Cycling UK Forum

[Moderator note - Discussion of the Brompton chain tensioner design split off from the thread 'A new Brompton, the G Line' - viewtopic.php?t=162606]

Brucey wrote: ↑23 Sep 2024, 11:20am I think we are all agreed that it is a bit of a crud magnet as it is. If I was KOTW I would have a tensioner mounted about half-way along the RH chainstay, where it would pick up much less crud.

There are similar tensioners on hundreds of thousands of standard Bromptons with as far as I'm aware limited issues, and its not doing anything with gear changes, so probably a lot more grot tolerant than a derailleur. I think Brompton know what they are doing on that aspect of things.

that's as maybe, but it is still too close to the ground (quite needlessly, I might add) for a machine with offroad pretensions.

You could do it this way, which is what I think Brucey was describing. That would be far superior to a derailleur. I'm not sure if there is enough chain take-up for a Brompton.



This comes from the middle of the page about Pinion P-Line gears. The C-Line gears are lighter and come with various numbers of gears.

plancashire wrote: ↑23 Sep 2024, 10:48pm You could do it this way, which is what I think Brucey was describing. That would be far superior to a derailleur. I'm not sure if there is enough chain take-up for a Brompton.

This is, perhaps, missing the point of why the Bromton has a chain tensioner, which is nothing much to do with the bike being ridden but the chainline when the bike is folded becoming substantially shorter. When the back is flipped under the chain tensioner extends pretty much directly along the line of the chainstays to give enough extra chainline to stop the chain falling off.

The pic of the Pinion above has a belt which will generally not be kept shorter by a tensioner in use (I suspect that is just a re-router to help keep the belt out of the mud and rocks).

As a point of possible interest, Ben "Kinetics" Cooper's various conversions for a Brom include a belt drive option, which uses the basic Brompton tensioner arm but a belt-specific idler wheel (IIRC he 3d prints those). As with the chain version, the tensioner doesn't really do much when riding but keeps everything together when the bike is folded.

Pete.

plancashire wrote: ↑23 Sep 2024, 10:48pm You could do it this way, which is what I think Brucey was describing. ......

I was thinking more of something that looks a bit like the standard tensioner, but is mounted on the chainstay, further back than the pinion one; this way the fold would still work. Probably the lower arm would swing ~180 degrees during the fold, and it would be best if the upper pulley is able to float sideways if there is more than one sprocket. All very doable.

Brucey wrote: ↑24 Sep 2024, 11:51am

plancashire wrote: ↑23 Sep 2024, 10:48pm You could do it this way, which is what I think Brucey was describing. ......

I was thinking more of something that looks a bit like the standard tensioner, but is mounted on the chainstay, further back than the pinion one; this way the fold would still work. Probably the lower arm would swing ~180 degrees during the fold, and it would be best if the upper pulley is able to float sideways if there is more than one sprocket. All very doable.

Look at where the tensioner arm sits when folded. If you mount that in the middle of the chainstay it'll need to be ~ twice as long to keep the chain in place when the bike is folded. Doable, maybe, but I'd say it's pushing it to announce that as an obviously a better way to do it.



Pete.

I said it was 'very doable'; if it were 'obvious' then perhaps someone would have done it already.

Brucey wrote: ↑24 Sep 2024, 3:29pm I said it was 'very doable'; if it were 'obvious' then perhaps someone would have done it already.

Perhaps more useful than "very doable" or "obvious" would be "is it actually a good idea?" or "is it worth doing?"

So, what would one actually gain by making the tensioner arm roughly twice as long, and would it offset the issues of making it roughly twice as long?

I'm not really interested in change for the sake of change, I want my life to be marginally easier.

Pete.

pjclinch wrote: ↑24 Sep 2024, 3:53pm...So, what would one actually gain by making the tensioner arm roughly twice as long.... I want my life to be marginally easier.

the tensioner arm doesn't need to be 'twice as long'. What might make your life marginally easier is not jumping to conclusions.

Brucey wrote: ↑24 Sep 2024, 4:32pm

pjclinch wrote: ↑24 Sep 2024, 3:53pm...So, what would one actually gain by making the tensioner arm roughly twice as long.... I want my life to be marginally easier.

the tensioner arm doesn't need to be 'twice as long'. What might make your life marginally easier is not jumping to conclusions.

If you trace the arm back from its full extension when the rear is folded under to a midpoint on the chainstay it'll be about twice as long as if it mounts on the hub.

Rather than being petulant at me please draw a diagram of what you mean.

Pete.

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pjclinch wrote: ↑24 Sep 2024, 5:25pm...If you trace the arm back from its full extension when the rear is folded under to a midpoint on the chainstay it'll be about twice as long....

no it isn't. When the bike is unfolded, the lower pulley can be close to the chainwheel. When the bike is folded, the pulley moves to take up the chain slack. There is no reason to make the arm longer than it is; if anything it could be made shorter. I won't be drawing any diagrams for obvious reasons.

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.

Cyclothesist wrote: ↑24 Sep 2024, 7:55pm I suspect that's why the tensioner is in rear derailleur position.

I suspect it's there because Brompton have spent 3-4 years and no doubt hundreds of thousands of pounds on research, design, development and testing of the new G-Line, informed by their 35 years of experience in building the original Brompton and any second guessing by folk on here (who have never even seen the bike in the real world) is totally pointless and probably thoroughly wrong.

It's just a bit of fun rareposter, and a workout for the grey cells. Some trigonometry for working out if the tensioner arm can be shorter in the middle of the chainstay. Some visualization of how that would look and what might be the problems.
Maybe Brompton have tried it and it's going on the Mk2 G line