Lowering For Handling

[FJCamper]

I know some of your guys lower your cars for looks, and that is a catagory all by itself. To each his own. This subject is about lowering for handling.

Lowering changes lots of important suspension geometry that is not evident until something breaks or you crash the car.

A little lowering can be a good thing. It drops the center of gravity and makes the car handle better, and it reduces wind resistance for better top speed.

The affected front end geometry of a lowered Type 1 is caster (which makes you go straight), and bump steer (when the tires steer the car as they travel up or down).

The lowered rear end (on an IRS) begins to suffer axle angularity as the transaxle drops in relation to the centerline of the wheels. The axles are cocked upwards, adding stress (and robbing power at the CV joints) as they rotate.

How low should your rear end be? No more than what gives you straight axles at rest.

How low should your front end be? About two inches. Turns out that matches what will keep your axles straight in back, too.

Caster is easily misunderstood. It is just a slant. The bottom torsion bar tube on a Type 1 actually sticks out ahead of the upper tube just a little. At the front wheels, this creates the same condition as the casters you have on a shopping cart buggy or the front steering fork of a bicycle -- it makes the car tend to roll straight.

Lowering the front end, even in perfection with the rear, requires that caster shims be added under the bottom torsion tube, to jut the bottom tube out a little and add rolling stability.

At 100+ MPH this means the difference between life and death.

The other front-end lowering gotcha is bump steer. Remember, your steering tie rods are of a fixed length, and as the suspension moves up and down, the only compensation is the ball-joint pivot of the tie-rod end.

When the suspension travel is at such an extreme angle that the tie rod's ability to pivot is reached, the tie rod then either pushes outward or pulls inward on the steering knuckle, and the car becomes self-steering.

This is very, very dangerous at normal highway speeds. At racing speed, it can cause crash and burn spectaculars.

There is not much you can do about bump steer except not lower to such extremes that it becomes a problem.

Lower with care, and it'll help you go faster and win.

FJC

[helowrench]

have you seen any need to "flip" the tie rods over to ensure that no bump steer is present? Or is that lower than you are running?

Rob

[FJCamper]

Hi Rob,

Flipping the tie rods is to create more suspension travel. The rods ride so close to the frame head as is, that when you go into extremes of lowering, they are where they can rub and strike the frame head as they move.

Flipping the rods as a means to attempt some bump steer adjustment (when you are not into an extreme) changes very little.

A normal job in suspension race prep for any car is to first measure the bump steer by removing the suspension spring, and move the wheel up and down with a flat sheet of plywood held up against it, to see at what point the wheel either presses against the plywood or retreats from it.

Then you start figuring out ways to minimize the in-out movement.

On some cars, you can move the steering rack up or down, on some cars you can slightly alter the tie rod length.

On our Type 1's, you solve the problem by not creating it.

Imagine screaming down the hill, max throttle, in traffic, at Road Atlanta with concrete walls on either side of you. You're turning right. The suspension is bottoming out.

Bump steer at this point is disaster. Watch us not crash.

http://www.youtube.com/watch?v=bCESjlQs-04

FJC

[raygreenwood]

Bumpsteer is usually not just the result of suspension bottoming out. It is also the result of over-compression of bushings within the steering/suspension train.

This method of bumping up against the edge of the plywood....technically you are measuring extreme arc of the wheel travel as it relates to fully compressed versus normal or fully extended....yes/no?

Is the affect you are measuring not similar to camber change as caused by wheel jacking like you would get in the rear of a swing axle vehicle?...or am I viewing this in my mind from the wrong axis?

What generally "really" causes bump steer at that same fully compressed (spring and shock off the car to see travel) condition.....is a combination of Toe-in/out change and castor change.

Part of this problem is that static (at rest) castor is designed into most front ends....by mounting the front subframe at an angle. This helps the steering knuckle...which is otherwise designed as a 90 degree joint....to tilt the tire at an angle through a turn to keep greater contact patch on the road.

However....

Since many suspensions are not fully designed to be at a certain castor angle.....they are designed IN the 90 degree world....once they are tilted when installed.....the bosses where control arms and radius arms pivot are now no longer perpendicular.

What this causes....is....control arm arcs that move in TWO dimensions...as they approach their maximum degree of arc at full compression.

To put it in a shorter explaination, many manufacturers have the bosses for control arms and double A-arms tilted at a forward end up and rearward end down. While this causes control arms and steering knuckles to have a forward tilt...accentuating castor and limiting tramlining and bumpsteer....it also makes less compressive toe-in change to control arm bushings than you would get if the bushing was installed dead level with the bolt being horizontal to the car pan. This works well....because instead of forward wheel force being absorbed along the length of the bushing and prying on it or distorting it in its tube.....a tilted boss and bushing for a control arm or A-arm causes foward wheel forces (those that cause tire spread that you compensate for with toe in) to simpy pivot the bushing slightly on its axis instead of distorting the bushing. Other than slightly compressing the suspension and maybe causing an oscillation...it generally will not cause a cahnge to alignment angles.

BUT.....in order to make a tilted control arm boss work and be correct.....it has to have its control or A-arms be produced or welded on for a specific sized tire and wheel arrangement (height issue vs travel). If this is done...when the A-arm or control arm set is compressed to its full compression...it has simply rotated in one dimension (vertically) around the control arm bushing bosses with no changes to toe-in. But if you have then taken this suspension after the fact....and added more castor....tilting the bosses further and the A-arms further........when viewing from the top....as you compress the suspension......the control arms or A-arms will also pull toward the rear (generally..depending upon which way the bosses were tilted) by a couple of degrees. This causes toe-in change as well as whatever castor and camber increase you have already measured. This is sure fire bumpsteer.

If you had a method of re-adjusting the angle pivot mounts for your A-arms or control arms...back to normal after adjusting your castor.....you could get rid of this. The fact that the steering knuckle simply rotates fore and aft on the ball joints to compensate just hides this. The average street car has few probelms with this because the suspensions are planned to never really be anywhere near maximum compression or travel. Other than lack of ability to keep correct contact patch to teh road....this is why if you have ever driven a total wreck home that has been sitting for years...dead springs and dead shocks.....near maximum travel....you will be fighting from lane to lane. Bump steer city.Ray

[Ozzie]

I have a pithy response to all this...

use dropped spindles.

You can lower your front end 2 1/2" without changing steering and/or suspension geometry. Drop your rear 2" to match and even gain a touch of rake. That little bit of rake is very nice when you are trying to chase out the understeer built into the system. You might use the change to convert to disc brakes if you are a sedan owner. If you are lowering to be cool & you need more, use both a lowered beam and spindles. That way, you don't have to build in such extreme angles to get down.
I've got a set waiting for rotors & calipers to go on the 5-Ball.

[raygreenwood]

I don't think your answer is "pithy" at all.....drop spindles are smart. They maintain most of the original geometry without...as you note...resorting to extreme angles.

A little rake to the front end..usually does help "bump steering"...because technically it generates a slight positive of static castor. Where it goes wrong is when people go overboard with forward rake. At some point it can change the center of gravity too far forward when you stomp on the brakes. There will always be a forward enertial shift during braking. If the front drops excessively due to adding more rake...then the rear is in effect already higher....which can imcrease lift in the rear even more. Ray

[crocteau]

I have a pithy response to all this...

use dropped spindles.

That's what I'm talking about, a "tersely cogent" response "having substance and point"; good to see you back Ozzie. I'm thinking 1.5" dropped spindles would be just about right, but I reckon 2.5" dropped spindles and an adjustable beam with some room to raise might be a reasonable compromise, or overkill, for a given wheel/tire combo.

[Bruce2]

2½" dropped spindles are all that exists.

[crocteau]

2½" dropped spindles are all that exists.

True that. I have a wheel/tire combo (195/65 on 5½x15" Fuchs) that seems to have helped the way my '71 Ghia handles, and now I'm planning to improve it further by lowering it a couple of inches. After reading lots of "lowering" topics I still have a couple of questions. If 2½" dropped spindles were installed along with an adjustable beam, would raising the front ~½" or so with the adjusters (for more rub-free suspension travel) introduce a bad bump steer issue? With such a setup would adding a couple of caster shims make bump steer worse or is that just the lesser of two evils? Thanks in advance.

[karol]

Don't buy new "puma" beam with adjusters already welded in if you plan to raise your suspension. Adjusters are positioned wrong - if you raise your car all the way up it is barerly in stock height. There is much more travel downward, but you can't really use it because of binding balljoint. My car, with adjuster in the middle of its range was alreardy near balljoint limit.

[crocteau]

Don't buy new "puma" beam with adjusters already welded in if you plan to raise your suspension. ... There is much more travel downward, but you can't really use it because of binding balljoint. ...

Good to know, thanks karol! Are you using the ball joints said to have more travel for a lowered front beam (eg http://www2.cip1.com/ProductDetails.asp ... C10%2D4192)?

[karol]

nope. they are non available through polish vendors. just stock lemoforder ones. Even if i could use them i won't go any lower due to poor roads we have here (and i have small dia. tyres 175/65/15 which also made car sit low)

[david58]

What would be the handling effects of a 13 inch wheel with a 175/80/ R 13 on it?

[redhot]

At what angle does the stock torsion arms sit when the car is in driving condition, that is with a driver and full fuel tank etc.

What causes the stiffness of the lowered suspension. Is it the attack angle of the upward forces to the torsion arm, the length that is reduced of the force to the pivot point, etc?

At what angle does the forces attack? Vertical, on an angle etc when the car encounters a bump in the road? Of course there is always the friction from the road...

[FJCamper]

Hi Red hot,

With a driver and full fuel tank, your front trailing arms should sit nearly level. "Nearly" usually means a slight downward slant toward the rear if no luggage or load is present in the trunk.

Lowering the front ride height too much has the same bad side effects on a VW as any other car, loss of ground clearance and loss of suspension travel ... but on a VW it also affects roll, effectively stiffening it, and caster, reducing it.

Too little caster and you wander as you try and drive straight, and you lose your self-centering correction as you transition from a turn into a straight.

Lowering the front end on a swing axle car without an equal drop in the rear promotes dangerous braking and cornering instability. The effect is less on an IRS, but still exists.

FJC