Friday, August 12, 2011

127> Really, Really Fast

As anybody does much design work knows, you can always do better. Second guessing one’s own designs is something that is best not rushed, however. So here I am starting from scratch once again, with fresh eyes and a few different conclusions.

There are several considerations that motivated me to rework the bogie design. First, I think I put too much emphasis on a system that could use off-the-shelf tires, even at high speeds. This led me to motorcycle tires. Actually, though, what is the function of a tire? It is for vibration dampening, shock absorption, and traction. Since we are talking about running on smooth (finger-jointed?) steel, it is mostly just traction that we’re worried about. The problem here is that to achieve it, tires create a flat spot where the tire meets the road. Taking the wheel “out of round” in this way increases rolling resistance. In other words, it wastes energy. Any emergency stopping should undoubtedly be done by clamping the track, and no standard tread design is going to climb very steep slopes anyway.  Finally, I suspect that it would be easy and cheap to outsource, even in small quantities, a solid rubber tire designed specifically for PRT. 

Secondly, there is the matter of flanged wheels. I don’t like flanges for hard and fast use because I if they are of a hard material, they will make noise and vibration. If they are of a soft material, the area making angular contact will wear quickly. This is because a flange is in effect, a wheel with more than one diameter. Since any given diameter will make a wheel travel just so far per revolution, if wheel portions with more than one diameter make contact at the same time, one or the other must skid to compensate. Thus you have designed-in a wearing surface. Position-locking angular contact can be made with equal diameters however. Consider the example of a rounded pulley wheel on a square bar. There, two point contact can be made and, if the materials are hard, there is little frictional tradeoff. Anyway, I have softened my position to consider using flanges because they so simplify the mechanics involved. There are better plastics these days, (such as Dupont’s Hylene) and with large diameters and geometries that minimize load, it’s worth a look, even for continuous high speed applications.  
Another matter that I have been recently considering more is the matter of aerodynamics of the bogie itself. If the bogie takes up all of the room inside of a box beam track, then it must push all of that captive air in front of it. Any bogie design must take this into account, and obviously smaller is better.
The design shown fits into a track with and internal height of about 20” (500mm) high. This is where I may have gone a bit overboard. You see, I wanted to fit the wheels with a commercially available hub motors and although I have seen many Chinese offerings from companies I have never heard of, these don’t even come with technical data sheets and are hard to design around. Unfortunately, western motor manufacturers seem to only want to design for a very large customer base, and really haven’t tried to get into the direct-drive vehicle business, so I was left with a somewhat oversized British offering. 

Protean motors are very powerful wheel motors which are designed to fit on ordinary cars with minimal modification or loss of power. Since I don’t want to design track that is too small to transport people at speeds they have already become accustomed to, and I don’t want to design a system that will constantly require wheel changing, these 16” offerings seem like a reasonable top end, as far as rim diameter goes. This does, however, make it into one heck of a hotrod. 
The Protean wheel motors, you see, produce (together) up to 320 HP continuously. (240 KW) These motors ARE the wheels, of course, so there is zero drive-train loss. So the thing can pretty much go as fast as we want. (For comparison a Tesla Roadster goes 125 mph (0-60 mph in 3.9 seconds) pulling  a roadworthy steering and suspension system, a transmission, and a 450kg battery pack at “only” 288 HP. (185 KW) So we are talking fast. Very fast.  Note that a motor’s power draw is proportional to the work it does, not its potential, so you still use very little power while cruising if the vehicle and bogie are well designed aerodynamically. In the pictures these motors are seen in green. 

The geometry that I am exploring in this design centers around eliminating upper guide wheels by having the vehicle press against a “ceiling” within the track to eliminate tipping or derailing when turning off of the main track onto a fork. The steering guide wheels are angled and flanged to fit more compactly. These guide wheels could also be external to the track, something that I have avoided for noise reasons, but my fears may well be overblown on that issue. Anyway, I have shaved a few inches from the track girth and, well, made a rocket.