Custom lots-of-titanium Brompton

Hey just passing through and I see you guys are interested in making your bromptons as light as possible, don't know if it will help but was passing by this site and they had a cool titanium part have a look. Looks pretty good as well.

http://www.welovebromptons.co.uk/index.php/shop/titanium-suspension-block/

Here's my effort. Stock Ti Brompton two speed with titanium'd everything I could to get under 9kg. See picture captions for details. Remaining bits to do, chain and crankset. https://www.flickr.com/photos/53601801@N08/

Hello

I happened on the following post by googling for "bromton front-rack":

http://www.lfgss.com/post3556801-110.html

I just joined so can't PM tilover to learn more about that front-rack he apparently built for a Brompton.

Is this custom-made?

Thank you.

I'm sure someone will correct me if I'm wrong but I think it's just one of these

http://rover.ebay.co.uk/rover/1/710-134428-41853-0/2?icep_epid=-999&icep_msku_flag=n&mtid=1673&icep_meta_categ_id=888&kwid=1&icep_item_id=191191748827&icep_id=67&mpre=http%3A%2F%2Fwww.ebay.co.uk%2Fitm%2Flike%2F191191748827%3Flimghlpsr%3Dtrue%26hlpv%3D2%26ops%3Dtrue%26viphx%3D1%26hlpht%3Dtrue%26lpid%3D108%26device%3Dc%26adtype%3Dpla%26crdt%3D0%26ff3%3D1%26ff11%3DICEP3.0.0%26ff12%3D67%26ff13%3D80%26ff14%3D108%26ff19%3D0&device=c&icep_cond=New&adtype=pla&rpc=0.14&icep_ctlg=-999&ipn=icep&icep_etrs=Y&gclid=CPnA1Njh3b8CFcrpwgodsbQAjg&targetid=73623431226&rpc_upld_id=32952&itemid=191191748827&crlp=43044613506_563391&icep_cbt=n&crdt=0&cguid=562013c01420a5e16af363e3fb4cab34

it was one of those with a new tab welded on to attach to the luggage front (although in that pic i think it's just as standard and doesn't quite fit right

The Brompton of Titaniuminess has been providing sterling service on my commute, the only real problem being its tendency to snap rear spokes. I think it's a combination of teeny rims, a rather chunky hub, the resulting spoke angle, and using CX-Rays in a rather challenging setting. They've all snapped at the nipple not at the J-bend, which suggests that it's the spoke angle where the spokes go into the rim which is the problem.

Naturally, the easiest way to solve this is to build a new wheel. And having decided that I definitely, absolutely had to have 5 gears to get me up and down the Cols and Massifs of London's famous London, the new one's fixed.

But there is a good reason for this, because it is a truth universally acknowledged* that every folding bike is deficient in not having a power meter. After all, if you didn't log it then it didn't happen, but if you don't have power data then it doesn't matter if it happened or not **.

So the new wheel is based on this:

a very reasonably priced 28h Powertap SL+ track hub I got off eBay. I've modified it to reduce the OLN from 120mm down to 112mm using custom end caps like this:

which is just some 19mm A/F stainless hex bar with an M14x1.5 thread down the middle and some machining on either end. Built up into a wheel with some 16" Velocity Aeroheat rims and some custom-length 13g Sapim Strong spokes (hopefully no more snapped spokes) it comes in at a relatively chunky 956g without rim tape:

However, that's lighter than the 5 speed hub by itself, so the bike is lighter than it was. Other than having to machine up a little ali top-hat to keep the chain tensioner in place, it all fitted together reasonably easily.

It works, but at the moment there's too much slack for the chain tensioner to take up when the bike's unfolded for it to work well fixed. However, I reckon that's fixable with some new parts. One of which will be a 14t sprocket to replace the 15t one that's fitted at the moment, because (not surprisingly) a 50/15 gear is a bit too spiny with 16" rims even just for pottering around town.

Once I get the new parts (new chain tensioner, pulley wheels, 14t sprocket and chain) I'll try and get it running better. But, like I said, it works...

/* this is not true
/** also not true

yup, it was a steco with a the headtube rest chopped and rewelded with some bolt holes for the Brompton mounts- not an ideal solution though :)

The problems with the gearing being too low were easy to solve with a 13t sprocket, the smallest that will fit on the Powertap hub. Combined with a 52 tooth chainring, and given the 16" wheels, that gives a 64" gear which seems fine so far. I did get a 14t sprocket as well, but I think I'll stick with the 13t one for now.

The main problem was with the chain tensioner. If you tried to resist through the pedals, the chain tensioner would swing upwards, ramming the jockey wheel furthest away from the sprocket into a welded boss on the titanium rear triangle. The jockey wheel, being made out of relatively soft aluminium, was losing this ongoing battle. Also, because the slack in the system introduced by the chain tensioner, the chain would skip if you tried using the drivetrain to slow down.

The problem, in a nutshell, was that the bottom of the chain, running from the rear sprocket to the chainring, wasn't straight. Leaving the sprocket it first headed upwards over the first jockey wheel, then down over the second.

However, the chain tensioner doesn't actually do anything on a Brompton, not when you're underway anyway. The chain tensioner is really only there to take up the slack which arises when the bike's folded. The top jockey wheel doesn't need to be there at all, and while the bottom one needs to be in contact with the chain, so that it can swing into action (quite literally) when the bike is folded, it doesn't need to do anything else provided the chain's reasonably taught.

So, a wee bit of re-engineering and we have this:

The chain tensioner is a modified 2/6 speed one, as this puts the lower jockey wheel further way from the rear sprocket. The lower jockey wheel is fixed in place (I cut down the chain tensioner mount and machined up a small aluminium bobbin to mount the jockey wheel on), and the upper jockey wheel no longer exists. It's been replaced by a bracket, some penny washers, and a machined aluminium spacer.

It's not the finished product - more of a proof of concept - but after a quick test ride it seems to work reasonably well. If it does, then I'll make a rather more polished version later.

Strong work!
I guess you have seen this one, similar concept:
http://www.fixedgeargallery.com/2007/dec/3/HuwGwilliam.htm

Mental

However, the chain tensioner doesn't actually do anything on a Brompton, not when you're underway anyway. The chain tensioner is really only there to take up the slack which arises when the bike's folded.

I must have miss this, but what about the suspension block?

Yes, that was what persuaded me that a fixed Brompton would be possible, despite everyone saying it isn't. On his Flickr pages he says he can't skid it though. Pity there are no close-ups of the chain tensioner.

The rear axle mostly moves up and down, due to the location of the pivot, and the range of suspension travel is pretty limited. Due to the fact that the rear axle travels in an arc rather than just up and down there is some alteration in chain length due to suspension movement, but it's tiny. If you ran with the chain really, really tight then I suppose it might cause issues, but it's well within the limits of a reasonably slack chain to take up the fore-aft movement caused by the suspension.

So are you still running this fixed?

Yep. All good. As long as you don't try to skid, because if you do the Powertap rear hub starts unscrewing itself internally, which is bad. Use the brakes and all is well. I can't really skid anyway.

Badass. Great read.

Slack chain

Fixed Powertap on a Brompton.

Just because it's possible, doesn't mean it's right.

Well, the Powertap's batteries have died, and I haven't got any SR44s in stock, so at the moment it's just fixed.

Wait, it requires batteries? Needs moar dynamo.

Sorry for the dredge!

Upgrading/re-vamping a Brompton, had read this thread before and enjoyed it. Upon trying to do it myself I suddenly realised how many little issues you've overcome.

Mdcc mentioned a problem early on with the chainset using non standard bolts, was this overcome with the modification of the larger chainring?

I currently have a 24mm axle carbon chainset that I was looking to use but I'm getting mixed reviews on external bb's and their compatibility with bromptons - did you know if this is possible?

The other option I had was to get a campag double chainset but was unsure I'd be able to machine the same solution as you've been able to.

Im going to spend my Saturday night reading every page of this

Only Saturday?

Oh my child.

Oooops, overlooked this one. I didn't have to use non-standard bolts - the chainguard is really just another chainring with the teeth taken off, so as far as the crankset's concerned I'm running two chainrings so I could use the normal bolts.

I've never seen a Brompton with an external BB, although I can't think of any technical reason it couldn't be done provided you've got enough clearance between the crank and the rear triangle to permit The Fold to take place without contact.

I've got a few updates to do with new shiny bits I've fitted, so I'll try to add to your reading list shortly...