Brompton 130mm with normal external gearing
 

I'm going to get alot of hate I think but here goes.

I'm looking to widen the rear triangle to 130mm on a Brompton to accommodate a normal hub like all normal bike do.

The gear ratio in a Brompton internal 3 speed and external 2 speed is very impressive. But I just want a normal hub and derailleur on a Brompton. (Let's not get into the argument of other hubs solutions and why I'm doing it).

There are two ways to achieve normal hub in a Brompton, either you buy a new rear triangle from Kinetics that's been designed to accommodate it or you modify the existing rear triangle. Since Kinetics rear triangle cost £395 and a aceoffix rear triangle (replacement triangle) only cost £70. I just thought the widening route has very low risk, if it doesn't work out it only cost £70 to get it back to normal. That's the beauty of modifying a folding bike, just need to unbolt and change the rear triangle.

Before all the comments about it's difficult for a DIY job. I'm not doing it myself, I'm going to comission a Steel Bike Customisation/Repair workshop to do it. So they will have the full knowledge in what can be done to a steel bike and what can't be done and what other things need modifying to the rear triangle to get it working.

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All I want to ask if there are anyone who have done it before? What is there to look out for?

My concern are the wheel base, would the widening shorten the wheel base and rear triangle so the wheels no longer fit? If so, has anyone tried soldering a new longer drop out to return the wheelbase to the same length ?

As for the folding, i think it should be ok as Kinetics is evidence that it should work. As for the chain slack, I've seen solutions with tensioners attached to the jockey wheel and adding a chain guide/hook at the chainring (similar to Mezzo Bike) to make the folding work.

Kinetics not only sell his own triangle (in several types) but also do the widening of the rear triangle to 135mm, as well for the steel triangle than for the titanium triangle.

Kinetics still do this job on demand. You just need to send him your triangle and he can enlarge your triangle (that's what I did with my original Brompton rear triangle that was widened to 135mm by Kinetics and still accept a wheel with a Marathon Plus tire). Kinetics is by far the one that has the most experience in Brompton rear triangle widening.

Kinetics has self made tools to do the widening that guarantee that the widening is symmetric (i.e. that he wheel remain centered and well aligned with the frame axle) and that the triangle isn't shortened.

For the chain tensioning when folding the bike, I think that the best solution is to use the tensioner of the Birdy III located around the chainring. Adding a arm on the derailleur cage doesn't work well, it was the solution used on the Birdy II and the chain was often derailing when folding/unfolding the bike.
Such an additional tensioner around the chainring is especially needed if there is a big difference in cog sizes on the cassette (a 9-32t or 9-34t cassette is the best for a bike with a small rear wheel like the Brompton or Birdy it provides good gear inches with a 50t or 52t chainring).

Note that there are also 135mm Chinese titanium rear triangle with a derailleur hanger.

Thanks, this is the best and the most brilliant answer ! I don't see the tools for widening or the service to widen on Kinetic website, I'll email them and see if they still do it.

Do you mind telling me how much did they charge for the widening ?

I think I will go for the Birdy III solution, I'm looking to fit a 56t chainring pairing with 11-28 but if there is a 9-32T then I don't need to go that big in the front. What speed are you running the 9-32T on, I'm just curious to find this cassette?

Also see this recent thread:

https://www.bikeforums.net/folding-b...tensioner.html

Thanks this is interesting, it looks like it need alot of testing to find the best gear ratio and tension solution

Just checked with kinetics, they don't offer the service to widen the rear triangle only anymore. They will only do it as a custom build. But at least I know it's doable and will try to find another workshop ☺️

I didn't bought the widening only.

I bought a kit with the widening of the original titanium triangle of my superlight Brompton and a Rohloff rear wheel. It was in 2016 just before the brexit poll and the price for the kit was 1500GBP (I mention that it was before the poll that decided the brexit because just after that the GBP drop a lot against the Euro and the price of many Kinetics products increased. In my kit for instance a big part of the price is the Rohloff hub that Ben must buy in Euro).

If you ask Ben of Kinetics to provide you the complete kit, enlarged rear triangle with derailleur hanger + wheel, he might be interested.

For the cassette, the original gearing of the R&M Birdy Touring is 52t front and 9-32t (Sunrace 10s cassette) rear. It provides a good range of gear inches also for a Brompton since the Birdy Touring has 40x355 tires and the Brompton 35x349, so very similar wheel sizes.

This Sunrace cassette requires a special Sunrace hub, so I do not recommend it.

The best option is the 3T 9-32t Bailout cassette (made by Ethirteen, there is also an Overdrive version not very good for small wheel) which is now difficult to find in Europe, there is only one online shop that sell it at a very low price

The other option is Ethirteen Xcx plus 9-34t 11s cassette its only drawback is that there is no 10t cog, it goes directly from 9t to 11t (9,11,13,15,17,19,21,24,27,30,34) while the 3T Bailout has 9-10-11-12-13-15-17-19-22-26-32.

All those cassettes with a 9t smallest cog need a SRAM XD or XDR hub.

Another different solution is to use a SRAM Etap AXS 12s rear derailleur with the SRAM 12s 10-36 cassette. Its expensive but fully wireless, no cables at all which is nice for a folding bike.

I didn't have the guts to do it myself, so paid a frame maker to do it for me. It took him 20mn and I was charged the grand sum of €30. It's no big job for a pro.

1. Hold the bike around the bottom braket into a vice so you don't have to remove the triangle off the bike
2. He didn't do it, but for extra safety, you could hold the seat stays into clamps so only the chain stays are extended, and also clamp the chain stays just before the curve
3. Grab a long metal bar, and first push out the right-side stay outwards; Repeat with the left-side stay. Each stay must be bent at a different angle to match the original ratio — the right-side stay is slightly more bent (the triangle isn't symetric)
4. Use a wrench or hub to straighten the dropouts.

https://cimg7.ibsrv.net/gimg/bikefor...c49a7067b3.png

Sorry to say that, but this is the quick and dirty way to do it without any warranty to end up with the wheel centered nor in line with the frame axle and front wheel !

Kinetics has a jig that hold the rear triangle at several points and produce a controlled, reproducible enlargement of the frame that warranty that all enlarged frames are identically enlarged with the same final shape for all tubes.

He of course dismount the rear triangle from the frame before enlarging it.

He posted pictures of this jig on FB.

Unfortunately, Ben from Kinetics is not interested and I'm not looking to get a new hub. But all response seems to point to the direction of relative ease if you find a competent steel bike workshop. Just need to make sure to brief the person properly (i.e. only to widen the bar near the drop out area and make sure all parts remain the same geometry).

The new rear triangle that I'm going to widen is on the way and should arrive at the end of March. Will keep you guys updated !

Thanks for all the response, it really put my mind at ease to hear that there are others who have done it before !!
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I've been riding it fine for over 2,000 km. Looks like quick and dirty, at least done by a professional, does the job.

Since it's a spare triangle, you have nothing to lose.

Just using a skewer and washers isn't recommended, for that reason (asymmetry).

I guess it's Kinetics' post on FB refered to above: https://tinyurl.com/y7ean5k2

unless you need 420 to 520% range, I wouldn't touch a 9T cassette. I run a 11S 9-46T e13 cassette because I need 500% and I can go 46T max but I would prefer something like 11-52.

On a 16", the limitation might be the derailleur cage length but short cage derailleurs tend to be specified up to 30T (in actual fact they take 32-34T) so I look for 11-30 or 11-32 before considering 9-25 or 9-28 from 3T/e13 or Shimano capreo (which requires special tools and hubs). a fit a larger ring to offset the cassette range where you need it to be.

What about the tire wear?. When I had a repair done on a moped, that left the frame slightly misaligned, the rear tire started to wear off at a highly accelerated pace. It was not quite the fault of the garage that did the repair as they declared that it was outside their expertise and I asked them to go ahead anyway since I practically had no choice at the location.

Even leaving the left-side dust cover untouched on the Nexus, ie. it's still 132,5mm wide, there's enough clearance for the tire and it doesn't rub againt the frame.

I only had to raise the fender with a short flat bracket, since widening the triangle means the fender moved down a bit and could rub against the tire when pumped up at around 6-7 bars.
https://cimg0.ibsrv.net/gimg/bikefor...4a03a84c44.jpg

This is the old jig that was made for steel rear triangle.

Ben made a more sophisticated new one in 2016 for titanium rear triangle. At that time, he was selling his own steel triangle and wasn't interested in enlarging steel triangle anymore but was interested in enlarging titanium triangle for people willing one of his kit and not accepting the weight of his own triangle (its heavier than the Brompton steel triangle because its made to accept a disc brake, so the weight difference wrt. the Brompton titanium triangle is quite big).

Medium cage works fine on a ETRTO349 or ETRTO355 wheel (don't know if it will work fine on a 16" ETRTO305 wheel).

Medium cage exist in Shimano Ultegra and Shimano 105. Old version RD-R6800GS is specified for 32t max and work well with 3T 9-32 cassette, next version RD-R8000GS is specified for 34t max (3T website claims that it works up to 36t). The same is valid for the same generation of Shimano 105 that are almost the same as Ultegra, only slightly heavier.

well, I tried both Sora medium and short on 35-406 and the medium cage was too big, the bottom polly was leveled with the rim (clearance to the ground was the tyre thickness). While it mechanically functions, it make the derailleur very vulnerable hence my 20”ers fitted with short cage (same as manufacturers).

since I have very little clearance on 20” and cannot see how it can work properly with 16”.

Any link?

No, I do not have it.

The problem was that bending the base without flattening the tube of the titanium triangle was more difficult than with the steel base due to the higher elasticity of titanium.

Just wondering anyone tried a rear derailleur with clutch to solve the chain retention issue ? I've seen a over sized pulley solution which seems to only work on very close gearing, I was wondering if a clutch system might work for a more normal 11-28T solution ?

No, I do not think that it will work.

The design of the folding rear triangle is that when the triangle is folded, the distance between the bottom bracket axle and rear wheel axle become much shorter than it is when the triangle is unfolded.

This is due to the position of the rear triangle folding axle that is placed relatively far behind the bottom bracket axle.

So, the chain tensioner must be able to take over quite a lot of chain length to maintain the chin in tension and avoid the chain derailling/falling when folded.

Other cats have done this with their commercial versions of Andrew Ritchie's folder design:

Aceoffix 9 speed:

https://cimg0.ibsrv.net/gimg/bikefor...0d713c15ae.png

Billiton 7-speed:

https://cimg3.ibsrv.net/gimg/bikefor...d1245904d4.png

Crius Trifold:

https://cimg6.ibsrv.net/gimg/bikefor...598d8d43ed.png

And MIT's 8-speed:

https://cimg9.ibsrv.net/gimg/bikefor...b5afdbbe51.png

Crius Trifold 10-speed: lotsa detail, close shots.

It has a chain tensionner lever around the chainring, same type of chain tensionner as on the Birdy 3.

With so much tensioning so close to the ground, you are replacing one problem by another. You may even say that you replace a less serious problem with a more serious one.

I've seen reports that says the Crius solution doesn't fold well as the chain is too close to the chain stay where the front wheel hinge attaches to the bike. The Mint trifold version uses both tensioner at the bottom bracket and rear derailleur which seems to clear the chain away from the chain stay but add additional tensioner to the system.

I was just wondering if I can get away with just the front one with either oversize pulley or a clutch rear derailleur to make it work instead of another chain tensioner. The Crius YouTube (see below) shows that even with the bottom bracket tensioner it still have alot of slack to make it work properly.

https://cimg8.ibsrv.net/gimg/bikefor...06a36a6f7.jpeg
https://cimg0.ibsrv.net/gimg/bikefor...8909d103b.jpeg
https://cimg7.ibsrv.net/gimg/bikefor...38cd5774ec.png