Mechanics

SUSPENSION LOWERING

1992 Mazda b2200 • 2.2L 4 cylinder RWD Manual • 166,555 miles

What are some of the downsides to lowering my truck? I think it looks better, but I have heard that it destroys the performance?
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92mazb22
February 27, 2014.




You need to be an informed owner.

You will greatly increase tire wear, and decrease your stopping ability and handling. All vehicles are designed with very carefully set suspension height. Those numbers are published in all service manuals and in guides all alignment mechanics use. We get very picky about correcting ride height that's as little as an inch low because we know we can't do a proper job with your vehicle, AND we know not correcting it can land us in court.

First you have to understand what happens to your braking ability. With lowered suspension, you will have the illusion you're able to stop quicker because you're not being thrown forward as much as normal, but in a side-by-side comparison you'll see the lowered vehicle takes much longer to stop. Your brake's hydraulic system has a proportioning valve for the rear brakes that is fine tuned for your vehicle's weight distribution and weight transfer on braking. It reduces the rear wheels tendency to lock up under hard braking. A skidding tire has no traction and no steering control. The engineers knew how much weight would transfer onto the front wheels and they made the front brakes considerably larger to accommodate that. The calibration of the proportioning valve will be different for vehicles with different size engines, with or without some options like air conditioning, and anything else that results in a different percentage of weight on the front. They also know how much additional weight will transfer to the front during braking.

When you lower the suspension, you reduce the amount of weight that transfers during braking. That means the front brakes can lock up easier resulting in a loss of traction and steering control. In response, to stop the fastest, you have to avoid locking the wheels, and that means not pressing as hard as normal on the brake pedal. You have just increased the stopping distance beyond what was designed in by the manufacturer. Since the rear brakes are designed to avoid locking during hard braking, they have less braking power, but with lowered ride height they keep more of the vehicle's weight on the rear tires. The rear brakes need to do more of the braking now, but they're designed to do less, AND you're not pushing as hard on the brake pedal as you're supposed to be able to do. You don't get the weight transfer onto the brakes with the most stopping power so you can't use that higher stopping power.

Lower braking ability is easy to understand. Handling is not so easy. Again, you may have the illusion of better handling because the body will roll less in a corner, but you have to understand how the suspension geometry works. First of all, as the suspension goes up and down over bumps in the road, the wheels are designed to tip in and out on top a little. Failure to do that results in increased tire wear from the tires sliding left and right on the road surface as the control arms go through their arcs. To accomplish that is why older cars and trucks had the "short-arm / long-arm" (SLA) suspension systems. The upper control arms were shorter than the lower control arms, and they were angled down while the lower ones were perfectly parallel to the ground. When ride height changes, "camber" changes. That is how much the top of the wheel tips in or out on top and is one of the three main alignment angles. On most domestic cars other than Ford front-wheel-drive models camber can be readjusted to specs, but that setting only applies to a vehicle that's standing still on the alignment machine. Alignment computers don't correct for the wrong geometrical changes the suspension will travel through as the car goes down the road. We call that "aligning for the rack, not for the road". What that means is the computer is going to say the alignment is set for the best tire wear, but in practice you'll have terrible tire wear. Those alignment numbers are only accurate when the suspension is at the correct ride height, and it's why all alignment mechanics get so picky about correcting ride height before they start any alignment. Most newer alignment computers are able to measure ride height and tell you when it's wrong, but there's no way to make any adjustments that will overcome that.

"Scrub radius" is the even harder alignment angle to understand. It is a non-adjustable alignment angle that is designed in for proper handling. If you stand in front of your vehicle and look back at the suspension system, draw an imaginary line through the two steering pivots. That can be the upper and lower ball joints on large trucks and older cars, or the lower ball joint and the upper strut mount on cars with strut suspension. That line must intersect the road surface exactly in the middle of the tire tread. If you look down on the left tire, the half of the tread from that point to the left wants to "scrub" on the road and make that tire want to turn to the left. The half of the tread to the right of that point wants to scrub to the right, and the two forces offset each other to make that tire roll straight down the road.

Scrub radius is altered a little on front-wheel-drive cars with the split-diagonal brake system. They have the left front and right rear brakes on one hydraulic system, and the other two brakes on the other hydraulic system. Many years ago with heavy rear-wheel-drive cars, and still with most trucks, the front brakes were on one system and the rears were on their own system. Every once in a while we would run into a car with a ruptured front brake hose and the owner plugged it off so he could keep driving. What happened though was that one working front brake would tear the steering wheel out of your hand under just light brake pressure. There was no way you could stand to drive the car like that for very long. Now when we get to front-wheel-drive cars with the front brakes on two different hydraulic systems, a failure in one system would cause the same horrendous loss of steering control and people would be crashing all over the place, ... Except that the engineers redesigned scrub radius to take care of that. When the brake is applied, that tire wants to stick to the road, and that makes it pull back and turn out. If only the left front brake is working, that tire will make the car turn hard left. But, scrub radius has been changed to put the intersecting line out further. There's a bigger percentage of tire tread trying to pull to the right, and that offsets the brake pull to the left. Chrysler has that so well perfected that the only way to know there's a problem is by the "Brake" warning light, and the slightly lower brake pedal. On all other cars it works so well, all you might see is a little twitch in the steering wheel, if you look for it. Scrub radius offsets brake pull.

When there is no defect in the brake system, the forces acting on the left tire are also offset by the forces on the right tire. What changes though is when the forces change on just one tire, as when it hits a pothole or a bump in the road. Suppose the right edge of the left tire hits a small rock. For an instant that part of the tire has more weight on it and the steering linkage responds to that increased force. It is not offset equally by the left part of the tread or by the other tire. You'll get a twitch to the right in the steering wheel. You might overlook that one time and not even notice it, but we call that a "busy" steering wheel, meaning it's always dancing around and doing something other than staying straight and vibration-free as it was designed to do. That makes for a very tiring car to drive.

Alignment angles change too during cornering. One wheel moves up into the body and one drops down. The suspension geometry is designed to basically keep both tires flat on the road or to lean slightly into the corner. Too much leaning is just as bad as not enough because either condition reduces the amount of tread on the road surface. With less tread contact, there's less ability to prevent sliding in a corner. Ask any guy who sets up a NASCAR racecar. They aren't so concerned with the resulting poor tire wear on the straight stretches. They want those tires flat in the corners. To achieve that the trade-off is the less-than-ideal wear when going straight. You want just the opposite for your vehicle. You want the most tread contact when going straight so you can stop quickly and have a smooth, comfortable ride, but you want to minimize the loss of tread contact in the corners to reduce the chance of sliding. The best of both worlds can only be achieved at the ride height all those things were designed for as a complete system.

There are ways to alter the distance the body sits from the road without altering the suspension geometry, but that doesn't help the weight transfer issue when braking. Also, people usually don't stop there. They also install silly-looking huge wheels with low-profile tires. Anything that changes the outer circumference of the tire or where the center of the tread sits on the road surface changes scrub radius. That means you can mess up scrub radius just by installing wheels that are wider or have a deeper offset, even if you don't alter ride height. I have some muscle cars from the '70s with easily-adjustable front torsion bars, but I keep every one of them at exactly the specified height. If you attend any old car show, you'll see almost every car sitting the way it was meant to sit. For those that are altered, ask the owners about scrub radius and they'll give you a stupid, unknowing look. Hot rods and home-made cars are custom jobs with no factory specs to go by, and you'll find the owners of those cars never use them as daily drivers, and except for driving to car shows, they never drive them long distances. They just aren't that fun to drive, but that's what you want to turn your truck into.

The next thing to understand is that if you look at your steering linkage, you'll see that it is parallel to the lower control arm, and the pivot point at the inner tie rod end is inline with the pivot point for the control arm which it its bushings, so it goes through the same arcs as the suspension goes up and down, ... Almost. Those pivot points are off by just a little to cause that wheel to do something in a corner to aid in directional stability and control. By altering ride height, you're going to get that "aid-in-cornering" all the time while you're driving straight. The trade-off for that momentary cornering aid is increased tire wear. That happens so little that it's of no concern to the car designers or the guys who warranty their tires, but you will have that additional wear taking place all the time when the vehicle is lowered.

If you still think you want reduced handling and braking, and increased tire wear, consider that lawyers and insurance investigators know all about ride height and scrub radius and will use that against you. They will convince a jury that you were partially at fault for the crash when the other guy ran the red light, and they will be right. They will argue that you were less able to avoid the crash. They wring their hands and lick their chops when they find any modifications. We know from a common sense point of view that many of those things did not contribute to a crash, but when they find something unrelated to safety, they know they will find something if they keep looking. As a brakes and a suspension and alignment specialist, I've been asked numerous times if I would be willing to inspect crashed cars and testify in court. Luckily for me no one has ever followed through yet, but these are some of the things mechanics have to keep in mind when they work on a customer's car.

Caradiodoc
Feb 27, 2014.


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