Sunday, February 21, 2010

74> Tilted Design, Motorcycle Tires



Here is a logical offshoot of the last tilt-wheel design that I posted. It is still far from finished, lacking controllers, backup battery, brakes, etc. What is interesting about this design is that it is scaled to use standard motorcycle tires, which are speed and weight rated and specifically designed to take wear and pressure on the sidewalls. Weights ratings go to the 1750 lb. (794 kg) range and speed ratings are in excess of 175 mph. (282 k/h)

I am increasing worried, however, that perhaps one track doesn’t work for all applications. This design ties the interior dimensions to about 36” x 24”. (914 x 610 mm)  That’s not bad for long spans but is kind of awkward in buildings, under bridges, or in tunnels. While the swing-arm on the top of my designs should allow natural g-force correction and extremely steep slopes, it, too, adds to the height of the finished system. The result is very much taller than, for instance, the Skyweb Express system. 

In my brief exploration of underpasses in last week’s post, I was confronted by the disadvantage of having a tall system height (track plus vehicle) in this situation. I am not sure how many underpasses have room for both pedestrians and a fenced off Podcar lane and this troubles me. The Skyweb Express model not only doesn’t have this problem but it also appears that I may have underestimated Skyweb’s maneuverability somewhat. There is also the matter that I have several tweaks in mind that would greatly enhance their design (as I understand it) in this regard.

I remember in the old days I thought it would be pretty cool to be able to retrofit one road lane into four PRT lanes, packed two high and two across. One problem with that vision, however, is the need to keep people away from the track for safety reasons. This need becomes more pronounced with every incremental increase in system speed, so it really needs to be an integral part of the infrastructure from the inception. The Ultra system uses the inelegant chain-link fence strategy. Some bottom-supported systems (Vectus, Skyweb Express) use the strategy of keeping the track elevated and preventing access with the station design. Captive bogie hanging systems (such as I have been designing) have the clear advantage here, but it still seems prudent to keep the bottom slot out of reach even though it could, in theory, be very narrow, precluding, for example, insertion of an arm. How high is “out of reach?” Having given up on a combined track/vehicle height of under eight feet, (2.4 m) my next main worry is being able to fit between floors of a typical building. I will save the protracted discussion, though, of station design or optimal track height for a time when I have some illustrations prepared. Suffice it to say track height is not without consequence, so the large wheels may come at a cost. 

I have to say, though, that this design passes the “smell test” in that it appears to be a design that would be at home at very high speeds, which, of course, can be reduced. This is much better than starting with a design that has requires modifications to go fast. I also don’t see any inherent maneuverability problems in terms of relatively tight turning radii. Be forewarned though, that this design will take a while. Shown is an old version. Currently the frame (yellow) has been completely scrapped.   Simplify! Simplify! Simplify! 


4 comments:

qt said...

What size motorcycle tires are you using? If they're standard Harley-size, you might could shrink things a bit by going with a super-scooter. Honda Helix tires might be adequate with a smaller diameter, for instance.

Dan said...

Hi qt, thanks for dropping in.

I don’t know a lot about motorcycle wheels, but I gather that most are either 16 or 17 inch. I would be fine with the latter, assuming that they are rated for plenty of weight and are “commodized” enough to be inexpensive. That general size is for high-speed 4 wheel systems. If we want to do two bogie system, (two groups of 4 wheels, like a train car) there are an increasing number of direct drive “hub motors” being produced for the Asian scooter market. These are low powered but with 8, they’re plenty strong enough for sub 50 mph speeds and, of coarse, they are tire ready.

Right now I am having problems finding motors that are strong and fast enough for 4 wheel systems that will fit the “super scooter” tires. I must say though, that direct drive is the coming thing, and new motors come into the market almost daily. Also the switching design calls currently calls for all weight to be shifted to just 2 wheels while the other two cross the middle slot of the track, making the heavy load ratings imperative. If not for these limitations, which are certainly not written in stone, the size range you describe would probably be just about right.

Anyway, there are a number of related trade-off issues that really shouldn’t be rushed, before I can finally recommend a track specification, such as revisiting the maximum-slope/station footprint/swing-arm-length issue. This whole finished height business is going to have to be explored holistically…Maybe that will make a good topic for this week’s post…

qt said...

I'm not a superscooter expert myself. I mentioned the Helix because Craig Vetter is working on a low-drag fairing for that type of cycle and mentioned that the Helix uses 12-inch wheels. I don't know if they'll hold the load you're talking about, though...

prolix said...

Tires with high unbalance forces are downgraded or rejected. When tires are fitted to wheels at the point of sale, they are measured again, and correction weights are applied to counteract the combined effect of the tire and wheel unbalance. After the tire sale, the driver takes the responsibility to determine the unbalance or imbalance due to the operation of the vehicle.Wheels And Tires