15inch rear tyres for Cobras

[Alan Faulkner-Stevens]

For the many people who run 15inch wheels on their Cobras getting large rear tyres has been a serious problem due to a complete lack of suitable speed rated tyres. However I was at the Silverstone Historic Festival and passed the Avon tyre stand only to spot some large width section rubber. On discussion with the member of staff it transpires Avon have specifically manufactured some large width sized tyres for Cobra's and GT40's. These new tyres are road legal race spec. and are available now. I looked at some 295/50xVR15 which are an exact conversion for the 16inch tyres currently on the back of my CRS.
   The Avon guy did not have any spec. sheets on the tyres as they were so new they had only been delivered to him the day before, but he promised to put a spec. sheet in the post to me. As soon as it arrives I'll post more information for interested parties.
   regards Alan Faulkner-Stevens

[C9OBY]

Hi Alan
   
   Two questions:
   1. From the size that you mention, I assume that these new tyres require a 15-inch rim, hence new rims for the CRS (or are they also doing a 16-inch variant?)
   2. Do they have the equivalent (matching) front set in the same compound (225's on mine)?
   
   Separately, I am interested to know how the compound compares to the Avon ZZ-1's I have on the car at the moment which are almost useless in the wet and not much better in the dry! Hopefully the spec sheets will help with this info.
   
   Cheers, Bruce
   
   PS: Perhaps Avon will give you a free set to 'road-test' on behalf of the club?!!

[Alan Faulkner-Stevens]

Bruce, the new Avon tyres are designed for 15inch wheels. The 15inch wheels available are 7.5inches wide for the front and 9.5inches wide for the rear. Current 16inch wheels are 7 at the front and 8 at the rear. I parked next to a Lightweight at Silverstone last weekend and the larger wheels really charnge the appearance of the car. When I have the spec. sheet to hand I can make some suggestions for both a front and rear conversion for owners wanting to go from 16inch to 15inch. I'll keep you posted about this.
   As to you ZZ1 problems, I have the same tyres on my own CRS and find the tyres satisfactory, just how hard are you pushing your car? My only comment about tyres is this, I worked with Avons directly when I was working for Aston Martin and I have always favoured their tyres out of choice, but it may be your tyres are just getting to old. If they are the original set and your car was produced in say 2000 they are at least 6 years old. They may have been manufactured up to a year before being fitted to the car making them even older. The older tyres get the harder they go, thus making grip harder to come by. Also what pressure are you running in them?
   regards Alan

[MkIV Lux]

Hello Allen,
   
   Thanks for these complements of info. I just wonder how 295/50 VR 15 do compare in diameter and width to the original 255/50 VR 16 ?
   
   As mentioned earlier on, MkIVs were originally equipped with 15" rims, later switched to 16", as on the CRSs. 17" Halibrands have also been available, though not AC factory original.
   
   For 16" rimmed MkIVs and CRSs, the correct original tire spec is 225/50 VR 16 front and 255/50 VR 16 rear.
   
   Michelin are offering again the Pilot Sport in the correct 225 and 255/50 ZR 16 size (Y rated), in their Collector Tyre program. My dealer here in Luxembourg has quoted a price of 192 € front and 355 € rear apiece & mounting & VAT. Delivery within a week. Sizes of the Pilot Sport are: (width / ext. diameter / circumference)
   Front  225/50 ZR 16 : 242 mm / 642 mm / 1928 mm
   Rear   255/50 ZR 16 : 276 mm / 672 mm / 2019 mm
   
   PS for Bruce: 4-digit number usually in a “rectangle with rounded edges” indicates week and year manufactured  (example 2403 means 24th week of 2003)
   
   Best regards
   Constant

[C9OBY]

Hi Alan, Hi Constant
   
   Alan, do you by chance have any photo's showing the difference between the two cars with different tyres? Personally I think the 15" rims give the car a more classic look, but I do like the aesthetic of the 16" rims. The wheel arches do look 'fuller' with the 15's though.
   
   Constant, regarding your question of difference in tyre diameter, take a look at the following website:
   http://www.rochfordtyres.co.uk/tyrecalc.asp
   
   Per their calculator, the 255/50/16 has a diameter of 661.4mm (circumference of 2077.85mm) whereas the 295/50/15 has a slightly larger diameter of 676mm (circumference 2123.71mm)).
   The website suggests that you should try keeping the tyre radius within 2.5% of the original. The 295 tyre is 2.21% bigger than 255 tyre, indicating that the tyre is within this range, making it a suitable alternative.
   
   I do believe that the diameter will not be a problem when switching from 16" to 15" as there is plenty of space under the wheel arch. However, I wonder how the turing circle will be affected by the wider wheel rims (current 16" 7/8 front/rear rims versus the 15" 7.5/9.5). Alan, I look forward to reading your conversion advice.
   
   Regarding my tyres.... I don't know how to benchmark how hard I'm pushing my car.  It's normally pretty quick and I enjoy using the full range of power when suitable conditions / surroundings permit.  One reference point was taking the CRS around the Nurburgring yast year. Despite two laps of the track as fast as I could (lots of tyre screeching and a few 'moments') the tyres were barely warm. I parked up, jumped out and put my hand on the tyre; the rubber didn't have any 'softness' to it and had the same temperature after being parked for 20 minutes. This is vastly different to the michelins on my motorbike which get nice and sticky after a bit of action.
   
   Does anyone have any advice on suspension setup?  Where could I go / who could I speak to about checking that my car is correctly set, as I sometimes feel that there is not sufficuent feedback through the suspension telling me what the suspension and tyres are doing.
   
   Thanks for the tips on checking the tyres age; I'll check them tonight. I'll also check the tyre pressures, although from memory I am probably using about 2.5bar in front and rear (about 36psi).
   
   Cheers
   Bruce

[Alan Faulkner-Stevens]

Dear Bruce, I am looking at the 295/50x15's to go directly on the back of my CRS instead of its 16inch wheels. I believe the rolling radii between the two tyres to be so similar as to not affect the speedo reading. When I have the spec. sheet I can find a suitable front conversion. I also think the tyres used on the front will be a similar width to the ones used on 16inch rims so I dont forsee a turning problem. These I already know about using another tyre manufacturer but I was hoping to have 4 matching tyres. As to 36psi...much to hard!!!! Try 28 all round and see how that feels. Alan

[cobham cobra]

Hi C9OBY,
   I have a few photos of the wheel and tyre combo I took at the National Day and in Le Mans. Let me have an e-mail address and I can send them to you on Monday.
   John.

[C9OBY]

Hi Alan,
   
   I took your advice and reduced the pressure in all tyres.  What a difference - I'm not sure if I have exactly the right balance just yet as the tail felt a little 'squidgy'.  Maybe too little pressure? Perhaps I'm just used to the rasor-sharp response I had from the tyres at the higher pressure.
   
   The car also rides over rough surfaces/manhole covers with far more comfort than before.  I took it around some remote roundabouts on Sunday far away from traffic and I definitely had more feel. I couldn't tell if grip was better, but at least I knew when it was about to let go!
   
   Cheers, Bruce

[Alan Faulkner-Stevens]

Bruce, please also check your rear damper adjustment. It may be the soft feeling on the rear of your car may be down to softer damping by the shock absorber. I'm fairly sure the damping settings are adjustable, so go up one setting and try this on the road again. Does your car have a rear anti-roll bar? If it does fine, but if not adding one will help rear end stability. Alan

[Mark-Anthony]

Hi Guys
   
   I have the original tyres on my CRS (number 22), no rear anti-roll bar, and at 28lbs I think the tyres are still shockingly short of grip.  I now know they were manufactured in 1999 (Thanks Alan) and I hope that the lack of grip is down to the fact that they're now too old. 3000 miles in 7 years?  I'd hate to buy new tyres only to have to go out AND buy new tyres again and new wheels.
   
   Personally I think I'll try to find some sticky rubber for the 16" wheel first before I do the new tyres and new wheels route.
   
   It might be worth a mention that the adjustable rear shocks for the CRS are no longer manufactured so if they're weeping you have to get them re-built (always do both).
   
   Mark-A

[C9OBY]

Following on from Alan's investigative work (and numbers given to me by him) I have the following information on the Avon CR6ZZ tyres:
   
   Call Avon racing department (in the UK) and speak to Brian on 01225 357 694. He gave me the following info:
   
   The CR6ZZ tyres are effectively a road-legal race-compound tyre, promising much better grip in the dry and wet.
   
   The 215/60/15's are each £147+VAT and the 275/55/15's are £188+VAT.
   
   Avon are also doing a 295/55/15 tyre at £194+VAT, however it is suggested that the 275 rear is a better match for the 215 front, giving better balance.  Some customers have been requesting a wider tyre for the front (235/60/15) in order to balance the wider 295 rear, but it is unlikely that this will be manufactured within the next 2 years.
   
   They are based near Bath, but they will post them overnight to London for around £16.
   
   Take a look at http://www.rochfordtyres.co.uk/tyrecalc.asp to see the difference in tyre diameters etc

[MkIV Lux]

Thanks Bruce for the complement of info.
   
   What will the MOT say about this, or alteratively, is there any chance to get an official statement (even earlyer in period) from AC about different tyre measures that are homologated for road use for the MkIV?
   
   By the way I also have a set of 18" Halibrand rims that were developed by Uniclip some 3 or so years ago.
   
   Regards
   Constant

[jbottini]

quote:Originally posted by Mark-Anthony
   
Hi Guys
   
   I have the original tyres on my CRS (number 22), no rear anti-roll bar, and at 28lbs I think the tyres are still shockingly short of grip.  I now know they were manufactured in 1999 (Thanks Alan) and I hope that the lack of grip is down to the fact that they're now too old. 3000 miles in 7 years?  I'd hate to buy new tyres only to have to go out AND buy new tyres again and new wheels.
   
   Personally I think I'll try to find some sticky rubber for the 16" wheel first before I do the new tyres and new wheels route.
   
   It might be worth a mention that the adjustable rear shocks for the CRS are no longer manufactured so if they're weeping you have to get them re-built (always do both).
   
   Mark-A
   

[jbottini]

Originally posted by Mark-Anthony
   
Hi Guys
   
   I have the original tyres on my CRS (number 22), no rear anti-roll bar, and at 28lbs I think the tyres are still shockingly short of grip.  I now know they were manufactured in 1999 (Thanks Alan) and I hope that the lack of grip is down to the fact that they're now too old. 3000 miles in 7 years?  I'd hate to buy new tyres only to have to go out AND buy new tyres again and new wheels.
   
   Personally I think I'll try to find some sticky rubber for the 16" wheel first before I do the new tyres and new wheels route.
   
   It might be worth a mention that the adjustable rear shocks for the CRS are no longer manufactured so if they're weeping you have to get them re-built (always do both).
   
   Mark
   I had an issue here in the states finding rubber for the f & R of the 16's. While they are not optimal for racing i find the Dunlop SP8000's in 255 & 225 50x16 very nice, but then I'm comparing them to the 20 year old originals that came on the car

[Mark-Anthony]

Hi Jbottini
   
   Many thanks for the info on tyres.  I'll checjk them out and let all know.
   
   Mark-Anthony