Moulton TSR rear pivot

[Graham]

I doubt that such an experiment could simulate a shock load => maximal deflection equivalent to a big pothole.

OTOH. Some possibility of damaging the paintwork or even the frame. Don't do it Mick - it ain't worth it.

[Samuel D]

I like the idea of the experiment, however, using the maths, and assuming the cone FULLY compresses (which of course isn't possible), do we know what the angle of rotation is? If it's (as expected) very small, I'm unsure what that tells us, other than we'd like it to me more

Brucey did the calculation above (I guess the arc sine of 10 mm / 220 mm). But whether it’s 2.6° or a tenth of that or 26° may not make much difference. Is the pivot still from time to time? That’s more interesting, because the lubricant can’t be replenished in that state.

[Brucey]

I agree the exact angle isn't important; the point is that the pivot doesn't move far and it doesn't move all the time either. If the centre bushing is 12mm dia, the movement at the surface of the bushing when it articulates 2.6 degrees is about 0.27mm. Not far. This movement is unlikely to re-establish a lubricant film over the bush surface if it is absent.

cheers

[Mick F]

............ What is likely (or hoped) to be achieved with the experiment, other than a figure?

Knowledge.

When riding the beast, I cannot actually eyeball how the cone compresses. I very much doubt is compresses much at all. 20mm of rubber front to rear in its relaxed state. I would be very much surprised if it ever compressed down to 15mm.

That's 5mm reduction on a equilateral(?) triangle 80mm with of two 210mm sides.
Do the maths please.

I'm off walking the doggie now!

[Brucey]

if you want to see how much movement there is, put a blob of plasticine somewhere where it will be squashed by the suspension movement. It will clearly show how much movement there really is. BTW I expect you to be surprised.

cheers

[cycle tramp]

this bush sees high loads when it is static or almost static. This means that any film of lubricant will be squeezed down to nothing (so metal to metal contact is almost guaranteed) which is why I think you need a grease with a high viscosity base stock and solid lubricant additives if you want to avoid wear as best as you can. if the conditions are sufficiently adverse the bushings will always wear, it is just a question of 'how fast'.

cheers

Stupid question number 5;
If the bushes experience high loads when they're static, they why not forget the grease gun and try some oil impregnated plastic bushings instead? Oil impregnated plastic has come a very long way since it was first introduced, and given what's been written on this thread sounds like it may be worth some investigation- indeed if they do work, it may be of interest to other Moulton riders.

[Brucey]

I believe there are two sorts; plastic matrix with bits in, and metal matric with plastic in. The former are probably not strong enough for this job. IIRC Brompton use the latter sort in their bikes but they also ream the bushings to size in situ.

cheers

[cycle tramp]

I believe there are two sorts; plastic matrix with bits in, and metal matric with plastic in. The former are probably not strong enough for this job.

cheers

The oil impregnated plastic industry say that older engineers often say that... it's worth checking out their websites and plying them with questions

[gazza_d]

We seem to be getting away from the key point that the suspension components do wear naturally which is why bushes and service kits are available from MBC and dealers.
My TSR has done almost 9000 miles in under 5 years and is only on it's second set of bushes. My APB which has about 14k on it hasn't had many more. Both have no wobble or looseness at the rear and the suspension does work, You can see the movement in the bikes' shadow on a sunny day.
You do have to be careful fitting them and the bushes should be reamed out to make sure the sleeve can rotate with no tight spots.

It's a pain that the TSR bushes are slightly different from standard over the counter ones. Unless I had access to a lathe or similar I'd not try to cut the standards down with something like a file as it would be so easy to end up with them not quite perfect and result in more wear.

Bromptons have similar designed rear pivots which I understand also wear, and again service kits are available.

[Mick F]

It's a pain that the TSR bushes are slightly different from standard over the counter ones. Unless I had access to a lathe or similar I'd not try to cut the standards down with something like a file as it would be so easy to end up with them not quite perfect and result in more wear.

The idea .......... and I've tested it on a worn bush ........... is to mount it on a shaft wrapped in ptfe tape for grip, and then put it in a lecky drill secured vertically in a vice. Switch on and hold the file horizontally and file gently as the boss rotates at speed. It helps if you have skill-of-hand of course ........... which I do.

So long as the final products are the same flange thickness both sides and when inserted they are just narrower than the spindle, all will be well. Use the old bushes to measure the flange thickness, so you know what you're aiming at.

When I fitted the new bushes, the new spindle went through very nicely. I pulled it out and measured it with a micrometer and compared it to the old spindle. Hardly any difference, so I used the old spindle and kept the new one aside. When I do new bushes filed as I described DIY, I'll be using the new spindle with them.

Mine from new lasted 4,400miles before I found the play.
The bike has now done 7,600miles and there's a very slight play that disappears when grease is injected. It may last until 8,000miles or more.

The way ahead for me is to get a friend with a lathe and equipment ........... I'll have to wait until he goes back to work after the Covid crisis ........ and get him to manufacture a new spindle and to modify the cheap Simply Bearings bushes. If we could go into production with these, they'd be cheap as chips.

What this space.

[Mick F]

Just come back from a 20mile ride.
Experiment No1 complete.

I was on a few Cornish lanes with iffy tarmac surfaces, the odd patch of bumpy, and a few manholes and fire hydrant etc covers.
I aimed at them whilst freewheeling and reaching down to the rear suspension cone and feeling the movement. I put a finger on the small metal part on the rear or the rubber where it joins the rear triangle.

To be absolutely honest here, the movement was very minimal indeed. 1mm would be pushing it.
I was surprised how small the movement was. There was plenty vibration, so the rubber is a vibration absorber, NOT a suspension device.
This is why the rear pivot wears so much. It never subscribes and arc of movement to distribute the grease, just vibrates its way to rack and ruin.

Yet to feel if the play has returned. Just having a cuppa, then it's lunch.

Experiment No2 will be done this afternoon using the ratchet straps and plank technique.

[Brucey]

I aimed at them whilst freewheeling and reaching down to the rear suspension cone and feeling the movement. I put a finger on the small metal part on the rear or the rubber where it joins the rear triangle.

To be absolutely honest here, the movement was very minimal indeed. 1mm would be pushing it.
.

so your finger is calibrated....?....

IME the largest suspension movements usually occur on a moulton when you are climbing. Like a pogo-stick in some cases.

If you want to know how much it is really moving you need to measure it under representative conditions. The blob of plasticine I suggested is the simple way. The other way it can be done is to position a camera (eg on the back of the seat pin) to video the movement you are interested in.

cheers

[Mick F]

Yes, my finger is calibrated.
Isn't yours? Can you not feel movement and quantify it? I bet you can.

I couldn't put a finger on whilst climbing or pedalling, just freewheeling as it's a bit of a stretch. I made sure I sat well back to put sufficient weight aft.

After leaving home and climbing up the main road, I pulled over into Gunnislake Station carpark and went in and out of the empty parking bays. There's a dropped kerb into each of about one inch high. I bumped in and out and felt the rubber cone, and that's when I realised that there was hardly any movement. The roads and lanes were later, but bore out my initial findings.

No plastacene in the house, but there is some blu-tack, so that might be good enough. That will be Experiment No3.

Off outside for Experiment No2 shortly.

[Mick F]

Experiment No2 complete.

Moulton on a plank, and strapped down HARD. So hard, I had difficulty releasing the ratchet and considered cutting the webbing.
I sat on it and bounced my bum on the saddle, and it was a solid as a rock ............... the suspension, not my bum!

Measured the thickness of the rubber from rear ring to front ring when compressed = 15mm
After release, measured again = 18mm

Feeling the rubber when compressed, it was never like that during my ride today. I very much doubt it was like that when I didn't check either.

I still stand by my point, that the rear suspension isn't suspension, but an anti vibration system. I reckon that the rubber is too hard to be a good suspension.

You can get different springs for the front suspension. I wonder if you can get different rubber rears?
Maybe the more expensive Moultons have better rubbers. Mine is as hard as ............... a heavy dog ball.

Which one is the compressed one?

Yes, it's the one on the right.

[simonineaston]

Well done, Mick! (no wonder you're not a big fan of reading - you're always too busy!!)
Couple of comments: if you dig deep enough back into the history of the Moulton design, you read that the only reason Moulton wanted to fit suspension to the bike in the first place was to accomodate high-pressure tyres on small wheels. If the ride had been acceptable using the specially made high-pressure 16" tyres he and Dunlop worked up together, I don't think he'd have bothered with suspension, however...
I imagine that when he moved onto the APB (the TSR's daddy) he would have designed something with the smallest amount of movement that did the job - at least for the rear, 'specially bearing in mind he was moving up from 359 to 406, the bigger wheel arguably requiring less effective suspension.
The front suspension is something different as it affects the rider's comfort more, being where the hands & wrists are subject to the vibration passed up from the road. Softer suspension on the front helps rider comfort more than it does on the rear, I think.
Funnily enough, suspension blocks of different resistance are available for the Brompton, with some heavier riders resorting to stiffening up the block by popping a Jubilee clip round it.