Weight placement is a major factor, since it changes how the car reacts to weight shifting.
A car that weighs the same but has more weight in front will have less entry steering, the opposite of what many expect. This is due to the weight in the rear having less momentum or influence when the cars chassis unloads forward, or sideways while moving forward. Basically less pendulum effect.
The opposite happens under acceleration, the chassis can't bring the front up as much to transfer weight back, so on power, or exit steering is better with a front heavy car at the expense of rear traction.
It's always about a compromise of the two, regardless of where your motor is located. BUT, a mid motor car handles like a slightly shorter wheel base would since the weight isn't spread as far and remains inside the wheel base, which equals less inertia(pendulum effect) Great for some conditions.

There really is no right or wrong, no magic, just what works for the conditions, the drivers ability, the experience with and tuning of the vehicle.

Springs are used to control weight. But they also control feel, with stiffer springs feeling like there is less time to react to track irregularities. Softer springs make the car feel lazier or easier to drive, but can also be too soft. For instance if your left front tire comes off the track in sweeper turns, the rear springs are too soft. If your car is unloading the inside rear tires in sweepers, or tight 180's as well, the front springs are too soft.

Shock oil controls the speed the chassis, or suspension can move the spring. A lighter oil car recovers faster because it transfers weight faster, but it can bottom out easier and be hard to drive on high bite tracks since it will feel darty in those conditions.
Even rough tracks can sometimes benefit from heavier oil/shock valving since sometimes it's faster going over bumps then it is keeping the tires on the surface at all times.

Still always about compromise.

Over the 30+ years I've raced I find that the people racing several hours and every weekend at major events are the ones who have the most time in testing and sort most if not all of this out for us...Yet we still search for that magic trick to gain an edge on our racing buddies.

It is important though to have a good grasp on why we do what we do and the trade offs for each.

Happy tuning!

So I finally finished reading this about a week ago and I ordered a set of ft spring for my re re worlds car. It wasnt terrible setup with how the manual said, but when I balanced the springs and then I put #1 pistons in both ends and 30f and 35r and it is way better. It still needs more pack though so I picked up some losi pistons and I am going to compare to associated pistons and see how they match up. Thanks fredswain for giving us the knowledge to the how and why of setup. I have been searching for something like this everytime I tried to mess with setup. I just never knew where to start other that to try to counter react to something that I didnt like how my kit was doing. This method gives a good handle on what is going on and that knowledge will let me tune my kit with knowledge rather than just chasing the wind.

Tim

I love all this trying to use full size car theory on a model car, it works to a point.

On full size rally cars I was always work from the stiffest and lowest setup that you think will work for the conditions. That was from David Lampworth at Prodrive in the UK. They always would start a test with their most aggressive set up.

Most of the time trying to work figures out and theories is a waste of time. You generally know if you change x, y and z it has a certain effect and you are best starting with a locals setup or use the kit set up and start from there and change one thing at a time (try and get some practice days rather than race days to do this). If you have a plan of what you want to do, you can get through a full RC car in one days testing.

Its very easy to tune an RC car, not so easy on a rally car based on a production car. If I wanted to change the ackerman on my rally car we had to use the gas welding torch and a big hammer and change the rack arms. To get more rear camber we got the gas out again, drew straws and the looser had to heat the back axle tube up with a plate guarding a full fuel tank, while the lucky bloke had to use a scaffold on a hub puller to lift the car up and gain neg camber.

On a full size car we tended to reduce ackerman to reduce front inner tyre scrub on fast corners. Road cars are made so someone can park them at slow speed, so nearly always have a too tighter inner wheel radius. The only time you would want scrub was if the corners were very slippery and tight. Now if you take one wheel off the front of your RC car and run it on a flat surface you will see it turns a lot with the inside wheel, do that with a full size car and it will go straight on. RC cars use massive amounts of ackerman and inside wheel scrub, you will also see the chassis jack due to the wheel offset and castor by a massive amount too, all very unlike real cars (we ran about 3-8 deg castor and then it would rip your arms off).

Hideeho
Comparisons of actually setup between rc & 1:1 are completely useless because the differences in weight, wheel base, track width, & hp/torque are unbelievably huge. Back when the raptor bodies were 1st hitting the market for sct's I did some comparisons between a scaled up 1/10 sct & a real raptor. The sct overall length & wheel base are very similar (less than an inch off @ 1:1 scale), but the width has a HUGE difference. The sct is 35 1/4" wider (3 feet)! The wheels are even more ridiculous at 30" tall. Weight is even more incomparable. 4wd sct's weight 7-8 lbs, that scales to 70-80 lbs IN A FULL SIZE TRUCK! That would be compared to nearly 5000 lbs of an actual raptor. I haven't seen any actual figures for a long time, but iirc, a low turn motor is putting out .6 to 1 hp & 8-10lbs/ft torque. Scaled up, that is 10hp & 100lbs/ft at 1:1. The 1:1 raptor has 10lbs/hp & 10lbs/lbs/ft compared to 7lbs/hp & .7lbs/lbs/ft.

Physics works the same regardless of how much weight is involved. Because of that, the method to reach a setup should be the same at any scale. Because of the drastic differences in size & weight, the actual setup that works will be drastically different.

The physics and concepts always apply, regardless of scale. I'm more amused by the people that say they don't. You do however need to know how certain things are affected by scale. Weight and aerodynamics are good examples. A molecule of air is always the same size. A molecule of air is larger in proportion to a smaller object than it is to a larger object meaning a full sized airplane wing will have far more molecules of air travelling across of it as a model a fraction of it's size will. You just have to be aware of this. An rc car travelling at 50 mph is not the same from an air resistance standpoint as a full sized vehicle travelling at 500 mph. 50 mph is 50 mph. The key is to understand that you've got something a foot and a half long pushing through the wind at 50 mph as opposed to something 15 feet long pushing through the air at 50 mph. The concepts still apply but some of the variables have changed. Failing to understand that doesn't mean that they don't apply. Incidentally if you want to get a scale representation of weight, it isn't a linear relationship. Not even close.

Newton never said "F=MA, unless it' 1:10th scale."

If your inside rear tire is coming off the ground in a turn is a roll center adjustmeant (more roll stiffness) appropriate to get the tire to stay on the ground or is it just springs at the opposite end?

If your springs are balanced and you have a wheel coming off the ground in a corner, do not change the springs at one end! It could be a roll center issue but probably only to a point. It could be a chassis flex issue too. Having a wheel come up isn't necessarily a bad thing. It might be though.

There are lots of options, which is what makes it confusing, none are right or wrong, as long as you get the balance you like and lap times are on pace.
However...If the rear tire is lifting and your running low rear roll center already, then it has to be springs. Since low RC is the most chassis roll (least wheel lift potential)
Keep in mind all the roll center changes in the world won't help overcome the incorrect springs, since roll center changes effect the leverage the chassis has on the shocks.
Ie; remove your shocks and roll the chassis by hand, change roll center and repeat. You'll feel no "stiffness" change, but you'll get to see how camber curve and roll center go hand in hand.
The more the camber changes, the less leverage the chassis has on the shocks to compress them under load. High RC. Ideal with lighter springs in bumpy conditions. And vice versa.

What I would do is consider the feel I like, where it happens on the track and the available traction. Also, the springs I'm running at the time.
If I like the feel or balance of the car I would try one spring up on the opposite end first, since it's an easy change.

I've had great luck using roll centers and camber link & shock locations similar to what the factory guys are running in similar conditions and tuning springs and oil for the feel and weight transfer speed I want, as well as static camber and ride height.

.....
I remember seeing Matt Francis at the nats in Dalton on two wheels in the sweeper at speed. BOTH inside tires off the track, lap after lap! That's good balance! lol

Like you say the physics works the same, but the model is operating in very different parameters. The inside wheel and Ackerman is always one big difference that stands out to me.

The theory about go stiff and low and aggressive on the diffs first then work towards soft and high etc should work. The theory I believe is that you get a quicker car working that way round as opposed to getting too comfortable with a car on the soft side of the usable spectrum.

One big difference is that it doesn't hurt when you crash a model car and you don't spend months getting the thing rebuilt to crash again.

Separate issue now. Our track being run dry and blue grooved. My car is traction rolling everywhere. I can't push it at all... unless it's going straight.

I stiffened the shock oil incrementally up to the thickest oil the hobby shop had, 60wt Losi. ...and the smallest hole pistons I had (1.3x8 down to 1.2x8), cut some droop, lowered the car and went to a stiffer front sway bar (2.3mm to 2.5mm) and heavier springs. With these changes the car was massively better, but it's still not enough.

I'm not sure if raising or lowering roll center would help more or if longer links would help. I'm trying to apply the statement of how a 'low roll center is the most chassis roll (least wheel lift potential)' to my new issue, or if it does. Would more chassis roll make the car more likely to roll over? And also start lifting wheels off the ground since the chassis is rolling over?

Higher roll center to keep the car flatter. Lower on the inside/higher on the outside.

According to RC Crew Chief raisong the RC is what you get by lowering the inside, but it will keep the car flatter.

Yeah, I've seen discrepancies in tuning guides but my experience is lower inside link keeps it flatter and higher promotes more roll. How that affects your traction is another thing. Sometimes less roll is more traction or more roll is more traction. All depends on the track surface and what your trying to accomplish.