Just thought I would poke in here on the discussion of tires, weight and traction.

A lot of the literature that is being quoted is from books discussing road going cars (full size even). Now, the general principal that tires are 'load-sensitive' is true, but I think RC cars definitely behave in a more odd manner.

However, the analogy I like to use when explaining how weight can affect lateral grip is using a mass on the end of a thin string. The string being the tire and the mass being spun around is like the mass of the chassis. Now, for a given tire, it can only do so much work (even as load continues to increase) and this is the limit for when the tire begins to slide and induces understeer or oversteer (depending on many other parameters). Likewise, the string can only do so much work before it breaks (similar to breaking traction) as the mass is being spun around faster and faster. The lateral g's that the mass is pulling is analogous to a car going around a corner. So, if the string breaks at a particular rpm for the mass then if you decreased the mass you would be able to then spin the mass faster before the string breaks. Implying in the same manner that a lighter car is able to corner faster because it requires more lateral acceleration for a lesser mass to break traction.

No, this a good theoretical example that is better at describing touring cars (both full scale and R/C), but since off-road surfaces are typically more inconsistent other variables.

Hope that helps give another perspective on how to think about tires!

I would say a lighter chassis would have little to no effect on the shocks recovery from a bump if the shock is "Precisely" re-valved and re-sprung for the weight. Extreme weight changes could not be properly overcome though.

The problem area would be that the weight under the shock(unsprung) hadn't changed to effect the lighter packages recovery from an impact. (as has been stated)

If you changed the lighter chassis's shock, as well as the unsprung weight equally then the only remaining variable would be the shocks O-ring/piston fit friction and pivot points friction to overcome the lighter package.


I think this is why a lightened chassis can cause handling issues, many of these variables can't be overcome with BIG weight reduction.

But $125 screw sets are still cool for those with plenty of disposable income.

The only RC10 based car I built during the "Golden Age" that I would want back today was a superlightweight special that I built when I was living in Chicago and stock class had at least 4-6 heats and we were racing 3 days a week. I needed something special so... Composite Craft foam core carbon fiber chassis with the top layer and the foam dremeled out of 1/3rd of the top of the chassis. Anything that could be cut or dremeled was. The 3 screw front shock tower mounts ? Cut to 2 holes with the corresponding plastic gone, sway bar mount ? way gone. The battery cup was reduced to two posts. The arms had material removed. The rear bulkhead was like swiss cheese. The trannie brace was a carbon X. Every piece was modified. Even the servo mounts were cut down in half and used one screw per side. All ti and alu hardware. Those super small esc's and rx's that Kevin (Tekin) had. The only extra weight was that red kyosho motor plate for cooling.
The thing was ridiculously fast but also ridiculously difficult to tune. But when it was on, it was untouchable, really was. Made me look a lot better than I am. Kyosho gold shocks of course and using homemade flapper valves like todays MIP pistons. Man what a car.
Certainly the more knowledge of all types of suspensions and shocks and race cars you can get is a good thing but full size theory sometimes does not apply simply because the proportion of scale does not correlate as the scale changes. (like the strength of an ant is so high compared to his size) So take the books with a grain of salt.

I am a fan of light weight but the standard pistons don't let the tires move fast enuf PAC changes all the full scall stuff what is hard to deside for me is high PAC and low rebound and light springs or no PAC hard rebound and stronger springs and bump stops
The frist is vary agresive the second is calm and hard to up set the car

I think the resin for emaltion shocks is because we have PAC it helps with bump resistants

I have heard that analogy many times, and never agreed with it because it is too simplified.

A lightened F1 car will corner better than a 5000lb SUV, but not just because it has less mass...

That is not what the analogy implies. The example you just mentioned is referring to cars of completely different suspension geometry, aerodynamics and I'm sure many other things. Essentially comparing apples and oranges.

A more appropriate way of thinking about it would be to say a lighter and heavier SUV of the same make and model and both with the same tires or a lighter and heavier F1 car with the same suspension geometry, suspension setup, areo and tires. In these cases the car that weighs less will be able to corner faster because the tires still can only do the same amount of work but since the car weighs less it can pull more lateral g's until the force on the tire reaches its limit. This would also assume that the CG height is still in the same location, since you could remover weight from down low and effectively raise the CG of the car. Also granted, that the lighter car would be even better if the springs and dampers were adjusted for a lighter weight.

so i read the tire characteristics chapter in this chassis engineering book http://books.google.de/books?id=rY2u...eering&f=false

and in the summary it says the car has the best cornering performance when all tires are equally loaded, but when i try it on the track, my car has a lot more steering when I move more weight to the front tires.

The analogy implies that a thing with greater mass will break the string before a thing with lesser mass. Simple physics will agree.

But cars are not that simple. You proved my point in your second paragraph.

Of course it does. When you corner the tires are not equally loaded. In fact, any change in motion changes the tire load.

Placing more weight in the front will increase steering because you are increasing the load at that point.

Okay, so when the steering gets better when i place more weight in the front, i must assume there was less than 50% tire load in the front, because the steering would've to get worse if the vertical load of the front tires was already bigger than 50% of the cars overall vertical load. (cause you are moving away from the 50/50 optimum). Therefore i guess more weight in the front of the car only aids steering out of the corner because the weight shifts to the rear and you have less than 50% vertical load on the front tires. (going in to the corner you would've more than 50% front load because of the weight shift caused by braking)

Assuming does little to provide an actual usable answer.

Increasing the front bias over the rear does not automatically make steering worse. My sc10 was biased to the rear but still prone to hooking the corner entry and spinning out mid-corner. Eventually I had to add weight to the back (increasing rear weight bias more than it already was) to cure this.

After re-balancing the spring rate to accommodate the added weight, it is a whole new truck. Why? More overall weight on the rear tires (increasing traction) and more weight transfer to the outside tire in the corner.

well, IF the book is right and you tune every other aspect of the car to perfectly suit the change in weight bias (like increasing front spring rate accordingly to the added weight) and the car was 50/50 balanced in the situation you are looking at, going away from the ideal 50/50 would result in worse cornering performance/lateral g-force.

My question was: If that is true, why do i get more steering when adding weight to the front? it could've several different reasons like less than 50% vertical load on the front tires due to weight transfer, or more tire contact patch because the shocks (which weren't perfectly set up for the previous weight) suit the new weight bias better and thus work more effectively.

As with everything there is a limit. You can't have no weight but at the same time you can have too much. If you had 80% of your weight on the front wheels you would not have an awesome cornering machine. You'd have an understeering pig of a vehicle.

Adding weight to one end puts more pressure on the tires on that end. Spring/shock tuning does not change that weight, it simply allows the chassis to deal with it more effectively.

Remember that a 50/50 static balance goes away when you start moving. When you take a corner, your balance is thrown off and becomes biased to the outside and rear. You can counteract that a bit by slowing down or tapping the brakes which will shift some of that bias back to the front, but a majority of the weight is still being placed on the outside tires.

Adding weight to the front changes your cornering balance by keeping the front bias up and gives you more effective grip. You now have more pressure on both front tires, even though it is still not equal.

Keep in mind that everything is relative and there are a lot of variables involved. What works for one vehicle does not work equally as well for another. That perfect 50/50 balance really only works in theory and in a vacuum. To achieve equal tire load at all times you would have to have a very complex and active suspension setup.

if the shock was set up too hard, adding weight to that end would result in more tire contact patch because the shock suits the "new weight" better.

idd that's what i was trying to say. In a braking situation when weight is shifted forwards, adding more weight to the front would result in less lateral g-force. But when accelerating out of a corner, adding weight to the front will help to reach a 50/50 balance and thus have provide more lateral g-force