Access

I was thinking about this the other day.

When you are driving on loose dirt, let's say with big wheels that kick up a ton of dirt, is the path to optimum or 'best' acceleration still trying to minimize wheelspin or the loss of traction caused by turning the tires faster than the car? Or does the loose dirt that is being kicked up and thrown in the opposing direction to the car's motion change things enough? I'm not talking about rl driving but RC driving, 1/8th to 1/10th scale and beyond.

As an illustrative example imagine a RC drag race between two high-power electric monster trucks, huge wheels and out in desert. They both start at the same time, driver 'A' tries to walk the throttle up softly, maximizing traction in an optimum way on the loose sand. Driver 'B' just floors the throttle from the very beginning of the race. Who wins in this situation? Under hard-packed conditions driver 'A' is the winner. If the sand becomes looser and looser, is there ever a point where driver 'B' wins?

plasmaedge

Id say it depends on how soft(or hard) the grond is and what tyres you are using.

1320Fastback

Floor it, real life sand drags would use traction control if it made them faster.

rclugnut10

My Feedback: (1)

on sand floor it same with snow, floor it, on dirt and pavement there you gotta be careful i did this with my B4 a few times drove it out onto my dirt road so fairly hard packed with my SW 4600 combo idk what gearing 20/81 maybe, proline holeshots body on (so with a rear wing) and i dropped the hammer from a stand still it went no where the tires balloned but the car stayed in place for about 2 seconds then started to go, i did the same thing but spent 1-1.5 seconds bringing it up to WOT and it took off like a rocket, so it really depends on dirt, tires, gearing to a degree, a lot on the motor, even down to the suspention setup who will win an offroad drag race

the lug

guver

My Feedback: (325)

I believe that driver A wins on both surfaces (in most instances)

A couple things to consider are how much "overpowered" are the vehicles and what type of tire design there is and what is underneath the loose surface.

Here's my thinking anyways. Whomever has the most differential between tire speed and vehicle speed get the most forward motion.

ps ask a farmer this question if you think it applies.

Jeremy Z

Keeping traction is going to be quicker. Works the same as braking, but in reverse.

The only thing that makes it tricky is that keeping traction, but being right up to the limit is much more difficult to accomplish than just flooring it and getting OK results. If there was some kind of traction control, it would be easier to prove.

Chris_RC

Think about electric vs. nitro. Electric has instant power, nitro doesn't. Put them the same distance away from a jump and see which one accelerates fastest and jumps the furthest. Granted the electric won't slide out, the EP car will accelerate faster.

If you think about it, think of how slow you would go if you didnt roost. Especially wiht soft compound tires like Bowties where they act like paddles, accelerating fast is better and faster.

Access

The reason I asked is b'cos here is my breakdown of the physics.

case A: Functioning like a normal car (ie. on-road). Driving at the exact limit of traction, the friction between his tires and the ground provides forward motion. When the tires start to slip or 'spin', this friction is greatly decreased so his idea is to drive at exactly the limit of this friction and not go beyond it. We know this is the optimum way to drive accelerate on-road, and on a hard-packed off-road course.

case B: He has broken traction, so the friction between his tires and the ground is greatly decreased. Maybe 20-33% that of case A. But in theory he is also functioning like a rocket. His tires are spinning so fast they are 'throwing' or propelling sand in a direction that is opposing to his own motion. Since linear momentum is conserved, his hope is that the horizontal component of the sand thrown by the tires adds to his forward momentum enough to make up for a loss of traction.

In theory, case A's acceleration is limited to the amount of traction he can get on that surface. Case B's acceleration is not limited in this way. The more dirt he moves, the faster he can go, regardless of how good or poor traction on this surface is. Given a really powerful drive and super-size wheels, as long as he doesn't 'dig in' or bury his wheels he's okay.

The other factors to consider are that RCs are much smaller than real-life vehicles, and b'cos scaling is done in all 3 dimensions, the power-to-weight ratio is huge.

guver

My Feedback: (325)

I'm not sure that the debris or dirt thrown in the opposite direction enters into the race at all. I forgot about that's what you were asking. I think that imediately after leaving the tread it is "just debris"

BigTb17

Yes, but I think the idea is that for every action there is an equal and opposite reaction. So for each particle of sand, pebble or chunk of dirt that is being thrown backwards a force had to be applied to it. That means that an equal forward force is being applied to the tire. Throw enough dirt and that is some serious forward force, even though the tires have technically broke traction.

Access

This is the physics part, unlike kinetic energy, linear momentum is always conserved. For instance if I'm in space and I pick up a bowling ball right next to me (not moving in relation to me) and throw it in one direction - this will also cause me to move in the opposite direction. The actual velocity I end up moving in depends on the relative weights, the object I threw vs. my own weight, and so on.

GravityJunkie

Just throwing more material for an opposite reaction does not give any indication. Static friction coefficient is usually higher than kinetic friction. No one simple equation gives the true answer in real life.

I agree with others. It depends. How well do you need to control the car. You slap on the gas and you're basically drifting, with enough power. Drifiting is no where near as controllable as managed acceleration. For the first 3 feet the grip and rip may win. 15 feet later, it can be the other way around. If you can get the tread dug into the ground, grip and rip could be better. If not, flooring it can become a hinderance.

The best pro RC drivers are not the grip and rip, but easy and smooth on the throttle. Just a little food for thought.

Access

They do this b'cos of the demands of course/track racing and that most off-road race tracks are more of the hard-packed, prepared variety. A track made of loose sand would deform over time.

I do not think it is comparable to drifting, drifting is an on-road thing where you are not really throwing around a significant amount of meteriel.

I guess the main question comes down to 'in practice' does driver 'B' ever win. Can counterforces ever make up for the loss of traction 'B' experiences over 'A'.

Chris_RC

Take a look at Ryan Lutz. He is one of the fastest drivers in the world and he powerslides through every corner. here is a grea video I watch over and over.

[link]http://www.youtube.com/watch?v=zlWkmmW7sF8[/link]
Even extremely hardpack tracks with blue groove deform over time. At 2-3 day big races the track is perfectly prepared at the beginning, but at the end it is extremely rutted out because the tires act almost as shovels and eventually create potholes where people commonly accelerate.

monsterbombs

if your thowing enough roost it will move. every action has an equal
and oppossite reaction. you need to dig holes to move sometimes.
throwing the debris is what gets you going.

with alot of these new paddle tyres and sand paws it doesnt take much
power to get you going.

Access

Yeah in that video, he is able to drive a better line than any of the other racers. But powersliding around corners is a different philosophy, purposely allowing the wheels to break loose and then using that to your advantage.

Chris_RC

Never-the-less, this was one of the most interesting threads I have read on RCU. I never really thought about it before.

guver

My Feedback: (325)

Maybe it will help to remove the loose debris part of the equation?

Oh, I guess that was the point though. If we do remove the debris part of the equation and talk about coefficient of adhesion and traction and stiction then perhaps we can throw the loose stuff back in.

It may also help to think about water and air as well as ground and traction. 

GravityJunkie

Just because he powerslides does not mean his pounding the throttle. Controlled powerslides takes finesse. That track is dialed for sliding the corners loose dusting over smooth hard pack. If it's rutted, drifting corners is more of a gamble. I dunno if that was an actual race either, didn't see much along the way of marshals.

Like I was getting at, every situation is different. Not only does it depend upon the the surface and the tires, it also can depend upon what you're driving and how the vehicle is setup. I'll more often drift the corners in 4wd than 2wd. I many times use the front tires of 4wd to power me around the corners.

Chris_RC

Ya that track has this weird sand that doesn't pack down so you can powerslide really easy. I hate that track for that reason, it just feels weird when racing on that track. Definitly not a race, those other drivers sucked.

Access

Without the loose debris it basically comes down to the dirt-to-tire match we are all used to.
ie. For loose dirt, a harder tire with more tread or 'tread features'. The more dirt you can grab onto, the more traction you can get. If the dirt gets thick deeper down, longer pins to dig down to that level. If it's loose all the way through, something like paddles would be the most optimum. And driving style again, driving in order to not 'break loose' makes for optimum acceleration.

For the extreme case of driving through a loose medium, think of something like watercraft. Moving through water is, at least in concept, similar. In some of the first powered boats they used paddle wheels. Then propellers. And for some of the most extreme cases, pumpjets or waterjets. All these things were designed with one thing in mind, to take water and move it as hard as possible in one direction, in order to accelerate the vehicle (in the other direction). The reason we don't see the latter two (propellers or jets) being used in sand is b'cos the adhesion, weights, and variances are different than water.

hands without shadows

I think Access has the right idea. Stop think of friction and start thinking fluid dynamics. The more you throw, the faster you go.

RustlerTraxxas

Not putting it to the floor may be better in theory, but in real life my electric rustler cogs or doesn't reach the power-band if I do not first break the inertia with one or a few quick 1/4-1/2 throttle pulls in grass or dirt. The same inertia is there on asphalt, but it doesn't need to be on the power-band to accelerate on asphalt. In my opinion on-road racing is the same as off-road. You accelerate just enough to get up to speed with minimal traction loss. Staying on the power-band is more important than controlling wheel-spin if you can keep the car in a straight line. These are just my opinions, none am I claiming to be facts.