Brompton motor thought

[1fifer1]

Hi has anyone fitted a standard size motor to a Brompton front wheel, a Brompton has a 75mm motor (spindle width) between the forks and I believe a standard motor is 100mm, I remember articles a while ago using spreader bars to ease fitting of motors where needed.
If this is possible does anyone know where a Brompton wheel and standard motor could be fitted (laced) etc
many thanks

 

[Nealh]

25mm is too far to spread forks , 5 -10mm but not 25mm.

 

[saneagle]

Hi has anyone fitted a standard size motor to a Brompton front wheel, a Brompton has a 75mm motor (spindle width) between the forks and I believe a standard motor is 100mm, I remember articles a while ago using spreader bars to ease fitting of motors where needed.
If this is possible does anyone know where a Brompton wheel and standard motor could be fitted (laced) etc
many thanks

I've seen a few guys do it. There was one that took part in the bristol world championships nack in 2012(?) with a bafang QWSXK5 motor running at 48v. I think it might have won. The forks are steel, so they spread pretty easily. The only problem is that the fold isn't quite right regarding the way the cip hooks on when you fold it. When I did my 85mm Q85, I had to make a small adjustment somehow, but it must be a bit worse with a 100mm motor.

Why not use the Woosh kit or the Q85 motor, which I used?

 

[StuartsProjects]

If this is possible

Its very likely possible to do it.

Ignoring the views of those that want to win prizes\awards etc, is spreading the forks that much going to stress them to the point that they are (much) more likely to fail ?

Perhaps ask Brompton for their view ?

 

[saneagle]

Ignoring the views of those that want to win prizes\awards etc, is spreading the forks that much going to stress them to the point that they are (much) more likely to fail ?

No, because the forks are steel and you only do it once. After stretching, there's no stress on them other than the normal stresses of riding the bike. You pull or jack the forks apart beyond their elastic limit so that they take the new shape. If anything, the forks become a tiny amount stronger due to slight work-hardening.

 

[Woosh]

I tried spreading the Brompton fork before but gave up. There is a lot of skill involved to do that successfully. The fork is very, very stiff to start with, and there is always the risk that you bend one leg more than the other. Then there is the geometry how to mount the motor afterward. My advice is don't even think about it.

 

[AGS]

Yeah, the problem is that one side has a tendency to bend more than the other if you try the cold set technique with studding.

I cold set a rear swing arm for my Puch the other day from 130mm to 155mm. But I had to fabricate a jig to do it out of 18mm plywood so. I could stretch each side independently. Otherwise the wheel centre would have been way out.

It was a substantial stretch and I thought I would need mapp gas, but found it stretched easy enough cold. I just had to guess the overstretch to allow it to spring back to where I wanted. In my case the overstretch on each side was 50mm to get it to spring back to 12.5mm.

And then of course I had to do a second bend on each side at the rear to pull the axle flats back in parallel, one side at a time using the jig.

And I may need to fine tune it when I eventually get the motor and try a test fit. But it measures up pretty good at the moment with the original rear wheel mounted and spaced out with washers.

This is what I was working with:

Swingarm Puch Maxi S MLM Magnum style

Find your PSR Race swingarm Magnum Style online at Puchshop.de. The Puch specialist and parts webshop. Quick and safe!

www.puchshop.de

 

[AGS]

And also are you sure the bike will still fold properly if you stretch the forks by 25mm?

 

[saneagle]

If you look here in the Q&A, they confirm that a Brompton still folds correctly with 100mm forks:

https://www.kinetics-online.co.uk/folding-bikes/brompton/brompton-disc-brakes/