Acceleration and really good tires

Disclosure: non-builder here.

Under most circumstances, the static coefficient of friction between your tires and any surface is going to be less than 1 (I know it doesn't have to be, but I don't understand this well. I assume that to be over 1 it has to be adhesive in some way), so the amount of force you can actually put down before slipping should be less than the normal force (which will be your weight here; how many robots have spoilers or create more downforce in any way?), so therefore the maximum possible acceleration should be 9.83m/s^2 no matter how absurdly powerful your robot is... right? This is why there is a ceiling for road cars in 0-60 times below 3 seconds. (Bugatti Veyron 2.46 seconds, Porsche 918 Spyder 2.2 seconds, blah blah, but that's aerodynamic downforce magic again).

I was looking through the old Storm II diary though, and I found a citation of 20mph in about 1.5 meter of space. That's a rough guess I assume, but it seems a little high... though it's the sort of thing that's hard to eyeball.

20mph = 8.9408m/s top speed

accelerates to in 1.5m

therefore minimum average acceleration is:

a = v^2/(2x distance)

a = (8.94^2) / (2 x 1.5)

(8.94^2) / (3) = a = 26.64m/s^2

Implies a coefficient of traction between the tires and arena of a whopping 2.7


Where did I go wrong? Am I just nitpicking a really old guess? :) Or are really sticky tires in play? Tiny spikes?

How can you get a coefficient of friction higher than 1 between your tires and the arena? What is the perfect tire for robotic combat? The motors and batteries are up to scratch, but tires hold everything back.

Your calculations look right to me- i imagine the 0-20mph in 1.5 seconds is a calculation based on the motor torque, gearing, top speed, mass of the robot etc etc in an "ideal world" not taking grip into account. in reality... in an arena with a dusty floor, those kind of acceleration speeds are totally un achievable.

Some teams do use spikey wheels- but EO's frown upon it for obvious reasons.

Yeah, a lot of figures quoted by roboteers are either ideal numbers, rough estimates, or generously rounded up, so I wouldn't read too much into it :L

In the lower weight classes a few robots use either fans or magnets to increase down force, though this is more for extra pushing power than acceleration. Creates a fun side effect of being able to fight on the ceiling though:

https://www.youtube.com/watch?v=gJu7OI5uCmw

No, I don't think that any robots use sticky tyres - they would pick up dirt very quickly. Some arenas e.g. BattleBots do use "traction paint" though, which should increase grip, although this year they had some issues with it peeling off.

Razer did use spiked wheels with some success in the days when the Robot Wars floor was wood. These days it is steel.

Note that dragsters, on specially prepared tracks, typically have a coefficient of friction of around 4. The world record for a 1000ft track is 3.73 seconds, which is an AVERAGE of almost 4.5G. You probably could call their tyres sticky.

It's definitely worth noting the dirt on the arena floor as mentioned above - the floors usually are covered in a lot of rubbish, bits of robot, dust, swarf etc which drastically reduces your grip. Of course, an extremely grippy wheel theoretically would be better, but it'd pick up enough of that layer to negate the wheels pretty quickly. I'd go as far as saying you need to prepare your wheels to be 'off road' more than anything.

I believe that was one of Storm 2's issues in this case, don't quote me on that but I recall seeing them pick up a lot of debris and not being as grippy as a result.

Tried applying spray on belt dressing at a recent big competition (it stops belts from slipping by giving them a tacky feeling). Great for around 10 seconds then it was just the same as before.

Best tyres I've found in most circumstances is bike tyre tread, at least in heavyweight machines. Grips and cleans itself in the arena. Mountain bike tyres being designed to be used in dusty conditions and yet still be able to get grip is probably why they have a good balance.