Brompton Fork widening & cutout

[Pedalo]

Hi,

I'm thinking of fitting a Nano motor kit to a Brompton and was just wondering about the modification that needs to be done to the forks. The electric wheel co website mentions that as well as widening the forks, the fork cutout needs to be widened and lengthened by about 2mm.

The FAQ on the website also says that the whole process is reversible - i.e. a Nano Brompton can be restored to a normal Brompton.

I'm sure the fork would simply pull back together again but with widened and lengthened cutouts, would this adversely affect the fitting of a normal wheel? e.g. would it be more difficult to make the wheel run true if the wheel doesn't fit as snug in the cutouts? Anyone tried it?

Thanks

 

[mj8765]

Nano Brompton

Hi

The Brompton fork has a hole which a washer with a small lug fits into. This means that it is possible to refit the original wheel and it will be correctly located. Alternatively - a replacement set of forks costs about £50 I think - so if worst comes to worst you could just change these.

Martin - The Electric Wheel Company

 

[Pedalo]

Thanks Martin,

Good to see we have an EWC representative on the forum

I'm getting the expensive titanium version which is why I'm super paranoid about butchering the forks! I think replacing these are closer to £225

Is the widening or grinding any more difficult with titanium vs steel?

 

[jerrysimon]

Yes I agree nice to see Martin here.

Pedalo,

If you check my build thread here it shows you how to widen and grind out the fork ends to size. Also lots of pics showing how the lugs slot in once you get it right.


I think I would get a set of standard forks first. Cheap on ebay if you watch for them. A NOS pair went for about £25 the other day. Don't want to risk £225 titanium forks

Although the forks can be pulled back together and technically the old wheel put back in, the slots would be much wider after grinding them out so the slimmer shaft of the original wheel would rattle around in them until tightened. I guess when its all tightened up it might not matter given the washers with lugs also hold them in place.

As I said I would get a standard pair of forks and keep the titanium ones original. The weight saving is not that great in the forks but larger in the rear triangle.

Regards

Jerry

 

[daniel.weck]

A NOS pair went for about £25 the other day.

Yes, that was a bargain I thought

 

[Pedalo]

Yes I agree nice to see Martin here.

Pedalo,

If you check my build thread here it shows you how to widen and grind out the fork ends to size. Also lots of pics showing how the lugs slot in once you get it right.


I think I would get a set of standard forks first. Cheap on ebay if you watch for them. A NOS pair went for about £25 the other day. Don't want to risk £225 titanium forks

Although the forks can be pulled back together and technically the old wheel put back in, the slots would be much wider after grinding them out so the slimmer shaft of the original wheel would rattle around in them until tightened. I guess when its all tightened up it might not matter given the washers with lugs also hold them in place.

As I said I would get a standard pair of forks and keep the titanium ones original. The weight saving is not that great in the forks but larger in the rear triangle.

Regards

Jerry

Thanks Jerry, Yes I've been working my way through your posts. I'm sure you're right - probably safest to get a set of standard forks for the conversion.

 

[hma]

Thanks Jerry, Yes I've been working my way through your posts. I'm sure you're right - probably safest to get a set of standard forks for the conversion.

Or an other motorkit. Look here Crystalyte Europe :: Motorkits :: Folding bike motorkit

Rgds,

Hugo

 

[Pedalo]

Or an other motorkit. Look here Crystalyte Europe :: Motorkits :: Folding bike motorkit

Rgds,

Hugo

Interesting. Thanks for the link. At 3.9kg that motor looks to be a bit too heavy for me though.

 

[Pedalo]

Just picking up on this subject again, I saw in a thread a while ago a report from someone who had ground down the axle of a motor rather than widening the cutout in his bike forks. Could this be practical for the Tongxin 80mm motor? I guess the bigest potential issue is that strength would be compromised especially since the cable is passed through the axle...

 

[jerrysimon]

Pedalo,

I remember that post/pic too. At first I thought yes good idea but with more thought as you say I would be uncomfortable about weakening an already hollow shaft.

Are you apprehensive about opening up the fork ends a couple of mm ?

Much better than potentially ruining a hard to get hold of 80mm wide Tongxin motor!

PS Ah the penny dropped they are titanium forks right ?

Regards

Jerry

 

[Pedalo]

Pedalo,

PS Ah the penny dropped they are titanium forks right ?

Yes, that's right. The low risk option as you've said is to replace the forks with steel ones but since I have the lighter Ti forks I'm would really like to use them, so exploring all the options. How big is the cable hole in the motor? Would there be any material left if I were to grind down the axle enough to fit in the Brompton cutout?

 

[jerrysimon]

Here is a picture I took when I stripped it down

The side walls are already thinner on the flats. I really think its not an option. Its either get a stock pair of forks or hit that titanium with an angle grinder

What is the weight saving with the titanium forks ? I can't imagine its a lot. The main saving must be in the rear titaniunm triangle.

I am off to bed, night.

Regards

Jerry

 

[Pedalo]

Here is a picture I took when I stripped it down

The side walls are already thinner on the flats. I really think its not an option. Its either get a stock pair of forks or hit that titanium with an angle grinder

What is the weight saving with the titanium forks ? I can't imagine its a lot. The main saving must be in the rear titaniunm triangle.

I am off to bed, night.

Regards

Jerry

That photo is very helpful and convinced me not to proceed with that approach - Thanks!

 

[Pedalo]

That photo is very helpful and convinced me not to proceed with that approach - Thanks!

And I found the thread/link where grinding the motor axle was suggested - and indeed in that particular case the cable didn't enter the motor through the centre of the axle so wasn't an issue for that case.

 

[mj8765]

Forks

We would also not recommend grinding the axle down. We have done a couple of conversions on titanium brompton forks and have not had any problems with the adjustment needed. The Titanium forks are in fact more springy than the steel ones and need to be pulled out further than the steel forks to end up in the right place. No issues with grinding out the cutouts either.