At bikeforums.net, the Downtube Mini has attracted rave reviews as a 16" folding bike that has an excellent ride quality while being great value for money. Plus it has a special visual appeal. Pity these can't be bought in Australia, but my boss kindly brought it for me from the US while on a business trip.
The Mini wasn't quite right for me out of the box and I have made a number of modifications, some for bike fit and others for ride quality. This is to be expected; it would be rare for a bike to be perfect wrt fit.
I installed flat handlebars, Ergon grips, better brake levers and a slim, longer stem to increase reach.
The bike got a modified Brooks B17 Narrow saddle, cut to emulate a Swallow, a Carradice saddle bag and Crank Bros Smarty pedals. It has a Sturmey Archer 8-speed geared hub. While it is substantially slower than a road bike (about 15-20%), I have reached 74km/h on it, and it was stable and felt safe.
You can form an idea of how I fit on it. It could be bigger but it is close. Reach could be longer but a longer stem appears odd, so I lowered the bars to compensate a bit. I have done multi-hour long distance rides on it including some very steep rough off-road stuff and it is perfectly comfortable. I am 178cm tall.
One of the first things I did was to replace the suspension spring with a rubber bush. This drastically improved the suspension quality - it is well damped while retaining a nice plush feel. The spring was very bouncy for me. To remove the spring, remove this rubber end cap and unscrew the bolt under it to free the spring.
The spring, its concertina sleeve and 2 plastic support washers.
I installed both of the plastic support washers over the spring boss on the main frame side, to fill in the suspension socket a bit.
On the rear triangle side, I slid a mudguard washer with 10mm hole over the bolt to form a support platform for the bush.
I made the rubber suspension bush from this rubber doorstop, originally about 42mm long...
...and 35mm in diameter.
It has a 20mm wide recess at one end, precisely the right shape to go over the original spring boss. I drilled out the original hole to 9mm to fit over the 10mm suspension bolt.
The finished bush that was installed in the place of the spring. It was made from one doorstop plus a 10mm rubber disc cut off from a second doorstop, cemented together with rubber cement.
This is the bush installed with me seated on the bike, showing the rubber compression.
Another thing that I improved was the suspension hinge. There was some side-to-side play in it, which caused the bike to fish-tail while cornering fast. The hinge is made from 2 interlocking steel spindles which hinge in 2 plastic bushes. This shows how the original hinge parts fit together.
Especially one of the spindles fitted loosely in its bush, causing the side-to-side slop in the frame. I made 2 new bushes from Delrin (shown here behind the originals) in which the spindles fit very snugly. This completely eliminated the frame play and greatly improved handling at speed, especially cornering and climbing out of the saddle.
Here is a more detailed discussion of the suspension mods.
The bike is now almost perfect. Here it is on a light loaded tour. To my surprise, the effective chain stay length is only about a cm shorter than my Raleigh Twenty's, and I was able to install this luggage rack and hang my full size panniers with no heel strike problems. For this tour I didn't need much stuff so the saddle bag and rack top bag sufficed.
The rack could be fitted directly to the mounting holes and I had it like that, but for this tour I wanted some extra clearance to fit the rack top bag under the Carradice saddle bag, so I lowered it by using a small plate to lower the mounting position. The top steel supports were included in the seatpost quick-release screw. The suspension movement is no problem; the rack moves vertically and swivels at the quick-release.
The Mini fits neatly into a Samsonite 29" suitcase. I took it with me on a trip to South Africa.
Here I am, pootling near Pretoria. I also rode the Mini off-road on a very rough, steep stretch of dirt road over that mountain range in the background, the famous Breedtsnek Pass, which was severely damaged by recent storms and most would regard as unsuitable except for a MTB. The Mini handled it just fine.