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G R I P   T E C H

                                by Dave Headley

Today’s tires have improved by leaps and bounds over the tires of 25 years ago especially for the performance driving enthusiast. And so have today’s cars!

But what about your 35 year old MGB or even your MGA. Has it’s handling improved as much as tires have improved? The answer is no and the reason is that your MG suspension was designed for tires with about 0.65 G’s of cornering capability. Today’s D.O.T. performance tires should let you approach or exceed 1.0 G.

So what can you do? Well, short of fabricating a full tubular arm, coil spring over shock racing suspension as is the norm in SCCA championship road racing there are several opportunities, each commensurate with the size of your wallet.

The first thing that happens when cornering is the vehicle rolls. The degree of roll is a function of several variables; center of gravity height, track width, suspension roll stiffness, roll center height and most important - how hard the car is cornering. Being that 1960’s vintage tires limit the cornering capability the amount of body roll is modest and handling balance is maintained. But the car is relatively slow. With today’s higher grip tires, roll goes up proportional to the increased grip. But, Only to a point. The MGB ( and MGA) suspension geometry has insufficient roll gain to utilize higher body roll angles and keep the tire vertically aligned to the road surface thereby limiting grip and cornering speed.

So OK, I’ll put some negative camber in my front suspension. This will definitely help the outside tire grip the road. But what about the inside tire? Also, the straight line stopping ability will be compromised and while the car will respond quicker to initial steering inputs, the total grip is only marginally improved.

So let’s get back to body roll. Obviously, reducing the amount of roll during vigorous cornering should be a high priority. The first and most important step in reducing roll is to lower the car. MGB’s (MGA’s too) are easily lowered by modifying the front spindles & replacing or reworking the springs. In addition to installing dropped spindles, I prefer cutting the front coils as this also gives a desirable increase in spring rate which further reduces body roll. The rear leaf springs can be lowered by removing a leaf and restacking the short leaves and / or adding lowering blocks. I prefer to redo the springs as this will lower the rear spring rate, which in an MGB roadster improves rear grip. I’ll talk more about rear steer geometry later.

Lowering an MGB 1-2 inches ( or more on later rubber bumper cars) provides benefits in two ways. It reduces roll and weight transfer giving more grip. Why does reduced weight transfer improve grip? The total normal load on the tires has not changed! Most of us know that as the tire normal force (vertical down load or weight) increases the tire grip increases. However, this increasing grip is not equivalent to the increased normal load. Additionally the load vs. grip curve is not flat or straight so as we load the outside tires in a corner we do not gain as much grip on the outside tires as we lose on the unloaded inside tries, hence total grip is less. So reducing weight transfer improves grip. Lowering the C.G. and widening the track both reduce weight transfer and improve grip. This is why SCCA production race cars are trimmed very low and have extensively flared fenders enabling widened track allowed by the rules. On a racing MGB, the only limit to lowering is ground clearance.

On a street driven MGB, several other factors come into play. The first is front suspension roll center. As the front springs are lowered the roll center moves lower at least as fast as the C.G. - reducing benefits of the reduced vehicle roll rate. In addition, this further degrades the dynamic camber gain of the suspension. This can visibly be seen by looking at the lower A-arm on a lowered car. It will be visibly sloped with the outboard end higher than the inboard end. With this condition lateral wheel movement takes place during normal suspension travel - making the car feel "darty" over uneven road surfaces. The fix for this on MGB’s with lowered suspension is to raise the lower A-arm inner pivot point by up to 2- inches, maintaining the arm geometry parallel to the road and reducing the body roll. This also helps to improve camber gain with resulting improvements in grip. Stage II improvements require only the reworking of the lower A-arm. These arms are available for modest cost on an exchange basis.

For race track or serious autocross / solo competition, further stage III revisions to the front camber gain are recommended. This is accomplished by repositioning the lever shock to effectively lower the pivot point of the upper A-arm (shock arm). Modification to the cross member is required as well as further cutting of the coil springs.

With both stage II & III modification, bump-steering (adjusting roll steer) is highly recommended and will turn a twitchy handful into the proverbial "on-rails" car. So what is bump steer? Bump steer (called roll steer by car builders) is the term used to describe how the wheels turn relative to the car direction of travel as the wheel moves vertically through it’s suspension travel. It is measured as toe- change while the car (or wheel) is raised and lowered. It is desirable to have the wheels toe-in very slightly as the car is raised and toe-out slightly as the car is lowered. This is called roll understeer and makes the handling precise and stable. Roll oversteer is just the opposite and causes difficult-to-drive handling. Don’t confuse this description of over / under steer with the same result from limited tire grip at one or the other end of the car. Roll oversteer will occasionally lead to undesirable encounters with immovable objects.

Zero bumpsteer is a myth and only occurs on paper. Suspension geometry induced steer effects (over & under) primarily result from the combined effects of two parameters, roll steer and lateral force deflection steer. Roll steer as I just noted must be understeer because lateral force deflection steer is hard to measure and predict therefore requiring sufficient roll understeer to compensate. Many OEM (Ford, Chev, etc) vehicles have been designed with high levels of roll understeer to compensate for lateral force oversteer. The desired state for best handling is a low level of understeer in both roll and lateral force components. The term front-steer or rear-steer car relates directly to lateral force steer. Front steer cars with solid gear mounts (MGB’s & MGA’s) tend to have lateral force understeer and require less roll understeer. Solid lower A-arm bushings reduce lateral force steer effects but generally do not cause oversteer.

MGB (MGA) leaf spring rear suspensions do not exhibit undesirable dynamic steer effects. As previously mentioned, rear grip improves with reduced spring rates. Why? The factory installs springs sufficiently stiff to carry 2 people and luggage without sagging. In competition, the car is run with only a driver and no spare tire or tools and is several hundred pounds lighter in the rear. Desired rear spring rates are 100-125 lbs per inch per spring. This can be achieved by removing a long leaf. However, this reduces rear roll stiffness and may require addition of a rear sway bar. Further improvements in handling may be achieved by adding a carefully designed trac-bar system and revising the leaf spring front eye rubber bushings. This increases rear understeer and improves corner exit speed.

Cars with wider than stock tires and wheels may experience tire rub. Many people install a panhard bar to limit axle lateral movement. I personally do not like panhard bars for MGB’s. These devices are difficult to mount correctly, and usually cause uneven handling in LH & RH corners. Also, it is difficult to provide good anchor structure on the subframe in a location with correct geometry. These cars were designed to absorb lateral cornering loads through the spring mounting brackets. A simple low cost system is available to positively locate the springs through these points with out the added cost and weight of a panhard bar. Reducing body roll further improves tire rub tendencies.

Sway bars, sometimes known as anti-roll bars (not roll-over
bars / cages) are devices which add suspension roll stiffness without adding spring rate. the purpose of sway bars is to reduce body roll and balance roll stiffness front to rear. Sway bars do NOT reduce total vehicle weight transfer. When a sway bar is added to one end of a car, that end losses grip and the other end increases grip. Hence balance is affected. This occurs because the end of the car with more roll stiffness also gets more of the total weight transfer while the other end gets less. In most cases, MG’s included, a large front sway bar is added and then a smaller rear bar is used to re- balance the handling. Typically - 7/8" front, 1/2" rear, for track use. A vehicle with a higher than normal C.G. ie - coupe, GT body or a heavy / tall roll cage may want bigger sway bars. Extensively lowered cars may want less roll stiffness although these are typically the most raced examples and too much is just right!

The final touches to any competition car project comes under the category of tuning. The first step is corner weighting. Race cars with adjustable coil-over suspensions spend may hours on the scales aligning, leveling and weight jacking to achieve premium results. Your semi-stock leaf spring suspended MG will benefit from similar tuning. Setting ride height does not require scales, only a flat level floor. Typically , ride height can be measured from the bottom of the rocker pinch flange to ground level, preferably with the driver in place. LH drive MGB’s carry at least 100 lbs more weight on the left side so when you put the springs in the car, put the taller ones on the left side to help level the car. No more than 1/2" side to side height variation is the goal. Front to rear, I like to see 1/2" to 3/4" higher at the rear than the front measured at the rocker flange. If you have access to corner scales, further fine tuning can be done. If you’ve never worked corner weights on a car, you’ll need to keep this in mind. You can only change the sum of the diagonal corners relative to each other. You cannot change either front or rear axle sum or either side sum. For example: reducing the LF wheel load by 50 lbs. increases the RF & LR wheel loads by 50 lbs each and also reduces the RR wheel load by 50 lbs.

A typical MGB might have corner weights as follows:

      LF - 600 RF - 550                                             LF - 550 RF - 600

                                                Change to _

      LR - 500 RR - 450                                             LR - 550 RR - 400

 

              SUMS                                                                 SUMS

      Front = 1150                                                       Front = 1150

      Rear = 950          Sums do NOT change Rear = 950

      Left = 1100                                                          Left = 1100

      Right = 1000                                                        Right = 1000


Diagonal sums equal                                Diagonal sums NOT equal

Handling: u.s. - RH turn      Changes to
  
                 o.s. - LH turn    
                                                                    Handling: more neutral

These kinds of changes require reworking or spacing springs, and is more time consuming than with coil-overs. But the benefits are worth the effort !

Alignment is the final tweak to optimize handling. Front negative camber (static) can be added up to -3 degrees for stock front suspensions. Adjustment kits are available for MGB’s. With modified camber - gain geometry -1 1/2" degrees static camber is about right. Toe-in should be 0 to 1/16" toe out.

In the rear, I’ve found many MGB (MGA) banjo axles with toe-out. Toe-in is desirable and can be mechanically adjusted by a skilled craftsman with the right tools. At the same time, about 1/2 degree negative camber can be added which measurably improves grip. More that 1/2 degree can cause axle spline distress.

So now you’re ready for those sticky modern tires. Enjoy and see you at the races.

ABOUT THE AUTHOR:

Dave Headley is a mechanical engineer with over 30 years experience designing and developing products in the automotive industry. He first raced an SCCA MGB in 1966 and was the E-Production pole sitter at the 1996 SCCA National Championships at Mid-Ohio. Dave’s company, FAB-TEK (located in southwestern Colorado), was formed to provide racing hardware fabrication services to the racing world. Specializing in MGB’s was a natural and Dave offers products ranging from nuts and bolts to complete race cars. Give Dave a call at FAB-TEK (970) 564-5822 or e-mail fab-tek@live.com for advice or an estimate for your project.

Visit Dave’s web site at www.fast-mg.com

(personal info updated 8/01)