New lowracer prototype
I've been working on a new idea for a FWD lowracer that I want to race with this summer. If it works, then I could probably use it as a base for the new trans Canada HPV that I need to start planning.
I know this is probably going to lead nowhere, but for some stupid reason, I feel really compelled to see this exercise through.
There are two things I don't like about the traditional approaches with regard to putting a large front wheel on a front wheel drive short wheel base lowracer:
1. Very poor turning radius. This presents difficulties starting from a stand still at slow speeds because you need to turn that front wheel quite a bit to gain balance as you pick up speed. Once the lowracer is at cruising speed, it does not require much turning radius. A large front wheel (650 or 700 cc) won't turn very much before the wheel smashes into your crank, your legs, or the chain. Even the small front wheel on the M5 is a pain to get started.
2. FWD twisting chain. I would much rather simply route my chain directly to the gears rather than wind them around a pulley or intermediate drive. The extra gear requires uses up mechanical efficiency and you need to make room for it all between your pumping legs.
The first FWD lowracer I built back in November of 2003 worked great, but the front wheel was super tiny and I would much rather have a nice large smoothly rolling, low Crr front wheel:
above: the old FWD SWB lowracer with intermediate drive and twisting chain
above: the new concept FWD SWB lowracer with a direct chain drive to the front wheel and a mono fork.
The steering for this lowracer is a head tube placed directly BELOW the gear cassette. So - it is offset to the right hand side (rather than directly above the wheel). It is a 24" kids mountain bike wheel because it had the largest sized axle I could find of all the various bikes in my garage. I needed a large diameter axle because the wheel is mounted on ONE side only which allows the opposite side to freely turn right and left. The axle is a 12mm thru axle - probably still not beefy enough, but it will do for what I need this prototype for.
The purpose of the prototype was to determine:
1. If cranking on the peddles would cause any torque steer
2. If the forward motion of the bike would cause the wheel to want to turn right
3. If there was enough turn radius to balance when up to cruising speed
4. To see if the chain would stay on the gears while the wheel turned and the peddles turned over.
I was able to ride it down the driveway, but no further because I either need to replace that fat, knobby front tire with something very narrow to get a decent turn radius out of it, or add retractable trike wheels to the back so I can get it up to a decent speed before retracting the outrigger wheels and then determining if there was enough turn radius to balance and turn gradual corners. The tire kept hitting the chain, so it was very difficult to get up to any good speed (but then again, so is the M5 some days!).
The answer to concern 1, was definitely NO torque steer. Because the steering axis is directly through the middle of the gear cassette, there was no peddle induced pulling that I could detect. I am not sure yet about concern #2 because I couldn't get the bike up to speed, and definitely no concern with the chain staying on - it's surprising how much those chains will bend and still stay on the gears. It helps if the derailleur is turning with the wheel which is the case here.
The plan is to use a 650 triathlon wheel (same as the Rocket lean steer trike). It's a couple inches larger than the 24" wheel, but is very narrow and I can get tubular rims and good quality racing tires for it. I guess I am going to have to bite the bullet and purchase a 12mm thru axle mountain bike hub and have it laced to a 650 rim. The problem is, I am not sure 12mm is beefy enough for a mono support. Not sure what to do here, but I do need to get something to complete this prototype test with. Perhaps I need to get a custom 20 mm axle hub machined - but use the 12mm thru axle hub for the prototype.
As you can see in the photos, I did not leave enough room for the derailleur, so it's locked in one gear now, but I don't think I need to test that with the prototype. I can make sure to leave enough room for the full extension of the derailleur when I design the next version.
I know a few streamliners that have severely restricted steering radius and can balance OK when travelling in a straight line or are cornering slightly, so I am fairly confident that with a narrower tire, I will have enough turning angle to balance or turn at speed. In fact, I probably do now if I could get up to a decent speed. If it is unstable because of the steering pivot to the side of the wheel, then I know I can resolve this issue with a no feedback steering system like worm gear or a screw and ball nut. That's the next experiment.
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