Brompton rear frame replacement

Ms C’s twelve year old black Brompton L5 has a rusted-through chainstay-bridge. Combined with the twelve year old hinge, this had led to a fair amount of left-right movement in the frame. Whilst still rideable, it seemed like the sort of thing which should be taken care of sooner rather than later.

I had toyed with the idea of getting the existing rear frame repaired, but considering the cost of a brand new Brompton rear frame, the latter seemed like a better option. It also provided the opportunity to bring the back end of the bike up to the current spec, allowing the use of a Brompton six-speed set-up in the future if desired. An entirely new frame also offered the possibility of a different coloured rear frame, which would set the bike apart from the crowd due to Brompton’s policy of using the same paint colour on the rear frame, fork and stem.

The rear frame comes with almost everything you need, non-turn washers, hinge spindle and countersink socket head screws, except for the brass bushes. Whilst these may not need replacing on a less-worn machine (where the rear frame is broken but the hinge is fine), it is still irritating to have to buy an entire rear hinge kit just to get these brass bushes. It would definitely be appreciated if Brompton started including these as a part of the rear frame replacement kit, or at least made them available separately.

The rear frame comes with non-turn washers for both Sturmey-Archer and SRAM hubs.

Replacing the rear hinge requires a special 3/8 inch reamer (shown in this picture guide) which has a pilot section to ensure that the brass bushes are reamed in the same and correct plane. However, this special reamer costs around £200. For the first time in a decade, I was seriously considering getting this job done at a bike shop. I even got a quote for the job and parts from Bicycle Doctor, but thankfully I came across these instructions showing how to do the job with a conventional (cheap) 3/8 inch reamer. Still, this is not an easy or quick job and if you value your time, it might be worth getting it done at the bike shop.

Thankfully, my time is worthless, so I did it myself. The bolts which fix the rear frame to the main frame via the hinge spindle are fastened with a torque wrench in the factory, and held in place with a liberal quantity of thread-locker. The trick to getting these bolts to loosen up is to heat the area up with a hairdryer or heat gun, which weakens the grip of the thread-locker a bit. Once one of them starts to give, tighten it up again, heat the other side up and loosen that off whilst holding the other bolt in place. This may not always work, one bolt might remain stuck in the frame at which point a bit of drilling is required.

The chainset needs taking off to access the rear hinge bolts

Once the bolts are out, the rear frame can be detached and the hinge spindle should slide out relatively easily. The brass bushes are pressed into the frame pretty tight, with no protrusions which would help remove them. Thankfully brass isn’t all that hard and seeing as we aren’t interested in keeping the old bushes, they can be forcibly removed by winding in a 13/32 inch tap and knocking it out with a hammer. I didn’t fancy buying an obscure imperial-sized tap and so decided (after trying to bash the bushes out from the inside using a screwdriver) to file away what remained of the original bushes. This was probably a bad idea, but on this fairly old frame it actually worked out rather well, which will be explained in more detail later.

The new brass bushes can be press fitted with a quick-release skewer from a front wheel or a piece of suitably sized studding and some washers & nuts. I used the (non-QR) skewer from the Brompton dynamo front wheel. The first attempt didn’t go well and the bush started to bend, but thankfully I managed to repair the damage with pliers and it went in (after a bit more filing) on the second try.

Once removed, the damage to the old rear frame was even more obvious

Next comes the tricky bit, reaming the bushes without the special (and expensive) Brompton-specific reamer. The bushes are brass on the outside and some sort of yellow plastic-like material on the inside. The above linked guide suggests that it is important to not remove too much material, but I had to remove what seemed like rather a lot of material. Eventually I put the reamer in a battery drill, which helped to keep it straight. The reamer I used (made by Dormer) was just long enough to remove material from both of the fitted bushes at once, which kept the holes in the bushes in the same plane (the intention behind the design of the Brompton-specific reamer).

On a frame of this age, it is possible that even after replacing the hinge, there will still be some play in the new hinge. Thankfully, my cheapness in wanting to avoid buying a 13/32 inch tap, I may have inadvertently stumbled on a work-around for this. By filing away most of the old bushes instead of fully removing them, I had effectively left a brass shim in place in the frame which allowed the new bushes to be fitted with less play.

About halfway through the reaming process

The new rear frame decked out with the old parts

The new rear frame was bolted on with minimal fuss. Ideally the bolts should be held in place with Loctite 243, but I used Loctite 271 as it was al I had to hand. I will probably regret it if I have to do a subsequent replacement in the future.

Sadly, by this point I had noticed that the rim on the rear wheel had worn to the point of cracking and would not be safe to put back in service. I ordered a new rim and spoke set so that the 4/5 speed hub could be re-built before the bike was fully re-assembled. The old spokes and nipples were a bit past it, and I didn’t want to get 90% of the way through building the wheel just to have a spoke snap or for the threads to strip.

Once the rear wheel was re-built with new spokes and a new rim, the bike was put back together with relative ease. I am told it rides much better now.

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13 thoughts on “Brompton rear frame replacement

  1. “I am told it rides much better now.”

    I should flippin well think so, after all the trouble you went to to fix it!! Chapeau; that’s all a bit beyond my own meagre technical skills. I’m surprised you didn’t take it out for an extended test ride!

    • I thought about it, but without the telescopic seatpost (which I didn’t fancy swapping in) I wouldn’t have been able to judge as much. Plus there would have been a risk of discovering something else wrong with it if I rode it.

    • My L3 from the same era (2001) has the same problem after 11 years.
      The chainstay tubes are uncapped and, with the small wheels, it’s inevitable that water will get in if you ride it in the wet. I’m not sure whether Brompton bother with any kind of frame treatment either. My guess is that any water that gets in will tend to sit inside the chainstay bridge when the bike is folded up.

        • The lack of this problem is one of the main advantages of the titanium rear frame. I am looking at a bit of preventative maintenance for the steel frames and will report back.

        • It has happened on my titanium front forks! They have cracked. I will replace with steel it is far cheaper.

    • I believe that this was a bit of an issue with the bikes from the era these ones were made. Sadly, this is always the risk with second-hand bikes, I have no idea what the bike went through in its first eleven years.

      I’m not sure why Brompton use uncapped stays on the rear triangle, although they do use them for routing the wires on the dynamo. It could’ve been a weight-saving idea, but the saving would be pretty pathetic.

  2. There are two pre-drilled holes in the chainstay bridge that obviously contribute to the common rust problem in this area. Does anybody know why are they there or if they should be filled??

    • These holes find around the frame joints are exhaust ports for the fumes of the welding process. You can use some kind of water disperser to line the inner walls of the frame and prevent rust. You can also close them after this treatment. If there’s moist inside already, closing them wouldn’t be the best idea.

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