Automotive physics/theory: what is the best vehicle ride height for handling (on adjustable coilovers)?

General Discussion
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First let’s make some assumptions, we assume that our sedan is a popular mass market sedan with front Macpherson struts and rear torsion beam. The ground clearance from the factory is about 6 inches. We will also equip this sedan with height adjustable coilovers without any other type of adjustment. Thus we can lower the vehicle up to 3 inches. The factory setup is a good balance between handling, safety, and practicality. If we want to compromise practicality for better handling how low should we go?

We will assume that alignment, rubbing, and other issues do not come into play at all.

We know that the center of gravity will be lowered and this is good but there is also the issue of ‘roll center’ which becomes more likely to roll, this is bad. The angle upon which the lower control arm mounts from the wheel to the subframe body changes as the car is lowered and apparently this change causes the car to be more likely to roll. There has to be a sweet spot. If you have toyed around with coilovers before, what has been your experience?

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Lowering the roll center is good, reduces the amount of roll. I might go one inch, I wouldn’t want to compromise the suspension over large bumps any more than that. But if you’re adding coil overs, you also have the option of stiffer springs, which would help prevent bottoming.

What are you doing as far as tires/wheels? Bigger handling gains there, I’d think.

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So, the automotive theory is that very often lowering the vehicle height also lowers the ‘roll center’ is bad = makes the car more likely to roll.

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Retired suspension design engineer here… There is so much more to this that you have stated here. CG height is but one very limited issue as is ground clearance.

If the spring rates on the coil-overs are the same as factory, lowering the car will cause it to bottom excessively. Typically, spring rates are quite a bit higher with coil-over kits and lowering springs for exactly this reason. The softer the springs are, the more ride travel need be built into the car so the higher the ride height needs to be. Stiffer springs and more damping can reduce the need for that ride travel but ride quality will suffer.

You CANNOT assume this as it is a HUGE factor in the ride height, especially with MacPherson struts.

Lowering a strut suspension forces the tire to positive camber statically and that must be corrected with camber plates or crash-bolts. Lowering the suspension also forces positive camber as the car enters a turn - more than the strut would do at the designed ride height. This steals traction away from that tire. If the front uses struts, this promotes more understeer. Lowering the suspension also drops the roll center which makes the car roll more - the distance from the roll center to the CG at that end that adds even more understeer.

So now we look at the trailing beam axle… lowing the ride height increases the roll understeer because the trailing beam is now lower than the front bushings… so more understeer. Brake anti-lift is reduced so the car will pitch more under braking, offset a bit by the stiffer spring rates.

The drop in CG is a benefit but generally outweighed by lots of negatives that must be addressed.

Basically, if you are doing this for handling, you need to change a number of other things as well. Most people are doing this for Street Cred and so they can lower their cars… usually a LOT… And lots of folks buy the cheap Chinese coil-overs off Ebay (Maxspeedingrods) that have little to no engineering done on them at all. They kind-of fit but nothing else really works very well.

Do you have any further questions? This subject is VERY car specific, not just suspension design specific so ask away.

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Let’s say the car is a 2012 Mazda2 which has very loosy goosy handling from the factory. The issue of roll center is foremost in my mind. I can choose lowering springs (with performance dampers) that lower the car between 1.1 inch and 2.4 inches. Based on my reading, the milder the drop the better because center of gravity benefits are cancelled by the roll center deficit, so to speak (I’m not using the correct language here.)

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You are incorrect.

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He is correct. Lower roll center makes the car roll more, not less. The car’s roll is determined by the distance from the roll center to the center of gravity at that end of the car and the applied lateral acceleration.

This is a decent explanation;

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Not just the change in roll center height, but all the other negatives I mentioned above. You can easily lower a strut car far too much… most Mustang lowering springs!

A partial solution is to install ball joints with a longer stud to drop the control arm down a bit to correct the geometry. That can mess up the steering geometry so that bump steer becomes a problem. They sell bump steer kits for Mustangs, too. Not sure if either ball joint kits or bump steer kits are available for a Mazda 2.

Stiffer, lower springs and better dampers will go a long way to settling this car down as long as you don’t go crazy low.

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OK, I understand a bit. But it’s a tradeoff, correct? Jacking up a car makes it roll more, typically, right? So some amount of lowering helps, too much hurts?

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Yes, it is a tradeoff.

Back when most rwd cars had solid axles, lowering the roll center actually helped handling because it reduced understeer. The old cars felt like tricycles that wanted to tip over in turns and destroy the outside front tires. Stick a panhard bar on a 4 link or leaf spring car to force the roll center lower (66 Shelby Mustangs) and handling improved.

Trade offs abound!

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All right, so would you consider that the ideal handling situation is one where there is no lowering at all but the car kept at stock height but outfitted with stiffer springs and appropriate dampers and perhaps with upgraded anti-sway bars? So no lowering in the ride height at all basically.

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I think you are looking at this all wrong. The first thing to consider is what are your objectives and what are you willing to sacrifice to achieve them.

Track racing?
Better street handling? Summer only? All Season?
Comfortable ride?
Fuel economy?
Cool looks?
Other?

Rank these on order of importance to you.

The next step is to select tires that are most compatible with those objectives. When selecting tires, also consider whether you will change wheels to a larger diameter, greater offset, wider rim width, cooler looks.

The tire/wheel combination determines your next step for the suspension. When you go into a corner, the wheel, which is attached to the vehicle, pushes the tire to the outside of the turn. The tread of the tire, which is in contact with the pavement, pushes the tire to the inside of the turn. This causes the tire to “roll under” and lift the inside tread off the road, or at least reduce the pressure on the tread while increasing the downward pressure on the outside of the tread.

A good suspension will make the camber of the wheel go more negative on the outside wheel in the turn and more positive on the inside wheel just the exact amount to offset the tires “roll under” so the downward pressure on the tread is even all the way across the tread.

Two things do this. One is the castor of the wheel. This is the tilt in the vertical axis of the wheel. The more positive the castor, the more the camber will go negative on the outside wheel and positive on the inside wheel.

The other thing is body roll. As the body rolls, the outside control arm(s) move upward, making the castor go negative. The inside control arm(s) move downward pushing the camber more positive. So body roll by itself is not necessarily a bad thing. Too much change in the caster can cause the outside of the tire in the turn to lift. You have to balance the roll and caster to the tire.

The factory has done all this for you for the OEM tires. If you go with lower profile tires on larger rims and more offset on the rims, you need to reduce the caster change, usually through stiffer sway bars, higher spring and shock rates etc.

The rear is a different story. Unless you have 4 wheel steering, there is no caster. If you have independent rear suspension, the camber can change the same as the front, but because there is no change due to caster, wider tires are often used on the rear to compensate.

If the axle is solid back there, then usually the sway bars are smaller so that the tires can stay more vertical as the body of the vehicle rolls. The rear is more independent of the chassis. Also wider tires help too to balance between understeer and oversteer.

So everything is about your goals and finding the best balance to achieve them.

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There really is no ideal handling situation, it depends on what you want from the car. If you just want less float, install sportier dampers, done. If you want a bit better handling, after the dampers, buy springs that replace the factory ones. They will be stiffer and the car will be lower but the springs will be engineered for the car by Eibach or H&R or whomever makes the springs. Stablizer bars are the next step. I would not go farther for a street driven car with the suspension.

Better tires are the next handling step. Wider, lower profile, softer, same diameter so that means bigger wheels.

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For less float, it’s really the dampers that make the difference? Not the springs?

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Both contribute but the dampers are the first purchase. Stiff springs with soft dampers ride very poorly as they won’t have enough damping to control the bounce from the higher spring rates.

Firmer dampers with the stock springs will take out the float and control the motions.

If you then want quicker response, flatter cornering and less pitch, change springs.

If you want to just reduce roll in corner change just the stab bars. Stock springs with bigger stab bars will keep a decent ride but help handling, unless the bars are already quite large. You can go too big here.

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Thank you, this was information I was looking for.

I’m going for the stiffer springs (stiffer by 1K, or 56lb./in.) first and then sports shocks if those dampers aren’t up to the task.

The Mazda2 has a torsion beam in the rear, do you recommend an aftermarket anti-sway bar for this type of rear design? The torsion beam is already resist twist, would it make a difference? It seems like it wouldn’t to be honest.

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The suspension mods have been addressed so I pose this.

It’s difficult for me to think that this car came out of the factory with what you describe as loosy goosy handling.

The car is 9 years old with an unspecified number of miles.Have you not wondered if this loosy goosy handling might be due to worn suspension and steering components? Buy it new or not?

Shocks and springs mean nothing if everything else is on shaky ground.

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There was a kit for the Mazda2
Mazda2 / B-Spec Kit

Kit, Coil-Over Shock (Bilstein)
Front & Rear Race Springs
Rear Sway Bar
Plate, Strut Bearing
Exhaust System. CAT-Back
Brake Line Set, Braided Stainless
Pad Set, Front (Hawk DTC-60)
Air Filter, Performance (TBA)
Kit, Oil Cooler
Kit, A/C Delete
Kit Price: Approximately $2600 (2012 price)

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+1 to this comment, I can’t up vote this enough. Throwing performance parts on a without fixing what is worn out is a bad path.

A reply to your last question… The twist beam axle acts as a stabilizer bar as you recognize. Adding a stabilizer bar just adds to that and makes it stiffer.

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You can also stiffen up the rear roll rate by replacing the stock rubber bushings with harder polyurethane bushings. In fact, you can stiffen up the whole suspension by using harder polyurethane bushings.

The ride will be a little harsher but you will retain all the stock suspension specs for using OEM replacement tires.

One thing that can lead to “loosy goosy” feel in the handling is the wheel alignment. A little too much toe in can make the steering a bit sluggish but the stability will be great. Change the toe to be a little more toe out will quicken the steering response, but too much tow out will make the car feel twitchy, that is, it will tend to wander and will require much more attention to the steering wheel in order to keep it between the lines.

FWD vehicles usually have a little toe out or no toe at all because as the bushings load up under power, the front wheel are pulled in. In a RWD vehicle, the front toe is a little in because as the front wheels are being pushed, the road resistance loads the bushings in the opposite direction causing the front tires to toe out.

RWD vehicles set up for quick response will also set the rear wheels for a tiny amount of toe out. This is done by shimming the hub in the rear. You can’t do this with a RWD vehicle with a solid axle.