YouTube clip Ac 428 tyres

[Classicus]

Small YouTube clip on tyres for the 428....
   
   "AC 428. Dougal from Longstone Classic Tyres talks about AC 428 classic tyres."
   
   http://www.youtube.com/watch?v=wwJAtYjKeBQ

[cliffordl]

What's interesting is that the original Avon tyre has a diameter of 708mm, the new Cinturatos are 707mm but the commonly desired replacements such as the Avon and Michelin 215/70VR15 tyres are 677-683 mm in diameter. So to get better grip some of the diameter (and therefore the correct fill of the wheel arch) is lost.
   Will this downsize affect the speedo?

[lyonheart84]

Going to a lower profile does not necessarily give you better grip and as you pointed out Clifford does not fill the wheel arches as nicely as the correct size Avon or Michelin. The originals are known as 205 VR15 but they are in fact effectively 205/80 VR15, I never figured out why the 80 was left out on older tyres. Going to 215/70 VR15 does not quite make up the lost diameter and you are quite right in your assumption, will cause the speedo to read incorrectly and affect the gearing for cruising ( albeit neither will change much ).

[Emmanueld]

I would like to point out that tire technology has evolved quite a bit since the 60's and using and original size tires on a car like this is crazy (In this case, the chassis was originally designed to use much larger tires so there is no concern of suspension breakage). Mounting a larger profile tire will improve the grip as well as stopping ability. Using a 225/70 VR 15 will give you the best handling with the stock wheels. Even a 245/60 15 will work as the rims are identical to early Cobras. I think the best upgrade while using stock wheels is a 215/70 VR15 up front (to keep the manual steering light) and 225/70 VR15 rear. The best and safest street period tire is the Michelin XWX which appeared in the very late 60's and was installed on every Ferrari's and Lamborghini's of that era. They are available new from Michelin Classic (with modern gum) but are pricey!
   
   Emmanuel[:)]

[cliffordl]

I recently visited Vintage Tyres to sort out my ageing rubber. Great service (they let you into the motor museum while you wait for fitting) and good advice.
   
   Ended up with replacing the existing 20 year old 225 70 VR 15's with matching new Avons and keeping the 235 70 VR 15's at the back.
   
   Carrying a 225 as a spare - however this does not fit the spare wheel well when inflated so some adjustment now seems necessary (or keeping the spare un-inflated).
   
   Cliff

[Emmanueld]

Hi Cliff, great, there is plenty of room at the rear. You might find the front steering input a bit heavy, although I used that size before and it was fine. A custom shop may add a small wood frame under the wood panels in the boot and! "Et Voila" there is suddenly room for a big spare. This has to be done for both spare tire and battery panels so they sit evenly!
   
   Emmanuel[:)]

[dougal]

Hi
   Yes it is true that tyre technology has moved on, but it movede on with the technology of cars of the day. yes a wider tyre will give you greater road holding (provided you have the rims to suit) in stopping but i would suggest hat your problems are not locking up the tyres in straight line braking. In the corners it is possible to over tyre a car. Also wider tyres make a car track where by it picks up the uneveness of the road.
   Modern cars that are suited to more modern wider tyres have very different steering geometry to cars of your period. they have greater caster to overcome the tracking caused by wider tyres and then they have much cleverer poer steering to overcome the issues that this extra caster gives.
   wider tyres make the steering heavier, which in turn wears the steering components.
   I don't dissagree that fitting Michelins XWX tyres would possibly have been a period upgrade which will not effect the handling too much but then you loose diameter.
   The reasons for recognising the Pirelli Cinturato are:
   aesthetically they are perfect. This was the genuine period up grade to the Avon, as the Cinturato was a sports car tyre. the diameter is correct so it will keep your wheel arches full, gearing and speedo will be correct.
   Also with the Cinturato you have a high quality tyre that will give you better grip than was available in period because you have better modern rubber compounds, without effecting the handling that fitting different sized tyres will do.
   there is a difference between handling and road holding. fatter tyres will give you greater road holding (though may lead to aguaplaining). however the correct size tyres will make you car handle at it's best. in that the steering will feel the way it should, tracking will be kept to a minimum and the car will handle in a progressive manor. in that if yuou do push it round a corner it will drift instead of braking away violently in the way over tyres cars do.
   I think 215/70VR15 and 225/70VR15 tyres will probably be fine, though the car will not have as comfortable ride with less side wall. but without doubt the Michelin XWX is by far the best.
   I would respectfully suggest that 245/60Vr15 would be far too big for road use. what happens on the track is another story

[Emmanueld]

Douglas, as I said before, the Frua chassis comes from the MKIII Cobra which was designed with racing in mind, it should have been fitted with much larger tires from the start. Unfortunately, those did not exist at the time. Unlike most cars of that era, the Frua suspensions can be adjusted a great deal. This is not an underpowered Fiat or Alfa Romeo for which the cinturatos were designed. This is a 2900 lbs 7 liter engine monster with almost 400hp is stock form. I have spent years playing with the suspensions and tire sizes to make it work right and the way my car drives now is amazing. Just like a modern car. You are right with your comments regarding most 60's cars. I owned a 63 Sting Ray roadster with a 340hp small block, the car was original in every way. I thought the road handling was just scary with its original cheap tires. So I ordered a set of period  "Kidney Bean" knock off wheels from  a local builder and went on a quest to find the perfect tire for my project. I figured the chassis would not hold up to a modern sport tire, so I settled for Michelin radials Z rated which came stock on the big Lexus. I remember the size was 225/60X16. Well! grip was great but the chassis could not keep up, on very tight turns at high speed, the chassis would flex and the doors would open. It was too much tire for the Vette rudimentary 60's chassis, the added grip would flex the car instead of sliding. With the Frua, no such problem, the chassis is massive an despite much stiffer springs and bigger tires it is still rigid.
   
   Emmanuel[:)]