I've been wondering about making some little extenders to bring up the bottom bracket height for a theoretical old-british-folding-bike build I'm considering (which potentially involves switching out 440 or 451 rims for 406 rims for the sake of tire availability).

Why?
Particularly am thinking about a Dawes Kingpin - these came stock with now-almost-impossible-to-find-tires-for 440 rims. They can fit 451 which have an OK but limited selection of tires, 406 rims are OK but the bottom bracket height starts to get quite low and puts you in frequent-pedal-strike territory (a friend measured theirs with 2" bmx tires on a 406 rim and had ~242mm bottom bracket height). Raleigh twenties bottom bracket heights (according to other friend's measurements) are about 20-25mm higher with similar rims/tires so probably are out of the danger zone for the most part.

Some assumptions I have/things I already know:

• Buying a different fork with a longer axle-to-crown would be a more elegant and easier solution, I might very well do that, but that probably involves buying new or getting lucky finding another cheap folder or very unusually-shaped recumbent to scavenge a fork from so probably is going to be relatively expensive compared to a couple small laser-cut parts.
• Buying a totally different folder with a geometry actually designed for 406 wheels would be cheaper and a better idea. True but I'm currently having fun imagining a wacky vintage build. If the right bike floats my way I will probably buy it and never do anything described below.
• This might put the rim outside of the range of even long-reach calipers and necessitate a drop bolt or cantilever posts to be added, I'm fine with that.
• The design described would increase the fork spacing, but most of the bikes in question already have a narrower-than-usual fork spacing, that's fine, it'd probably just bring it closer to a normal spacing and then I'd cold-set it to a standard spacing
• I'd need to also raise up the rear with either a modified dropout or a similar dropout adapter bolted into the existing slot. That seems relatively straightforward either way since the rear-end geometry seems less critical to handling (if anything maybe I could also stretch out the wheelbase a little)
• The forks in question have pressed dropouts (i.e. the fork blade tube is crimped down and a u-shaped dropout is stamped out of it).

My basic idea is to design a small plate that can be bolted onto the fork dropout which effectively creates a new dropout a little bit lower. It could also be welded/brazed on but right now I'm thinking a non-destructive mod might be preferable at least for experimentation. Probably the total lift would be in the range of 20-30mm.

Questions:

• Is this going to be a deathtrap? I imagine if I make these out of sufficiently thick stainless or 4130 they'd be plenty tough and have a hard time imagining how they'd fail other than bolts working themselves loose.
• Would designing the shape of the extension to preserve the trail of the fork (option A) result in similar handling? Are there other factors I'm unaware of?
• Are there any similar mods/products like this out there? I did some searching around to try and find anything but didn't turn up with much.

Please note that this doodle is in no way to scale and is just intended to illustrate the idea.

"Death trap" is what is said after the worst happens but you're not there yet

My first concern is about a drop out extender thingy not being contained well enough to be stable/safe over the miles. I remember cutting off fork blades as a kid to extend another bike's forks with... For a short term trial when you are focused enough to check the hardware and fittings before each ride I see little issue But people tend to forget and think the "current state of affairs" will continue even if they stop paying attention (hence many "JRA" claims). If you do find the extender idea works I strongly suggest brazing/welding on the long term thingy.

The lifting up of the front end will change the steering geometry some. For the worse or better you won't know till you try. I do see the merit WRT to this aspect in raising the rear end.

However given all the mucking about mentioned as well as what the different rim brake track radius brings I suggest that you consider a bike with the wheels size you want already speced. Sadly most all the low cost folders are made of fairly heavy tubing and have components that are also both weighty and often lacking sporting qualities. I do have an Automoto 406 rim folder with a Sachs 3spd that could easily leave my garage for not too much $. Andy

Today is a good day for bolt-on fork extenders. Tomorrow, not so much.

Got a good chuckle out of that, Bulgie! My bet is that it will work particularly if the extender is thick enough to handle lateral forces. With three points of securing the extender to the fork blade it is unlikely to fall off.
Like Andy, we used angle iron to convert our Stingrays to choppers. Billy Martis extended his blades out 6 feet and rode it for a full semester like that. He went on to become a really good...nope, not engineer... guitar player!
FWIW, make the stuff from steel plate.

I can see and appreciate the thought going into this project, Bravo.

I would just make a long dropout out of appropriate material rather then use a two component device. I would then saw off the old drop off and braze in the new one...

Thanks for the feedback!

The more I think about it, the more it seems like the right move would be to just make it permanent from the get-go. New dropout would be an easy time to add eyelets and stuff. From a style perspective I'd probably go with something like option B and attempt to de-rake the fork a bit if I didn't like the handling. If I'm making bolt-ons _and_ a drop bolt or something just to test it, I'd rather spend that money and effort just putting in longer dropouts and better brake mounts


I'm probably not ready to buy anything just yet but let's see a pic!

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While we're on the topic of foolish and excessive things to do with a miniature bike fork, I had a couple other harebrained ideas:

1. What about adding a disc brake mount? I understand that generally the opinion is "don't add a disc mount to a fork that wasn't designed for it" but it seems to me the force on the disc would be a lot smaller because of the small wheel, so maybe not a disaster? If I'm already designing and making new dropouts it seems like a disc tab is not too much extra trouble and maybe could be integrated into the dropout.
2. Since really the goal here is to increase axle-to-crown, what about just brazing on another crown race seat right on top of the existing crown race seat (assuming I can find someone locally with a crown race cutter)? The kingpin, like the R20 and a lot of other folders has an extra long steer tube to accommodate the (unnecessary to me) quick-release stem clamp. Presumably one could shift some of that extra steer tube length below and still have plenty left over for the headset. Would having an extra 15-25 mm (I'm guessing, based on the apparent stack height of those clamps) of steer tube between the fork crown and the race be a structural liability? Might depend on how long the butted section of the steer tube is (if there is a butted section?)

There are really huge crown races out there:
https://ebay.us/m/jWPLPQ

Similarly, there's some headsets that intentionally add a lot of vertical padding.

I've been considering them for my Trek 5000 which I'd prefer was more like a hybrid than a road bike.

Yes this is the way to go! You don't necessarily need to braze on a new crown race seat though. If you have access to a lathe or a minilathe you could just make a "lower-stack-extender", that sits on top of the existing crown race seat, and then presents a new one 20mm higher up. The whole thing could be made out of aluminium which is much easier to lathe (but I do make metal crown race seats on my minilathe, so it can be done).

If pedal strike is the issue you could also try shorter cranks. Those are trending these days anyway.

No reason why the original idea of clamp-on extenders couldn't work but it would be tricky to make them so they got a good purchase and they would be a bit ugly.

Small-wheel folding bikes are among those bikes that have made tremendous advances. Further, good ones with 406 wheels have become commonplace, and lower in price, at least on the used market. Thus, I would recommend surveying the market and deciding what your long-term goals for the bike will be, and finding a nice used example as a canvas to paint on. What city do you live in? The markets vary greatly, a big bike city would offer better selection on craigslist.

I enjoy restoring old stuff. But in the case of a folding bike, the modern designs are so much better in performance.

An example of what I've done with a moderately cheap folder, a circa 2004 Dahon Speed 7; not as cheap as a Boardwalk, but has a frame that appears to be more torsionally stiff than the smaller-tube Boardwalk. Less expensive than a Mu.

Mods:
- Front and rear racks, rear that holds the panniers far enough aft for pedaling heel clearance.
- 2X gearing for 400% range (HUGE plus, I couldn't do the hills on the original 52x30 low), 21-85 gear inches on 50/34 x 11-30, plan to go 11-34 with next chain and cassette change. 2-piece hollow-spindle crank with external BB bearings. GS (mid) length rear derailleur with claw mount, as frame lacks a RD hanger.
- Improvised "Deltech" cable for better frame strength and stiffness (not shown in photo).
- Aero bars, bar ends on the flat bar, 2-position brake levers, mid-cable brake lever on left front of aero bar for front brake.
- All bearings rebuilt with careful preload adjustment, greatly improves durability.
- Wheels retrued to very close tolerance and with even spoke tensions, greatly improves durability.

One issue with adding a crown race seat further up the steerer is that it will drop the top of the steerer down and into the stem/head tube. In other words,the steerer might not be long enough to have this happen and still secure the headset adjustment and stem.

I agree with doing a bolt on dropout extender trial/test first, just don't leave the fork at that state for long. Andy

My folder is well out of the way, behind and above a bunch of other stuff and is far from what a sporting rider would want to pedal for very long. Instead I attach a sale ad to what is one of the better folders with 20" wheels. I have a good relationship with Rochester Community Bikes, the non profit offering this Bike Friday, and would be happy to act as your local guy to pick up, box up and ship it off to you. Andy

I see that my copy and paste of the Marketplace ad for a Bike Friday wasn't allowed to go through, darn politics between interweb players... It can be found on the Rochester, NY Marketplace. Andy

This is part of the formula for sure, I'm told that a lot of folks who ride these on 406 wheels lose the cottered cranks and go down to 165mm but still have more pedal-strikability than usual (and I tend towards wide pedals to fit my feet so that seems extra likely for me)

Yeah agreed - I'm in SE michigan and have been (and will continue) looking. Realistically this is probably a build-it-up-next-winter project so there is absolutely no rush for me. A right-priced dahon or similar seems very plausible - I do kind of want a 3 speed hub on this which seem less common on the newer bikes - haven't really decided yet though (and obviously not a deal breaker because I can always move one over or rebuild the wheel).

ooo I heard mention of those but couldn't find any actually for sale! I think the very-tall crown-race-for-a-sealed-bearing-headset really seems like the way to go at least for initial experimentation. Cheap and non-destructive, also doesn't change the brake (or steering) geometry and probably gives a little extra wheelbase as a bonus. Combined with a rear wheel lift I could probably get a decent amount of extra height - and easy to tailor to the height to match the available steer tube length. Presumably less structural risk too since the load is going down to the original crown race seat.

SE MI, my old stomping grounds!

If you like 3 speed IGHs, there are many Bike Fridays for sale used (at least in the Pacific Northwest, particularly around their mothership in Eugene Oregon, or Portland, and some in Seattle) with a Sachs/SRAM 3x7 or 3x8 IGH/cassette setup, that was popular with them for a while, but they sell cheaper now because most people want all external gearing, because it's easier to service. I would think different with ALL internal like a Rohloff 14, but to me, the 3x7/8 was the worst of both worlds, the drawbacks of IGH AND derailleur gearing. (It was designed for city bikes in Europe so could have a chainguard, but I have seen front derailleur bikes with chainguard on these forums.) Dahon made the Speed TR ("Touring") with that setup, instead of simply using a 3X crank in front. The Rohloff is notable in having great gear range, great seals, and oil lube where a drain and fill is easy, instead of $100+ teardown and lube of a typical IGH, there's an IGH specialist in the Seattle area that does that.

Bike Fridays are great bikes but built to order so often a semi-custom size; You need to know what dimensions will fit you, and can do that via the Bike Friday website I think.

The pic of mine in post #9 is an hour due west of Eugene OR. I was sorry I didn't have the time to stop by Bike Friday and look at their bikes, the shuttle bus between the train station and the coast, passed right by them.