SPRIDGET UPGRADE

Upgrade Your Sprite or Midget Suspension with Firmer Bushings to Give it that Extra Stick

by Will Perry

When the Austin-Healey Sprite was designed in the 1950’s, it was the first sports car to offer stiff, unibody construction. Its responsive handling (and low cost) naturally led to its use in motorsports. In fact, a Sprite was raced in America only three days after the car’s U.S. introduction. This stiff monocoque chassis and front suspension design continued through the life of the Sprite and MG Midget production run.

Properly prepared, Spridgets are still regular competition winners. Because these cars were designed during a period when, as they say, race drivers were fat and tires were skinny, they must be modified to take advantage of modern, high-traction tires. Their original design allows too much deflection of the front suspension’s rubber bushings, which doesn’t help a wide footprint work; plus, they have too much positive camber to begin with. Both of these problems can be cured with firm bushings offset to obtain negative camber.

Installation of a set of these bushings, generally made of MDS nylon, is relatively simple and provides great benefits. The shortcut method presented here is appropriate only if the suspension (except for the rubber-bushed pivots) is in good condition.

(1) First, firmly apply the parking brake and block the rear wheels. Raise the front end and support it securely. After removing the front wheels, you want to release all of the tension in the front springs. To do so safely, replace two of the spring base plate retaining bolts, at opposite corners of the plate, with 5/16 x 4 inch bolts with threads running the full length. (Figure #1) After these are tightened, remove the two remaining bolts. Alternating between the two remaining four-inch bolts, slowly loosen them until all tension is out of the springs.

(2 & 3) Then remove the fulcrum bolts at the frame end of the A-arm. Pull the inside end of the A-arm down just a couple of inches to expose the old rubber bushings and remove them. If the fulcrum bolt is worn or badly pitted, it must be replaced. Polish the outside of the bolts with emery cloth, and file the sharp edge off the step on the fulcrum bolt. Clean the inside of the frame mounts. Lube the new MDS nylon bushings on their inside and outside ends with moly grease. Also lubricate the bolts. Insert two of the MDS nylon bushings in each A-arm from inside the "A" (Figure #2), and then slide the A-arm back into position in the frame. Insert the two fulcrum bolts through the frame and bushing. Next, insert the two bushings from the outside of the A-arm by pushing them through the large holes in the frame mounting (Figure #3).

The special washers for the fulcrum bolts must be polished smooth on the side that faces towards the bushings. Polish them by sliding them across sandpaper that has been placed on a hard, flat surface - a heavy glass coffee table top works great. Clean and lube the face of the washer and nut; if a nyloc nut is used, use a new one. Tighten the nut to pull the bushing into place. Be sure the lock tab on the bolts and washers align with the holes in the frame. Also, be sure that the offset on the back of the washers fits into the frame cutout. Securely tighten the nuts, inserting the cotter keys if castellated nuts are use.

(4) Now we’re ready to do the top pivot bushing. Remove the nut from the top fulcrum bolt and the pinch bolt on the shock arm, and them remove the fulcrum bolt and old bushing. Polish the surface of the bolt until its smooth and file the sharp corner off the step of the bolt.

You can now install the offset bushings. For maximum negative camber, which generally produces the best performance on wide or radial tires, the bushings should be installed with the hole offset directly toward the outside of the car. This will move the camber one and a half degrees to negative (Figure #4). If less camber is needed, the upper bushings can be rotated to the other positions, but this is rarely the case.

(5) Lube the inside of the bushings, the outside of the bushing flanges, and the bolt. Insert the bolt through the bushings and the shock arm with the furnished flat washer on either side of the bushing (Figure #5). Install and tighten the nut. Now put the pinch bolt into place and turn the fulcrum bolt until the notch is aligned to allow the pinch bolt to be tightened without interference. Reinstall the cotter pin, reinstall the wheels and you’re ready to go.

We used a g-Analyst and a couple of Spridgets to test the effectiveness of our new, firmer offset suspension bushings. Based upon a biaxial accelerometer, the g-Analyst records forces ten times persecond. We uploaded six hundred turning g samples per run into a personal computer. Lotus 1-2-3 was used to average the gs for each run. (Thanks to Carl Blevins of TS Micro for g-Analyst and computer support.)

To determine if the bushings could improve performance of a totally stock street Spridget, we borrowed a 1972 Midget (thanks to Brandon Shirley) for a day of testing. A tight skidpad (autocross speed) was used to test the stock bushings; then we installed the MDS nylon offset bushing kit. Goodyear Eagle 175/70-13 tires on five-inch-wide rims were used.

Camber measured on-half-degree positive with the good stock bushings and one degree negative with the offset bushings. Pyrometer checks showed that tire temperatures across the front tires varied less with the offset bushings.

Analysis of the sample g readings revealed that the offset bushings yielded consistent, significant improvements - averaging about one and a half percent increases. With the offset bushings, cornering force averaged .86g. The soft springs fitted to the later Midgets allow too much roll to obtain serious gains from the firmer offset bushings and modern performance tires. Installing firmer springs (even going to the original equipment shorter, firmer springs as fitted to early Sprites and Midgets) would only improve performance even more.

Tests were also performed with a vintage race/autocross Bugeye Sprite fitted with the firmer offset bushings, firmer front and rear springs and a three-quarter-inch front anti-roll bar. On the Goodyear tires it averaged .94g. On a set of very used (about 1000 miles of vintage racing) 185/60-13 Yokohama A008RS tires, the car averaged 1.04g on the skidpad. Roll measured two degrees. Previous g-Analyst use in this car has recorded cornering forces as high as 1.16g during autocross and vintage racing. Improvements on the race car were in line with those of the street car.

While the performance improvements yielded by these bushings may seem minimal, there are other factors to consider. The pyrometer testing showed that the tire temps were more even across the tread with the offset bushings, which can result in longer tire life and a car that is easier to drive. So overall, this modification is well worth the time and effort.