Off Road RC Vehicle Dynamics - popular opinions or actual facts?
 

Hi guys, while I'm sure this information has been covered somewhere, or even in multiple individual posts throughout this huge forum I wanted to start a thread discussing off road rd vehicle dynamics and comparing theory with practical observations. For example some areas I am struggling to find correlation between vehicle dynamics theory and what people are posting of there actual experiences are:

1. Adding mass forward in a buggy increases the steering and grip both at corner entry and grip mid corner at the front. This doesn't make sense to me and in fact goes totally against my theoretical understanding. Adding mass to the front should reduce grip at the front (2WD) and create more understeer. Am I wrong?

2. Ditto for the rear. Adding mass increases grip both during acceleration (agreed) and mid corner ( again I undrstand this should decrease grip mid-corner and create a more tail happy oversteering car) ?

3. A heavier car will have more grip than a lighter one. Again totally in contradiction to my theoretical understanding. The more load you place on a tyre the lower it's coefficient of grip and the less corner speed you will be able to hold given that lateral acceleration builds linearly with the total mass of the vehicle. ? I could maybe see more tyre load increasing grip on a very loose low grip track if it meant that the tyre pins pushed through the loose top layer but then the issue is with tyre design / selection not the vehicles mass... On-road vehicles certainly wouldn't gain more grip by increasing mass all else being equal.


I'm very new to rc cars so forgive my ignorance. Also I enjoy thinking about the theory so if u are the type of person that just knows something works and doesn't care why don't let this thread frustrate you. Thanks guys.

Another one I keep reading:

4. Changing spring rates effects weight transfer during acceleration / braking. My understanding is that aside from a tiny amount of additional weight transfer due to pitching of the body more/less changing the CoG slightly, the spring rates shouldn't change weight transfer at all. The moment caused by acceleration, mass and CoG height will be the same regardless of spring rate and thus will create the same opposing forces at the tires. The rate may change but the total amount shouldn't. Right or wrong?

A lot of what you say is really relative to other factors. For example, track surface. A really grippy clay, compared to a very loose track with a layer of sand on top, will have enormous consequences, especially in corners. For example, weight can help on a loose track as it lets the tyres dig in deeper (more weight pushing them into the ground)

To make it easy, just think of a real car...
If weight over the axle would reduce grip on real car - then a FWD car could accelerate faster than RWD car with the same amount of power.
But with accelerating, the weight shifts to the rear end of the car (with a softer suspension a little more than with a stiff one) which causes a FWD to spin the wheels and a RWD to have more grip.

This is of course without differentials, anti squat etc.

A FWD on the other hand can get a lot of oversteer, when braking hard... since the weight shifts to the front and the back gets light (engine and drivetrain in front). Onpower it normally (without special diffs etc.) tends to understeer a lot with the front getting light and the wheels fighting with forward grip and lateral force.

Btw. I think it's not really about adding mass - more changing the balance of the car normally. (Or maybe adding mass to make it go smother over a track.)

I understand that weight over the front or rear will increase the total amount of grip on that axle (coefficient of grip x normal force) and thus increase the amount of traction available for braking / acceleration. I also understand the concept of weight transfer (I was just saying I don't see the spring rate making any real difference to the amount of weight transfer - just the rate). What I was trying to say is that I don't believe that adding mass to the front or rear should increase lateral grip mid-corner, or even corner entry, as the increase in total available grip due to the increase in normal force on the tyres will be proportionally less than the increase in lateral force on the tires due to the increased mass. This is because the coefficient of friction of the tires decreases as the normal force increases (at least in on-road tires / cars) while the lateral acceleration increases linearly; so the heavier the car the less maximum lateral acceleration it can develop with the same tires.... if that makes sense? Unless I understand it wrong. I just keep reading people saying that they need more cornering grip on the front or rear and so they are adding mass or changing the distribution to increase the percentage of mass over the axle they are trying to get more cornering grip on - I understand that this should actually have the opposite effect.

Hi Zylux, good question. There is a lot of mis-information out there, but also some things dont translate 100% from full-size to RC cars. The theory is generally still applicable, but the relative importance of some factors is not always the same (ie what may be a secondary effect in one becomes a dominant effect in scale).
Ive been fortunate enough to work in vehicle dynamics on full-size motorsport and road vehicles, as well as having some success in RC racing, so Ive offered my experience below.

1. Adding mass forward in a buggy increases the steering and grip both at corner entry and grip mid corner at the front.
Generally increasing mass at the front increases exit steering, and makes the car more stable on entry/mid. (In some cases, if the front is too light the tyres dont dig in to a soft surface correctly, but on harder surfaces this isnt usually the case).

2. Ditto for the rear. Adding mass increases grip both during acceleration (agreed) and mid corner ( again I undrstand this should decrease grip mid-corner and create a more tail happy oversteering car) ?
Adding mass will increase forward traction on exit, but usually helps with rotation mid corner and entry (ie reduces lateral traction). It does let you brake harder into a corner though.

This is a short piece I wrote on weight transfer a few years back:
http://keilorrc.org.au/reports/Techn...ion_report.htm

3. A heavier car will have more grip than a lighter one. Again totally in contradiction to my theoretical understanding.
This depends a lot on the surface type, tyre compound and also surface roughness.

Surface type: A hard surface behaves as you mentioned. Lighter mass gives more relative grip, and the compound has a huge effect on traction. Soft surfaces can behave differently though, as the mechanism of tyre traction generation is different. Sometimes you need more weight to dig into the surface as this is creating more grip than the compound/surface interface.

Tyre compound: If the compound is relatively soft, less weight will make the tyre more efficient. However in some circumstances (eg a control tyre that is relatively hard, or a cold day) more weight will generate more heat in the tyre which can be more effective at generating grip.

Surface roughness: On a smooth track, less weight is generally better. However as tracks get rougher, a light car will be more unsettled by the bumps and hence lose traction. A heavier sprung mass will help tyre contact patch variation.

Ease of driving: One of the other issues with a light car is that they can feel too lively and are edgy to drive. Heavier cars tend to feel more stable and natural, which for many drivers (even good ones) translates to faster race times. With EP cars, the main weight reduction came from lighter batteries. Unfortunately batteries were at the centre of the car, so while the mass was reduced, it was relatively spread further away from the cg (increased radius of gyration). I believe that this is one of the reasons they didnt feel as natural to drive.

4. Changing spring rates effects weight transfer during acceleration / braking.
This is incorrectly referred to in many full-size and RC texts. As you correctly point out, it mainly affects the 'rate' of weight transfer, not the final amount. Tyres can be very sensitive to the rate of weight transfer change (full-size and RC) so it is still important, just not described correctly. Usually on lower grip surfaces you want to speed up the initial weight transfer (with higher roll centres) then slow down the final weight transfer (softer springs). The large body roll of soft springs is confused with large weight transfer.

I notice you are from Melbourne. Im also from Melbourne, and run a help thread in the Aussie forums you may be interested in:
http://www.rctech.net/forum/australi...rt-thread.html

Would be good to meet up at the track one day. Which track are you racing at?

Regards

Ray

Thanks a lot Ray, you have confirmed my understanding. It looks like one of the primary difference between road / off-road is that on a softer surface sometimes more weight can increase coefficient of grip by pushing through the loose top surface - but again, surely a better way would be to use (or design if no-one makes one) a tire with thinner pins that push through the soft top layer ( less force required due to less area creating the same or greater pressure). The control tire and heat generation makes sense.

I actually haven't started racing yet having only put together a buggy a couple of weeks ago. I've only been to the track four times so far lol. I'm going to join the Keilor club and plan to race there. Hoping to get down there tomorrow afternoon to practice a bit actually. I think I saw some of your results on their website. Very impressive!! My ability is laughable by comparison.

I'm running a Kyosho RB6 in Mid Motor. Have tried to reduce mass and get it centralised (to reduce inertia), and particularly focus on reducing un-sprung mass so that there is enough sprung mass to allow the suspension to do it's job and control the wheels / tires (as you discussed) though there seems to be limited scope given the relatively large mass of the wheels and tires. I think the car handles well, although I obviously have almost zero experience at the moment.

Maybe see you down at Keilor one day! My name is Dom. Zylux was the brand of a central locking kit I bought years ago and threaten my girlfriend with as the name of our first born if we ever have a child lol.

1st one Adding weight will give you more grip but not necessarily more steering... steering is done with tire contact and angle... it's like this a skinny tire gives you more corner grip/wide tire gives you more steering... the reason a skinny tire give you more grip it put more weight in less total area. so if you have a lite car you need a skinny tire to make up the grip if you have a heavy car you need a wide tire to make up the steering/push. so yea a 2wd buggy has skinny tires in front so it does not need more wight and at speed would push more (you have more weight to moving in the opposite way you want to go).

2nd like 1st more weight adds grip but it will push more not slide. just like 1st rear oversteering is to do with tire contact and angle. yea on both you need some weight to get the proper contact (if the tire not on the ground it will not work)

3rd yes weight alway add grip this is why two car both with 1000hp one has weigh 2000lbs it will spin tires easy vs 10tons would have a hard time to spin tires.

think like a F1 with out a wing it would slide off the track... weight is like downforce the more you put the less it slides. but unlike downforce that only pushes down/ weight pushes to the side as well so the more weight the less less likely you are to slide but more likely you are to push. so there is a fine line what weight is best for each car/set-up and track type.

is this what you was looking for?

One thing I have noticed about the mountains of information on the internet is that you need to see if it's being written for on-road or off-road racing.

On-road racing cars rely much more on the chemical reaction of the tyre and the tarmac/carpet to generate it's grip e.g. the heat and friction sticking the tyre to the surface.

Off-road racing cars rely much more on the physical structure/design of the tyre's impact on the dirt/carpet to generate it's grip e.g. the pins/bars on the tyre digging into the surface to push against it.

These two very different requirements need different set-up techniques and vehicle dynamic parameters to get them to work effectively, so what works for touring cars won't translate to buggies for a lot of things.

That's why I've learnt it's important to see what application the advice is being given on.

Have fun picking through it all, though, it can be fun!

Thanks mate, but I'm not sure I understand or agree with all of your points. I think you misunderstood question 3 about more mass / heavier car. I understand that more mass on the rear = more grip (coefficent of friction x normal force) and that this allows you to put more power through the rear wheels. However the % increase in grip will be outweighed or at least matched by the % increase in power required to accelerate the heavier mass (assuming weight is added not redistributed) F=m.a, P = F.V so you won't actually accelerate any faster (slower if you assume a declining efficiency in tire grip with increased normal force).

I'm also not sure I understand your explanation of 1 & 2 in terms of push and slide. I assume you are talking about tire cornering stiffness; in that a wider tire will generally have a higher cornering stiffness and therefore a lower slip angle for a given lateral force and that you can therefore attain a higher maximum lateral force before exceeding peak non-slide slip angle? Can you explain what you mean by slide and push a little more clearly?

Also, Ray. Just had a look at your link. Looks like a lot of good info! Will spend some time going through the thread. Thanks again.

RC car tuning is mostly popular opinion and very little fact. If you've got a theoretical knowledge of vehicle dynamics then it's no surprise what people do and say in RC doesn't make sense to you. A lot of what you hear is either wrong or wild guesses when people talk about tuning.

If you want a very good read on how to set up a car from a guy with lots of knowledge on vehicle dynamics, check out the thread "Tune with Camber Links" and read FredSwain's posts. There's a lot in there, but I think you might enjoy the ideas presented within that thread and the theories discussed.

Lol. Thanks mate, I'll check out the thread. 👍

yes more weight is slower if to much and faster if just right. my Ex. ways extreme to give a better understanding. the point is you need the right weight.

#2 yes push and slide are the same///(it terms in racing tight and loose witch are the same as push and slide)it is hard to tell if your front is to tight(push) or to loose(slide sliding) they both make you run high(or hit the wall) in the turn the same way(it's is much easier to tell what the rear doing)... but when something to heavy is being push to the out side of the turn(kinda like brakes work on heavy vs lite cars) VS. if something is to lite it slide to the out side of the trun (kinda like a race car that lost his wing)... that why to stop a push you put wider tires and to stop a slide you put skinny tires. this is why on a 2wd buggy you need a wider tire to make it turn (not weight)(it has crazy skinny tires by adding weight(most of the time) will make it push more) this is why buggy front is easy to tune vs like a 2wd truck where the front has a higher chance to slide (where adding weight to the front will help you most of the time but not all the time). i'am not saying weight in the front of a 2wd buggy wont help but it a very low % of the time.

does this help?