Tallbike

Ron has bought himself a new bike. Naturally he brought his old one round and parked it in my shed.

I'm told Healing bicycles were standard NZ issue in the 1970s. I already had three of them so I thought I'd make a tallbike. I got a scrap of paper and spent an idle half-hour sketching this.

No particularly good reason other than the fact that the idea exists.

Advantages:
1. Visible on the road, so drivers give more space when overtaking.
2. Penny farthing height (amusin') but, having a longer wheelbase, reduced danger of a header.
3. Uses up bits from the Bike Heap.
4. Any weight can be carried low and between the wheels.

Didn't want to go too high cos
a) difficulties of ascending and descending, & 
b) broken leg = not much fun.

I took a perfectly serviceable gentleman's mountain bicycle frame from a nail whacked in the shed wall and removed the 35mm top tube, it being the same diameter as a Healing steering tube. Popped it in the lathe steady and machined the end square to the bore.

Not for off-road use or stunting. Wouldn't want to over-stress that 18-gauge 2" steel downtube.

Chopped the downtube out of a Healing Tenspeed, cut the headtube in half, machined the lower half to fit inside the 35mm Tarini top tube and plug-welded it and then butt-welded it in place and tidied everything up with an angle-grinder. Dremeled the inside of the Healing half-headtube to fit a scrap of 18 gauge 32mm and plug-welded it in place.

Thought for a few minutes about which side a front Vee-brake cable  might like to lie before welding the Tarini top tube as an extension of the Healing headtube. Guessed there'd be a problem getting the top and bottom cups parallel, so I used a square to nip two blocks of aluminium onto a bit of box-section, and popped the extension between them before welding.

Only other major problem is extending the front fork steerer tube. The fork bearing cups are now twenty-three and a half inches apart. To extend the fork tube I need to weld a long 1" tube, and at its top, weld a piece of threaded fork tube onto which to screw the top bearing. The middle bit of this steerer tube is likely to bend when the front wheel hits a bump. A Huffy yielded a stiff piece of top tube: those old Huffys had 16 gauge frames in anticipation of being ridden by sturdy Americans. Another fork yielded a threaded end, and had the thinner 21mm ID which was sometimes used to ensure frustrating incompatibility between bicycle parts, but meant that this was of thicker gauge tubing. Machined all the ends square to the bore, natch. I found a steel handlebar from a  perfectly good gentleman's mountain bicycle, chopped bits off it, and machined them as inserts in the various bits of fork steerer tube.

Steerer tube bits cut and machined square to bore

Inserted and plug-welded through 4mm holes

Butt-welded together

Tidied up with angle-grinder. Fingers crossed that it's not so angle-ground as to be weakened.

It occurred to me that from the fork crown to the handlebar is three times as long as a normal tube, so when the front wheel hits a road bump, that tube running between the steering head bearings is 3x3x3 as likely to bend. Which is a lot. The ends of the frame head tube are of smaller ID than the middle of the tube, so I can't slip a bearing around the middle of the fork head tube. I consulted a design engineer on the other side of the world who builds these:

He promptly replied

Remember that if you have a fork tube 3 times the length, the force on the top bearing is 1/3 of the force on a short tube. I think you don't need to do anything. Easy to test anyway. Set up a tube or bar between supports at the proportional distances and apply a load to the end and measure the deflection. Move the end support to the other position, apply the same load and measure it again. I'd use the fork itself if I had it made, a heavy weight and a dial indicator, but you could use any material with the same cross section along its length to see what happens.


I think I'm going to see what happens.