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TWEAKING the CHASSIS - COMP-103

The answer is always, "Read the rule book first (then negotiate)".

You may notice the same subtitle here as on the previous page, but it bears repeating. One step out of line with equipment mods can get you tossed out of your favorite competition class. A lot of this could be covered in the Suspension, Steering, Gearbox and/or Engine sections, and some of it may be redundant, but combining all related topics into one general article may be beneficial. This article was prompted by Ted Elwes in Tewkesbury, Glos, UK. He uses his MGA for paved hill climb competition and had lots of questions about suspension springs, bushing materials, axle tramp, lowering, sway bars, caster, camber, tow in, wheels and tires, engine upgrades, close ratio gearbox, final drive ratio, limited slip differential, roll bar and fiberglass body parts. That's quite a laundry list, but I'll take a shot at it in general terms. This is one of my favorite subjects, how to make an MGA go like scat without spending a fortune for building a race engine (and still keeping the car streetable). I have substantial experience in this area by way of autocrossing in Stock class with SCCA nearly every weekend (in season) for close to 10 years throughout the 90's. I have been there, done that, pretty much everything, except I have never had the problem of axle tramp.

You could start by reading everything available in the Suspension section. In particular, do watch the one minute autocross video, preferably the 17-MB high resolution copy if you have a fast enough data link to download it. Otherwise the 1-MB low res copy will do. This video shows my car running on street radial tires, not race tires. With the sticky treaded road racing tires the tires don't squeal and the lap times will be about 4-seconds quicker for a 60- second lap, as it doesn't need to slow down so much for the corners. You should notice that my car definitely does NOT lift a rear tire, not ever. The suspension is pretty much bone stock except for a fat front sway bar. You should know that competition driving is not so much a power issue as a matter of finesse in driving technique (requiring lots of seat time for practice).

The first trick is that you have to keep increasing size of the front anti-roll bar (we call sway bar for short) to reduce body roll until it keeps the rear tire on the pavement. You cannot tune the suspension for oversteer/understeer character unless all four tires are always on the ground. You may know that original MGA had no sway bar at all until it became an option with the 1600 model, and later standard with production of the 1600-MK-II. Tires then were skinny bias ply (with poor grip by modern standards) until radials became an option very near end of production. The only sway bar ever supplied by the factory was a 5/8" diameter bar, which may have been just about appropriate for the skinny bias ply tires. You already know that modern tires have enough additional grip to produce more body roll and lift the inside rear tire off the pavement. Today the minimum size sway bar for good use with modern street tires (especially radial tires) would be 3/4-inch diameter. When you push for competition, particularly with stickier competition tires, it will need a 7/8" diameter sway bar to keep the wheels down. Sway bar stiffness increases as the fourth power of diameter, so going from 5/8 to 7/8 diameter makes the bar itself nearly four times stiffer in resisting twist. The sway bar works its magic by changing weight distribution on the tires from corner to corner, not from side to side. Once properly set up some small adjustments of the sway bar end links can modify oversteer or understeer character at will.

So far I haven't said anything about lowering ride height or modifying springs or shocks, and maybe I never will. My car has all of the original suspension parts in original configuration, with exception of the sway bar. Original rubber bushings in front A-arms were woefully inadequate for competition and would deteriorate in a couple of years. I used polyurethane bushings in the A-arms for a few years in the mid 90's, highly recommend for any type of use, street and competition. I suppose they would never wear out, as they were like new after 3-years and 75,000 miles of very serious use. In late 1997 I switched to MGB GT-V8 type A-arm bushings, just for a trial, with similar good results, and those are still in good serviceable condition after 12 years and another 110,000 miles. Either poly or GT-V8 type will suffice, but do ditch the original soft rubber bushings.

Similarly I replaced the rear spring shackle bushings with poly bushings about 10 years ago. That wasn't much of a functional difference, except those also never wear out (also highly recommended). Silentbloc bushings in front end of leaf springs are still as original, but were replaced once in late 1997 (after 40 years service on original issue parts). Elastomer bushings for mounting a sway bar may be changed to affect roll stiffness up front. In this respect I like to use nearly rigid poly bushings for frame mount, generally firm poly bushings for the end links, and then tweak resilience of the end links for fine tuning of the sway bar.

After a few years of regular autocross competition I started breaking steel wheels (due to stress cracks). This ultimately led to procurement of my "special" alloy wheels. These were (expensive) special order because I needed narrow wheels to stay in Stock class with SCCA, and no one makes a 4" or 4-1/2" wide aluminum wheel with the correct offset.

In the early 90's I was running on Michelin X (or XAS) 165R15 radial tires, fairly hard rubber generally good for 80,000 miles of paved road. Starting 1992 season with half worn tires, they were bald by year end. In Spring 1993 I installed a set of Bridgestone Potenza 195-70-15 street radials. These were more predictable for handling and nicer to drive on, but only maybe a second faster around the track, and also bald by season's end.

In Spring 1994 I bought a second set of wheels and two sets of tires, Dunlop SP4 165R15 for street use and Yokohama A008RS, 195-60HR15 treaded road race tires for competition only, transporting the race tires to events in the luggage trailer. By season's end the race tires were bald too, and those only had 2 or 3 hours of track time, but oh what bliss! The difference is like night and day when you don't slow down much for corners and lap times are 4 seconds faster. Each year thereafter (sometimes more often) I had to pop for $500 for a new set of race tires, but it was worth every penny. Unfortunately that model tire went out of production around 1999, and there is no other race tire that will fit on my narrow wheels. So I have effectively been legislated out of the sport with SCCA, as wider wheels would kick my car into a different class where it would not be competitive. Since then I only autocross with local vintage clubs using regular street tires.

If race tires are allowed in your competition class, by all means grab a set with (preferably wider) wheels to match. It is likely that the smallest race tires you can buy for 15-inch wheels will be 205-50-15 size. Wheel width spec is 5.5"-7.5". They will look like these:
www.tirerack.com/tires/TireSearchResults.jsp?width=205%2F&ratio=50&diameter=15
Skip the first page and look only at "Track & Competition DOT" tires. DOT Approved means they have some minimal tread pattern that makes them (minimally) street legal, and therefore approved for use in Stock class with SCCA. Before you buy these tires, read the rule book for your racing venue.

I know Hankook makes lots of different tires. I suppose for tarmac competition you might want something like the VENTUS Z214 here:
www.hankooktireusa.com/Product/product.aspx?pageNum=1&subNum=1&ChildNum=6&FnCode=06
(click on DOT R-compound)

Autocross does involve a jack rabbit standing start to get up to speed as quickly as possible. On street tires it may involve considerable wheel spin if you're not careful on the throttle and clutch. With open differential expect the RR wheel to spin lots more than the other. With race tires you can apply more torque to the wheels on jump off and be more aggressive with power shift into 2nd or 3rd gear (and they also brake much better). Race tires grip well enough to cause some clutch slip with aggressive launch and power shifting, so you might expect the clutch disk to wear out eventually. There was a time when I needed to install a new clutch disc about once per season when running a standard clutch in the MGA.

Then I switched to a slightly lighter flywheel (20# vs. 28#) and competition pressure plate. Lighter flywheel allows the engine to spool up a bit quicker, allowing slightly better acceleration in the lower gears. The competition pressure plate grabs tight and doesn't let the clutch disc slip much, could even catch a little tire scratch going into 3rd gear. The stock clutch disc then lasted three years and ultimately failed only when the lining material worked loose on the rivets. A competition disc should solve that problem. With sticky tires and strong clutch, when neither one wants to yield, I used to replace a worn U-joint in the propshaft about once a year. Tough combination. I finally switched to a standard MGB pressure plate. This worked just about as well for competition, allowed lower pedal force with the disc lasting a couple years, parts were cheaper, and it didn't eat U-joints as often. Count me as one happy camper, still using the same combination of parts today (except no race tires).

I was running the 3/4" sway bar for many years. This worked generally well for modern street radial tires but would allow lifting of a rear tire when running on race tires. When the 3/4" bar finally broke (after 12 years of very aggressive use) I installed a 7/8" sway bar, and all of the tire lifting problems went away. Finally for one competition season the car handled like a real race car. It would first exhibit mild understeer going into a tight corner. Solution for that was to lift off the throttle for one second while turning in to transfer weight forward. As soon as the nose would hook up I could hammer the throttle and power steer out of the turn. With open differential the inside rear tire would spin a little in 2nd gear, but never enough to run engine past red line, and then it would hook up and go. I suppose the tire spin with one wheel was just enough to break the tail loose a little to allow a bit of throttle steer (with 100 BHP engine in 2nd gear and 4.3 final drive ratio). It was an absolute delight to drive around the tight track (usually in the 25-50 mph range), and I bagged every competition trophy available that year.

On the issue of axle tramp, I have never had that problem with any type of tire on the MGA. I suppose if you remove leaves from the leaf springs you should expect that sort of problem. I would definitely NOT recommend doing that. Softening the rear springs might allow the inside rear tire to stay on the ground better, but it also allows more body roll, which you know is a problem to begin with. Solution to reducing body roll is the larger front sway bar. Also I find nothing wrong with original Armstrong hydraulic lever shocks. Lots of people who change to tube shocks end up with those being way to stiff.

Meanwhile, if you do have a problem with axle tramp I suggest you let the clutch up a bit more gently allowing the clutch to slip a bit rather than breaking the tires loose. Spinning tires is not the best traction condition anyway. The only reason I spin tires off the start line is because the stock engine will bog down at low speed. When I spin the tires a little I can take advantage of higher flywheel speed and stored energy to give the car a better kick off the line. By the time tires stop spinning the car has enough ground speed in low gear to keep the engine speed above 2500 or 3000 rpm so it will never bog down. Wear on tire rubber is easier to fix than wear on the clutch disc. This should be less of a problem with an engine possessing higher torque, where it should not require so much clutch slip or wheel spin to get promptly motivated.

The only other reason for tampering with the original leaf springs may be to lower the ride height. I don't suppose that really matters much until you are near 98% of the peak of competition skills. Original MGA configuration has 6" ground clearance below the frame. 165-80-15 tires are around 26" tall with rolling radius around 12.5" and produce about 810 revolutions per mile. 205-50-15 tires are much lower profile, produce about 900 (+/-12) revolutions per mile. This translates to 10% smaller tire diameter, which will reduce ride height by 1.25" all around, resulting in ground clearance of only 4.75" below the frame. How low would you want to go? I would not want to be down to 4" frame clearance for a street car (with exhaust pipe hanging lower). I already have some ground clearance problems with the 195-60-15 tires (3/4" lower) and standard suspension height. Watch out for speed bumps (sleeping policeman?).

One thing to definitely avoid is, raising the rear ride height by installing new leaf springs that are too tall. With too tall springs the rebound straps run out of travel and go taught with minimal body roll, resulting in lifting of the inside rear tire and dramatic transition to gross oversteer (bad, bad, bad, and dangerous). With original ride height and 165-80-15 tires you can place three fingers between tire and fender flange in front (three fingers up). At rear you can lay your fingers flat on top of tire and touch knuckles on the fender flange (one hand flat). Low profile tires will increase this top clearance accordingly with reduction in tire radius.

The MGA suspension is actually very good in stock configuration, if you have the appropriate front sway bar to dampen body roll. With too small sway bar you get too much body roll causing the outside front tire tilting to positive camber and lifting part of the tread off the pavement, resulting in loss of grip and unwanted understeer. With the right sway bar body roll is minimized, keeping the tire straighter and flatter for better grip and reducing the tendency to understeer. For serious competition it may be beneficial to introduce about 1/2 degree (up to 1 degree maximum) negative camber in the front tires. This is done by installing negative camber lower a-arms. It doesn't require much change in length of the lower arms, only 1/16" to 1/8" longer for 1/2 to 1 degree tilt of the wheel.

Also to keep the tire tread flat on the pavement it is best to use wheel width about equal to the tread contact width. For 205 size tires this would be a 6-1/2" wide wheel. When tire sidewalls are vertical the tread will stay horizontal with parallel flexing of the sidewalls. A rim too narrow will result in lifting of the inside part of the tread and undue wear on the outside shoulder. Wider wheels actually work marginally better, but most people would stay with wheels no wider than the tread width. When changing wheel width you need to pay attention to keeping the same wheel offset as original to maintain original track width (center line of tires). In particular do not place the tire underneath the fender flange. Original wheel offset is 1-3/8-inches (35-mm) for bolt-on wheels. When you have the correct wheel offset there is plenty of space for wider tires under the MGA fenders (especially lower profile tires).

While wider tires are a little better, you can over do it. When tires are too large with low loading they don't warm up enough or quickly enough, especially on short runs. Tires like to have optimal running temperature for best grip, and cold tires are not good. Also larger tires are heavier, introducing more unsprung weight (poor handling on bumps) and increasing rotational inertia (reducing acceleration). The best tire size for the MGA might actually be 185 wide, but you can't buy race tires that small. So get the smallest race tires you can (205-50-15) and wheels to match (6" to 6-1/2" wide).

If you are allowed to run a positraction differential it may be somewhat beneficial (but correct tires are much more important). Locking type differentials (those having friction plates) try to make both rear wheels turn the same speed in all conditions. This is good for inhibiting tire spin in hard acceleration in the lower gears, but not so great in tight corners. In tight turns the inside tire wants to turn slower than the outside tire. Forcing the inside tire to turn same speed as the outside tire will cause the inside tire to slip and lose grip. Not that it matters all that much, as the outside tire is doing about 80% of the cornering work anyway. However, over driving the inside rear tire also causes understeer (front wants to go straight ahead rather than turning the corner).

If you can get a Gleason Torsen differential (Quaife differential in America) you may be better off. See Rear Axle tech RA-302C. This is an all gear drive torque sensing unit with no friction clutches. If you understand why a worm gear will not back drive you can understand how a Torsen differential works. The primary requirement there was to make the driven crown wheel smaller diameter than the worm gear. It will allow the rear wheels to assume their natural respective rotation speed in tight corners without over driving the inside tire. It does however depend on both rear wheels actually touching the pavement at all times. If you lift the inside tire it will spin free (and turning it by hand appears to be an open differential). If the inside rear tire stays in contact with the pavement it will give amazingly effective traction control, significantly better than a locker type or friction based differential. If one tire should lift off the pavement or lose all traction, you can touch the brake slightly to make the differential resume its noble duty, even with one wheel in the air.

As to leaf spring modification for changing ride height, you should keep in mind that a leaf spring should be essentially flat in the loaded condition to take high side load without distortion. When you do things to the leaf spring to lower the ride height without changing the end mounting points, you will be warping the spring past the point of being flat and being deformed with arch in opposite direction of the free condition. When the leaf spring is not flat side loading will cause it to twist and yield sideways. Any sideways motion of the rear spring may affect an oversteer condition. Additionally, if the pavement is undulating in the turns the changing load on the springs will cause varying sideways distortion, varying side motion, and a dynamic sideways wobble in the rear axle. None of that is any good, so you really want to keep the leaf springs as flat as possible. If you feel the need to change rear ride height, you can lower the car by installing spacer blocks between spring and axle housing, or you can raise it by installing longer spring shackles at the rear.

There are similar issues with changing ride height in front. If you shorten the coil spring you are changing angle of the suspension arms. This affects the roll center of the front end which can cause dynamic (and unpredictable) variations of oversteer or understeer. It can also cause bump steer problems when steering tie rods are not level. The better way to change front ride height is to change height of the bearing spindle (knuckle) on the swivel pin (king pin), while retaining all original geometry of other front suspension parts.

Maybe that's enough rambling for now, should keep you busy for a while. Let me know if you have more questions.

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