough to stay safely in place while the bike is folded. With the tensioner hinged from just behind the hub at B, that means the tensioner has to be the length it is, the red line AB.
If you mount the tensioner at C, roughly in the middle of the chainstay, you still need to get the chain to A. I'm not seeing how you can do that without the tensioner occupying something like the yellow line AC, longer than the existing tensioner.

That's my thinking, if I've missed something I'd like a diagram to show me what. Brucey says it's obvious why he won't draw any but to me an engineer not willing to explain something with a diagram is rather odd.

Pete.
(edited for typo)

[Cyclothesist]

I can see what Brucey means and I think he's right, you don't need a longer tensioner arm. The problem I see with moving the tensioner towards the midpoint of the chainstay is it's then within striking range of your feet and the cranks. If you're at all duck-footed that could be a problem. I suspect that's why the tensioner is in rear derailleur position.

I can't and I suspect he isn't, that's why I'd like a diagram or annotated photo to clarify what he means.

When unfolded the tensioner arm is lined up in-board of its mount and at that point there's no particular reason you couldn't mount the tensioner on the chainstay, all it has to do at this point is deal with a little bit of suspension bob or if it's got a derailleur-ette (as the G-electric, T line and 6 speeds have to some degree).
But when one folds the rear under the chain length is very significantly reduced so now the tensioner has a different job, and relative to the chainstays it rotates to almost the completely opposite direction, more or less parallel with the chainstays and extending well beyond them.

Here's a Brompton rear in fold position...

Brompton fold-under.jpg

The chain needs to get pulled to point A in order to be taught enough to stay safely in place while the bike is folded. With the tensioner hinged from just behind the hub at B, that means the tensioner has to be the length it is, the red line AB.
If you mount the tensioner at C, roughly in the middle of the chainstay, you still need to get the chain to A. I'm not seeing how you can do that without the tensioner occupying something like the yellow line AC, longer than the existing tensioner.

That's my thinking, if I've missed something I'd like a diagram to show me what. Brucey says it's obvious why he won't draw any but to me an engineer not willing to explain something with a diagram is rather odd.

Pete.

I've sent you a PM. Thanks for the picture. It does look like the tensioner is in the optimal place on the 16" wheeled Brompton. The 20" is likely going to fold and shorten the chainstays by a similar amount so I concede. Mid-chainstay maybe could be done with a 2 pulley wheel tensioner similar to a rear derailleur cage but that wouldn't be an improvement. Looks like rareposter is spot on

[Cyclothesist]

Brucey do you agree? If not would you mind trying to educate us with a good description of what the mid chainstay tensioner you have in mind would look like?

[rareposter]

Looks like rareposter is spot on

Going to print that out and get it framed!

[sizbut]

Brucey do you agree? If not would you mind trying to educate us with a good description of what the mid chainstay tensioner you have in mind would look like?

Please NO. Start a new thread if you folks want to go down a endless argument on just the chain tensioner.

[Cyclothesist]

Brucey do you agree? If not would you mind trying to educate us with a good description of what the mid chainstay tensioner you have in mind would look like?

Please NO. Start a new thread if you folks want to go down a endless argument on just the chain tensioner.

Apologies. I've reported myself to the mods, started a new topic and asked if a section of this thread can be moved to the G line tensioner thread that I started. HTH.

[slowster]

Threads now split.

[sizbut]

Oh no. I'm kinda obliged now it's been split., so here goes.

1. Brucey is right. So long (pun intended) as it's position allows the same angular range of movement, the tensioner arm doesn't have to be longer.

2. Brucey is wrong. The tensioner doesn't need to be moved from its current position. The benefits would be very marginal (imperceptible to most riders) and certainly not worth the impact on bike manufacture and assembly.

[djnotts]

^ sizbut: "Brucey is wrong. The tensioner doesn't need to be moved from its current position. The benefits would be very marginal (imperceptible to most riders) and certainly not worth the impact on bike manufacture and assembly."

I ride 2 of my 6 speeders off-tarmac, tow-paths, bridleways etc. The tensioner is no problem anywhere that a Brompton will go, irrespective of the tensioner! The terrain limiter is mud and its interaction with possible tyre size/tread and mudguards, not the tensioner.
The G line's wheel size and clearances obviously alter the limiting factors, BUT I suspect that larger wheel diameter balances out any need to change tensioner position.
The G is no more a true mtb than the existing models. Fiddling with a tensioner ain't gonna change that.

[pjclinch]

Oh no. I'm kinda obliged now it's been split., so here goes.

1. Brucey is right. So long (pun intended) as it's position allows the same angular range of movement, the tensioner arm doesn't have to be longer.

If you put it in the middle of the chainstay it's not the angular movement that's the problem but that it just can't get far enough to take up the increased amount of slack.

Look at the photo, it's pretty much parallel to the chainstay and sat beyond the hub, so the chain can extend well beyond the hub & chainstay. Put it in the middle and while it can extend in the same direction at the same angle it can barely reach the hub, let alone extend beyond it to take the chain where it needs to go.

The key point is that the job when folded is radically different to when riding: there is far more chain slack to take in when the bike is folded than when it is not and the various bits are in relatively different positions because the fold hinge is significantly offset from the bottom bracket.

Ideally, look at a Brom in the flesh and experiment with the fold, the positioning of all the pieces is quite "tuned".

Pete.

[Cyclothesist]

The only way I can see that the tension arm can be shorter is if it threads the chain through a sprung Z type path. I don't think that's an improvement. I'm not sure if that's what Brucey is thinking?

[plancashire]

The Brompton chain tensioner has two parts: the swinging sprung arm and a small fixed arm. Both have small toothed wheels around which the chain runs. The fixed wheel is very close to the sprocket. The chain slack is taken up by changing the relative position of these two wheels. When the bike is ridden, the chain runs fairly straight, taking just a small bend past each of these two wheels. When folded the chain wraps much further around both.

If you move both wheels somewhere else along the chain line they would take up exactly the same amount of slack. The reasons you might not do that are many; the main one I can see is that there isn't any space to move them to when the bike is folded. You would also lose the 2-gear derailleur possibility. I suspect Andrew Ritchie played around with a lot of prototypes before settling on the current design.

If anyone on here can demonstrate something on a real bike that works better I'd be interested. Theoretical ramblings and drawings interest me less.

[2_i]

If anyone on here can demonstrate something on a real bike that works better I'd be interested. Theoretical ramblings and drawings interest me less.

Having a front triple 50/34/20 and a rear range 16-11 on BWR, I experimented a lot with taking away slack, and it is hard to come up with anything better than Brompton. My modifications are slight. The axle of the tensioner is slightly longer than the Advanced tensioner's, and I have a pin that catches the chain when the bike is folded, taking off some slack. The length of the tensioner arm is, however, the same as in the Advanced. The setup works great for me in practice despite any theoretical reservations about that arm's length.

[Cyclothesist]

Kinda disappointed Brucey hasn't elaborated. I've a feeling we're maybe missing a trick. Or maybe Brucey has revised his thoughts?