| Author | Message | ||
     KC Saayman Frequent User Username: kc_saayman Post Number: 36 Registered: 10-2004 |
We have had this discussion before. Cars before chassis no. 18269 take narrower 205 tyres, and cars after take wider 235 tyres. My question relates to the profile. On the later cars the correct size is 235/70 15. That tyre is about 710mm tall. What is the profile on the narrower tyre? If 205/70 15 the tyre is only 668mm tall. However, if 205/80 15 the tyre is 709mm tall. So, which is the correct tyre for earlier Shadows? 205/70 or 205/80? Do the earlier cars have smaller tyres, or should they be of the same height, just narrower? Kind regards KC www.ClassicRollsSA.co.za | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 1003 Registered: 4-2003 |
My, this subject has been done to death. The tyres for all cars from the Silver Shadow are of the same 710mm outside diameter, but vary in wheel diameter and tyre width.. The only exception was in early Turbo Rs, where shoddy Pirellis of a smaller diameter were slapped on as there was no tyre available to suit those cars for a year or so. The Pirelli cars had a different speedo drive gear fitted until they were reshod with the correct tyres in service. Although the sidewall marking gives a nominal diameter of: wheel dia inches x 25.4 + 2x(width mm x profile aspect ratio), the figures are nominal only. In practice, the manufaturers standardise to a nominal value close to the calculated value to preserve gearing ratios between tyre ratings. They choose a width and outside diameter first, then juggle the nomenclature with aspect ratios in 5% increments to suit the tyre as closely as possible. If you look at the table below, you will see what I mean. Any of the tyres listed, apart from the Cloud and 16" R-Type tyres listed, will not affect gearing by very much and not at all in most cases. A 225.75x15 will show a speedo reading 1.4% too low, and a 225.70x15 will show a reading 1.4% too high. These are both well within range. Many an early Shadow is shod with 235.70x15 tyres. They are fine so long as they don't foul the guards on individual vehicles. I had 235s on our T-Series for years, but do now prefer 215.75x15 as they are a little sharper in response. The Shadow rims are way too narrow even for a 205 in truth, and that is the reason why Michelin refused to sell tyres to Crewe for new cars as they insisted in rims of at least one inch additional width before they would consider Crewe as a customer. Unfortunately 215s are becoming hard to source. All the listed 15" tyres up to width 235mm are fine for these cars. The wider wheels and tyres will have no hope in those guards.  | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 1004 Registered: 4-2003 |
To answer the question directly, a 205.70x15 has a design diameter of 665mm (calculated 668) unsuitably small for a Silver Shadow, and will make the motor run more than 6% faster for a given road speed. A straight 205x15 (ie that is a 205.80x15 in fact) is as designed, and with these tyres the cars are already geared towards or beyond the ideal. A 225.75x15 tyre would be a safe bet. Witness that it is possible to overspeed the motor's 4500 RPM limit in top gear on an unrestricted highway at 120mph on standard tyres. | ||
| KC Saayman Frequent User Username: kc_saayman Post Number: 37 Registered: 10-2004 |
Thanks for your detailed and informative response Richard. You have cleared it up for me. Regards KC |
