I'm sure this topic has come up a number of times but is there a definiative source on the race car specs? I'm interested in the wheel/tire details obviously they put larger wider tires on there, almost looks like it has a lift kit to accomodate the size.

 

If it's not in this, it may not exist.

 

In that book, for 1984 the wheels are given as Dunlop Denloc double bead run-flat rims, 17" diameter. 10" wide at the front, 11.5" wide at the rear. Tyre size is said to be 275/625/17 front and 300/625/17 at the rear

 

Thanks, does it mention adjustments to ride height? The fender lip appears quite high I wondered if they used custom length springs.

 

I don't see ride height mentioned. Front spring rates were 1500-1800 lb/in, and rears were 400-450 lb/in.

 

Gives the spring rate, but nothing about ride height.

 

The front suspension was not entirely stock. A TWR spring shocker unit was used in place of the OEM springs and dampers and it was mounted on a modified sunframe, which itself was bolted directly to the body with no rubber bushes. The placement of the spring shocker unit gave different leverage on the wishbones from the stock springs and also enables fast changes of springs and ride height. All in Allan Scott's marvellous book about the racing TWR XJSs.

 

One of the reasons it wasn't lowered much is that in spite of the larger than stock wheels and tires, Group A required that the stock wheel opening and inner fenders remain the same size. The book describes how they "massaged" the inner fenders and used various sales market slight variations in wheel opening specs in the homologation papers to get the maximum size they could within the rules. Walkinshaw was famous for stretching rules to the absolute maximum, and getting away with it.

 

Plenty of motorsport teams follow those rules, right up until somebody lodges a complaint based on "why didn't we think of that" sour grapes - and the rules change until the next round of sour grapes. most teams won't document such stuff, better to check the group A rules of the day as you can be sure they were right on the limits.

Group A rules from 1985 Group A 1985

Lists a rim width of 13" max for cars over 5 litre, doesn't stipulate a max dimension but clearly states that the entire rim and tyre assembly must fit below the bodywork - hence the slight flaring to the arches on the TWR XJS, looks like the tread was in line with the bodyline of the car - looks like 60 series rubber so that on a 13" rim would put it into the 26" approx overall on an 18" rim - I don't know if they ran staggered though and the combination would likely have changed depending on the venue or conditions anyhow.

You can always buy number 5 .... Number 5

 

thanks ben, great article #5!
ron

 

That's definitely a lot of tire under the wheel wells / arches, as you all call them. She does seem to stand a little tall compared to most XJS'. Seems to be a little more ground clearance??? HMMMM?

Jack

 

most spring rates are less than 1/2 of that. Even Daytona rates aren’t that high.

 

The ride height is discussed in Allan Scott's book.

The TWR cars were lowered in comparison to a stock XJ-S, NOT raised. The exact amount of the initial front/back lowering was not disclosed, but probably just over an inch, say about 30mm. Per TWR's Allan Scott, the front was then lowered an additional 20mm in comparison to the rear, resulting in it's distinctive forward rake. Finally, a "secret" modification was performed up front that lowered the front rubber spoiler closer to the road. My guess is that a 1/2" or so spacer was installed between spoiler and body, or the lower panels could also have been stretched downwards. They were really trying to get rid of the forward lift they were experiencing.

At any rate, he said that the final result of their efforts was the lower spoiler being 60mm closer to the road surface than before. So if we were to guess around 30mm of initial front/back lowering, plus 20mm more in the front to give it a rake, plus a 10mm spacer above the spoiler mounting point to push it downwards...you'd get about 60mm. If the initial all-around (front and back) lowering was less than 30mm, it just means that the spoiler modification would have to be greater than 10mm to arrive at his 60mm figure, but I don't think the spacer would have been any thicker than 1/2" or the modification would likely have been noticed.

Also note that though wide wheels and tires were used on the TWR cars (275/300 front/rear racing tire tread width, which equates to about 305/335 DOT tire section widths), the overall rolling diameter of the tires was only 625mm/24.6", in comparison to the 26.9" rolling diameter of a car running the (then) stock 215/70/15 tires. In other words, rolling diameter of the front AND rear of the TWR car's tires was about the same as that of the front wheel/tire on the XJR-S cars, which had a 24.9" diameter resulting from their 225/50/16 front tire (the smallest rolling diameter of any factory XJ-S tire). This significant reduction in rolling diameter, by itself, lowered the car by a little over an inch in comparison to stock, before they even touched the suspension components to actually lower the car for real. Also, with only a 3.8" sidewall, this is somewhat LESS side profile than a 35 series tire, and no where near the thicker side wall of a 60 series tire.