Tweaking your MG for the road


Paul’s tips on how to go about it!  

Part 2 

But what about the corners?

As with the engine, useful gains in handling can be made for quite a modest outlay.

Before looking at the options it is useful to consider some of the factors that contribute to the way a car handles. These fall loosely into two areas:  controllability and stickability.

The controllability of the car arises from a number of factors - its responsiveness to the steering wheel;  the extent to which the wheel needs adjustment in order to maintain the car’s line around a corner as it accelerates, brakes, encounters uneven road surfaces, etc; its balance between front and rear  (does it tend to understeer or oversteer?);  and  how it behaves at the limit of tyre grip  (is it sudden or progressive?)   Some factors affecting controllability are difficult to change as they are a consequence of the basic suspension type and won’t be addressed here.  Others are much easier, and include:

Check that the various rubber components and shock absorbers are in good order. Pay particular attention to the rear axle by road testing to see if the car tends to veer one way and then the other as the accelerator is fully depressed and released. Do this several times in a row along a straight bit of road - the car should remain true on line. If it does not (and it is not uncommon!) it is a sure indicator that the rear axle is loose due to tired spring pads and bushes. As with the engine, there is a world of difference between an average “done a few miles” MG and one which is recently overhauled!

Gains can be made to steering responsiveness and rear end stability by fitting synthetic components (eg nylothane) instead of rubber in the various suspension mountings and bushings, though at the expense of a small increase in road noise transmitted to inside the car. Whether this noise increase is a nuisance is a personal preference question - some find it so, others don’t mind. If a quiet ride is an issue for you, then it’s probably best that you avoid this modification.

The front to rear balance can be adjusted by variations in tyre pressures - increasing pressure increases grip (Note: Every tyre/rim combination has an optimum pressure above which grip will reduce. The lower the aspect ratio of the tyre, the lower this pressure is; however for tyres usually found on pre 1980 MG’s this point is comfortably above the normal range of road pressures.)  If your car feels a bit tail happy (oversteer) then increase your rear tyre pressure to give the rear more grip. Similarly, if it feels a bit understeery, increase the front pressures. Changes in pressures should not be in increments greater than 2-3 psi at a time without trying the car first. If seeking to reduce understeer, pay particular attention to how the change you’ve just made affects the straight line stability at higher speeds. If it does so adversely, you’ve gone too far (this is why it is important to make only a small adjustment at a time)

The second major component of handling is the stickability of the tyres to the road. The type of tyre you fit is obviously important, but having made this decision there are also other influences at  work. The effect of tyre pressures has just been outlined above, but while this is ok for adjusting front/rear balance, increasing the tyre pressures to gain more grip has limitations such as a harsher ride and the existence of that optimum pressure beyond which grip reduces.   

Also important is the condition of shock absorbers and their efficiency at keeping the wheels in contact with the road. For the 1985 International Rally, we were required to retain the standard shock type, so fitted uprated new OE units all round. The rugged forest tracks gave them a hard time and were a very good test!  The front shocks performed surprisingly well, and saw several years service after that. As a result, at the front it is fair to say that uprated reconditioned original units will be quite satisfactory for road use. The rear was another story altogether; performance was well under par and the event wore out two pairs of new shocks. Although many will find that uprated standard rear shocks are satisfactory for their requirements, there is plenty to gain from rear telescopic conversion if you are mostly on the open road!

For a given tyre at a given pressure there are two very significant factors affecting its grip: The weight the tyre supports, and the angle it makes with the road. Both of these stem from the fact that the tyre is rubber, and as the rubber loads up towards its friction limit it distorts and eventually buckles under, causing it to lose grip. Because of this, tyres do not show the usual proportionality of friction surfaces and the amount of grip available does not increase by as much as any extra weight supported.  

For example, if the weight on the tyre increases by 50% then the amount of grip increases by less than 50%. With 50% more mass to persuade round a corner, but less than 50% extra grip to do so, the tyre is going to let go earlier.  

The angle of the wheel to the road produces a similar major effect. If the tyre tread is not sitting squarely on the road, then part of it will lose contact and leave the rest more heavily loaded - with a resulting reduction in available grip. Sounds so simple, why not just stop there?  Well, unfortunately with independent suspension (which has a great many advantages in other respects) as a car leans in a corner, so do the wheels. On the other hand, wheels on live axles don’t lean - this is a very important consideration for an independent front/live rear combination like most post war MGs as we’ll see later.

All this leads to the following principles of basic suspension tweaks:

1.  weight transfer from one wheel to another => higher loads => less total grip.

2.  reduced body roll => wheel stays more upright => better grip (indep susp only)

3.  pre-set independent suspension wheel angle with negative camber => the outer wheel becomes upright with body roll. (Even though the inside wheel is at a poor angle, the total grip is increased because under heavy cornering it caries very little load and may leave the ground altogether, leaving the outer one to do all the work.)           

Without major redesign, beyond the scope of this consideration, suspension modifications generally centre around uprating spring rates and antiroll-bars in order to reduce body roll. (Anti-rollbars achieve a given reduction in roll with less ride harshness than firmer springs)  This helps through principle (2) above, but also transfers load from the inside to the outside wheel at the end of the car where the stiffness is added, and so reduces grip at that end - for example, a front anti-rollbar will transfer load at the front. If the grip is altered at one end but not the other, we have to be careful about such changes in order to retain or improve the front/rear balance.

In general, extra front stiffness (via springs or anti-rollbar) generates opposing tendencies: gains from better wheel angle v losses from weight transfer. For initial increases in front end stiffness of the MG models under consideration (without changing the wheel geometry) the plus outweighs the minus and there is a net improvement at the front, along with a gain at the rear because it has shed some of its roll reaction to the stiffer front. Better at both ends, and feels great on the road! Before going to yet heavier springs/anti-rollbars at the front, it is necessary to introduce a bit of negative camber so that the outside wheel is better placed to take the extra load. Having done this, further total grip is achieved at both ends and  good balance is retained.

This sequence is the basis of our kits. As with everything, it involves compromises as the performance increases (otherwise the factory would have done it from the start), so you have to decide what level is right for you. The downsides apart from initial cost are increasing ride harshness (most find the first steps of 3/4”antirollbar & 480 lb/in spring to be very acceptable) along with reduced tyre life once negative camber is introduced (the inside edge of the tyre wears more rapidly than normal).

A final consideration lies in the independent front/live rear axle combination of MG’s from the TD through to the MGB & C. As the solid rear axle keeps the back wheels at a fixed attitude to the road (basically vertical) then reduced body roll gives no increase in grip at the rear - all the gain is at the front.   

A rear anti-rollbar  therefore has the following effect: weight transfer is increased at the rear only, so rear grip is reduced; wheel geometry is improved at the front only,so front grip is increased. The net effect is that the car feels better initially due to being flatter in the corners but reaches its limit earlier and more suddenly. The back breaks away and tries to overtake the front....as a number of people have found to their dismay. It is in my view potentially dangerous, and I advise strongly against it. Yes, I know the factory fitted one to the late rubber bumper cars in an effort to counteract the increased ride height - presumably this was to avoid the small extra harshness of a stiffened up front end at a time when the B was being made more “refined”. A better result on these cars is achieved by removing the rear bar and increasing stiffness at the front along the lines indicated earlier.

Handling is a complex topic; the foregoing is a simplified summary to guide you through some of the readily available ways of uprating your MG.  I hope you enjoy the result!