so first off - Happy Holidays to everyone. and 2nd - does anyone happen to know the specifications or where to find them, for the OEM supplied 18- TR 415 wheels that were standard on the 400 series? (i.e. equivalent lug spacing, offset, equivalent diameter, and equivalent rim width)? trying to determine which wheels and tires to purchase as a replacement in either the 115, 16 or 17 inch size. thanks. ed
The TR are Metric items and stands for Tension Reparte in French. Long story but the TR system, more correctly the TRX never caught up with the rest of the other tyre manufacturer other than Michelin. Besides Ferrari, the only other manufacturer that ran TRX system on their cars were BMW for the high end 5er and 7er in the early to mid eighties and also Alfa Romeo for the 1.5 Veloce Sprint QV. Therefore those 'funny' numerals like 380/415 are actually millimetres rather than the normal inches as specified, i.e 15/16/17 etc.. So, to convert the TRX system to a 'normal' system is easy actually. Get the diameter right firstly. Means : What's the stock tyres with the Metric rims attached. For example, if the 400 runs 240/55/415 means 240mm wide with 55 section, mounted on 415mm diameter wheels. 2(240X55) /100 = 264mm (total tyre sidewall height) + 415mm Therefore total rolling diameter is equivalent to : 679mm. When you extrapolate a 16" (15"s too small for a BIG CAR) or a 17" wheel, you can double check the rolling size using the same formula. 2(225 X 55) /100 = 247.5mm total tyre sidewall. A 17" wheel is 431.8mm across. Therefore 247.5 + 431.8 = 679.3mm A 17" tyre combo should therefore be 225/55/17. Wheels preferably 8" wide minimum or in rimspeak, 8 X 17 Do not worry about the 240 contact patch being larger than a 225s (another rule of thumb is NOT to go less than the original specced tyre width) however because, another long story : Width of the tyre, quoted in millimetres is also dependent on the width of the wheel. If a 240mm wide tyre is mounted on a 7" (25.4 X 7 = 177.8mm) wheel, there's excess of 62.2mm or in other words, 31.1mm shoulder on either side of the wheel. It's considered a waste because the excess sidewall height will actually 'squirm' under pressure and more often than not will slump...no good for handling. Rule of thumb, a maximum of 15mm shoulder per side of the rim. Not only makes the car/wheel and tyre combo look good, but also the handling's affected if you put too fat a tyre on a 'slimmish' wheel. A Ferrari runs a Pitch Circle Diameter (PCD) of 108 X 5. As for offsets, I believe the 400 should be in the region of 25-30mm for an 8" wheel but maybe someone can chime in here, I haven't the details on a 400. Sorry for the longish post but here I am in China, on holiday till tomorrow and I guess I've too much free time..... Merry Christmas !!
Well first let me wish you a belated Merry Christmas and yet to come Happy New Year!. thanks so much for the info, the calculations, and the amplifying info. I new that folks were going to what appeared to be less wide tires after converting wheels but didn't know exactly why. I recently found a handy tire conversion excel macro that does a great job of letting of put in original size and trying alternatives. Even shows a graph of the original vs new footprint. I'll have to repost it here as I am on a different computer at the moment. Have a great trip home.
