Thursday, December 24, 2020

...But Why....?

According to the social media techno-gossipers, I'm doing everything wrong! They don't know what I'm doing, so clearly I must not know what I'm doing...

Roadracing motorcycle design has evolved itself into a dead end - they can't accelerate or decelerate any harder without flipping, or corner any harder without running out of clearance. Any changes to one or more areas to improve one aspect will result in an overall loss of performance. Tires are optimized to accelerate or brake, not both, leading to cooling/overheating events. Suspension has to be optimized for load transfer extremes. All the above leads to both slow roll response and high polar inertia about the roll axis. And that results in terrible aerodynamics. ALL of the above problems can be successfully addressed IF you discard the old configuration and derive a functionally superior new configuration.

Here are those problems (And causes):

  1. Stoppies (High CG, short WB) 
  2. Wheelies (High CG, short WB) 
  3. Lean angle limitations (Oversized rear tires)
  4. Roll rate limitations (High roll axis, high polar moment)
  5. Suspension sub-optimization (High CG, short WB) 
  6. Tire sub-optimization (High CG, short WB)
  7. Aerodynamic sub-optimization (High CG, short WB)

And here are the solutions:
  1. Long WB, low CG, linked brakes
  2. Long WB, low CG, 2WD
  3. 2WS, appropriate tire selection
  4. 2WS, low roll axis, low CG
  5. Long WB, low CG
  6. 2WD, 2WS, linked brakes, long WB
  7. Long WB, low CG 
And here's the how, why, and all that:

1. Pretty obvious, really. If load transfer under braking can't flip the bike, then there must still be some load on both tires. The lower the CG, the less load transfer, and the more both wheels will do the work. It also means that trail braking will be a lot more effective as the tire's loads transfer from braking to cornering.

2. Also pretty obvious. Same as the above, but from the opposite direction - just a lot more difficult to implement. Doing it beats complaining about it. The ultimate solution to that problem is addressed near the end of this post.

3. Not obvious, at least with the 2WS part. The tire part ought to be obvious: With 2WD and linked brakes, a big fat rear tire is no longer necessary or even desirable. A narrow tire at both ends provides enough footprint area - more about that in #6, below.

With 2WS, cornering force deviates from perpendicular to the roll axis, thus reducing lean angle (Simple trigonometry, if you must). If 2WS is taken to an extreme, a single track vehicle dynamically transforms into a purely 2-tracked vehicle known as a "Di-cycle": NO lean angle required at all for cornering. That isn't practical for obvious reasons, but steering the rear wheel in the same direction of the turn at any proportion much over 25% (Ideally over 33%) contributes to the same effect, thus significantly reducing lean angle. 

Both the narrower rear tire and 2WS effects greatly offset the negative effects of lower CG on lean angle. No, this isn't in any of the books. Yet. 

4. Not as obvious as it ought to be, sort of: Obviously, the reduced polar moment makes changing lean angle easier and also importantly, makes stopping changes to the lean angle easier. Less obviously, the lower the roll axis, the greater the lean angle changes with displacement of the contact patch relative to the roll axis when countersteering. 2WS normally wouldn't help at all with a high roll axis in that regard, since most of the effort in countersteering is exerted in overcoming wheel inertia - but with the lower roll axis and that increased displacement effect, countersteering becomes a LOT faster with the same effort.

5. Extreme load transfer is no longer happening, which means that a suspension range and rate to accommodate those extremes is no longer required. And with less suspension travel comes less sag under cornering, thus improving ground clearance and increasing potential lean angle even more. Lower CG and long WB, and less travel also means drastically reduced chassis pitching to bump response, acceleration, and braking, resulting in far greater chassis stability. Steering geometry can be optimized full time. And greater chassis stability reduces the appeal of conjuring pro/anti(Fill in the blank) magic geometry, thus making that pursuit irrelevant.

6. With 2WD, linked brakes, 2WS, low CG, and long WB, both tires are working all the time - a relatively constant load should result in a relatively constant tire temperature - tire cooling/overheating shouldn't be such a major source of drama anymore. And since extreme load transfer isn't happening anymore, much lower tire pressures (With the obvious benefit of a larger footprint) are both possible and desirable without developing stability and control issues. Having interchangeable front and rear wheels and tires is a welcome benefit. I doubt that existing tires are close to optimized for such implementation, but the choice of racing slick tires with different compounds and carcass stiffness is a huge help.

7. Brutally obvious: Much lower frontal area results in much less drag. Like totally free horsepower, man. Really. No kidding. 

The rationale (And obvious physics) behind all 7 points guided the design of my next racer. All 7 points are necessary, however inconvenient, to work. No, I don't expect said techno-gossipers to grasp any/all of those points. Nothing in the real world depends on their comprehension or consent. 

Yeah, OK, so what else is it good for? Well, electric motorcycles exhibit a LOT of functional shortcomings relative to their internal combustion relatives. Horseless carriages were usually terrible cars - gasless motorbikes are usually terrible motorcycles. But a lightweight streamlined low CG, 2WS, 2WD electric street motorcycle would solve range, speed, cost, packaging, and weight issues while providing uncommonly high comfort, performance and protection right now. Electric 2WD almost designs itself. Truly effective regenerative braking is only possible with 2WD, low CG, and long WB. With 2WS and 2WD - and smart steering control - it would be possible at a stop to steer both wheels to the side to some degree and balance the bike without human intervention - no dippy outriggers or ludicrous gyroscope system necessary. A minimum turn radius of half the wheelbase would make it far more maneuverable. Then park it by lowering it all the way onto an integrated stand. Wind gust response would be counteracted by that smart 2WS. Smart 2WS (Or even active steering dampers) would also allow the use of tires optimized for traction rather than ease of steering. And so on and so forth - the design and dynamic potential is mind-blowing - something far beyond my next internal combustion powered racer. None of that is in the motorcycle advertorials.  Yet.

January 2023 Update: Over a year after the above was written, many of the above points were made in a FortNine video about "Feet Forward" motorcycles. No, none of my work is featured, but it is still a fun video.

In the mean time, work is progressing nicely if not quickly on the next racer.