Sunday, April 11, 2010

81> ZZZZZZZ.........


Many readers might want to know what it is like to live the life of a famous PRT blogger/designer…
“How do you handle the glamour, the prestige?” Some might ask. I would like to assure you all that I put on my pants one leg at a time just like you. Away from the spotlights, amazingly, sometimes my PRT work can even be a bit, well, tedious. That’s right. Tedious. Why, in the course of writing the following piece I actually dozed off! P.S., Sorry, my non-American friends, for all the non-metric units but I want get this out… I’m getting.. (yawn) sleepy….

I have been looking into smaller wheel diameters because I really think full-sized motorcycle wheels are too much of good thing. True, they are capable of 120 mph speeds, and the larger wheel diameters improve the rpm/velocity ratio, leading to longer bearing and tire life.  There is also the matter of motor availability. There are a fair number of stock motors in the 3 to 7 kilowatt range (hub and torque motors) that tend to be designed for RPM ranges that are bit slow. Making the wheels bigger makes the vehicle go faster for a given motor speed.

But the exercise of the last post demonstrates the downside, that the full system height is a tight squeeze between the floors of a standard building. The previous design was 10’ 9” tall and getting that down even a few inches would be worthwhile. Furthermore the same is true of freeway overpasses.

With this in mind I started looking for very big scooters or small motorcycles to use in a smaller version of the system shown in post 74. While 16” and 17” are the norm for rear motorcycle rims, a Honda CN250 “maxi-scooter” has 10” rims. More importantly it has a weight of 346 lbs with no riders and a top speed of 72 mph. Since most of the weight on a scooter is to the rear, even without a couple of passengers, it seems safe to say that tires capable of handling the needs of a 4-5 passenger, single bogie, highway speed-rated PRT vehicle are already on store shelves in sizes down to 10”. The smallest scooter wheel that I have found so far that is rated for 100 mph is the Burgman 400’s, at 13.”

There is more to this than using stock tires. There are the wheel bearings. Here I have to admit to being behind the times. Improving manufacturing techniques and material science continue to change the rules. It used to be that bearings for our application would need to be roller bearings, probably tapered, and that they would tend to overheat. I have just finished looking at a bunch of videos of motorcycle bearing replacement, (including the heavy, small wheeled but over 100 mph Burgman 650) and it looks like they are running on simple sealed ball bearings. Hmmm. Ceramic balls, super finished, super hard steel races… Onward and upward! The bottom line is this. The large wheel size that has driven in my track designs has always been a guideline more than a rule. It would now appear that off-the-shelf tires and ball bearings can be had that are designed for 2000 rpm, 450 lb. per wheel applications. Therefore there is little justification for a 36” high track, at least in town. What I am shooting for is a finished vehicle-plus-track height of 10’, without compromising speed or comfort. I would note, however, that existing products, like motorcycle bearings or tires, do not necessarily represent the last word in what can be done technologically. Just because there is currently no 9” 120 mph, 500 lb. tire doesn’t mean it would be difficult to make. I just would prefer not to base a track standard around it. My inclination at present is to go with a 10” rim, (16” outside diameter) even though there may not be any tires currently available which are rated for speeds over 72 mph. Sometime down the road…err … track, if PRT on this track standard ever takes off, someone will make such a tire. The same goes for motors. Nearly every manufacturer advertises that they will design and build to your specifications and needs. Still, a preliminary design needs to have some solid basis for its dimensions…
One other… another…. the.. zzzz zzzzzzzzzzz zzzzzz…..the  zzzzzzz……

2 comments:

chronokun said...

So long as they can do atleast 100km/h er... around 70mph I think that should do for a start, I mean that should be enough to outpace road travel anyway.

Dan said...

Thanks for your input chronokun...
I have been working and reworking the geometry. No firm decisions yet.