nickb Posted January 30, 2008 Report Share Posted January 30, 2008 hi to all, am new here and wondering if anyone can help. I have porsche 968 club sport. I'd like a 2nd set of wheels (17") , so i can have a set for track tyres and one for road tyres. Getting hold of another set of originals is proving tricky (esp as the fronts are 7.5" which i believe were unique to the 968). So ive found an outfit called PWS who seem decent. They sell a new set of replica wheels, for a reasonable price, but the off-sets are different: front 7.5" original ET 65, replica ET 52 rear 9" original ET 55, rep ET 47 So what i need to know is to what extent the wider off-set will affect the geometry/set-up, because i want to be able to swap the sets of wheels, w/out havng to chg the geo (every time!) or affecting the cars handling in a material way. any suggestions wold be much appreciated thanks Nick Quote Link to comment Share on other sites More sharing options...
jon Posted January 30, 2008 Report Share Posted January 30, 2008 Hi Nick, You already know my thoughts on this (find a set with the correct offset so as not to change the SR) so I just thought I'd say welcome to WIM! Some of the regular geo guys are very busy with setting up Tony's new unit but I'm sure they'll be along to offer their opinion shortly. Quote Link to comment Share on other sites More sharing options...
Tony Posted January 30, 2008 Report Share Posted January 30, 2008 Hello nickb.... Jon is correct you can open a real can-O-worms doing this... The main concern is the change in the position of the scrub radius... You need to be wary if it bought the scrub radius too close to zero, if the radius changes sides during a manoeuvre then it will confuse the steering signals. Additionally it's possible to maintain the steering feel since signals in pneumatic trail changes as the tyre saturates so changes in the tyres aspect ratio and a small change in the mechanical trail will belay the zero SR (probabally) What sizes are the two sets of tyres you have? Quote Link to comment Share on other sites More sharing options...
nickb Posted January 30, 2008 Author Report Share Posted January 30, 2008 Hello nickb.... Jon is correct you can open a real can-O-worms doing this... The main concern is the change in the position of the scrub radius... You need to be wary if it bought the scrub radius too close to zero, if the radius changes sides during a manoeuvre then it will confuse the steering signals. Additionally it's possible to maintain the steering feel since signals in pneumatic trail changes as the tyre saturates so changes in the tyres aspect ratio and a small change in the mechanical trail will belay the zero SR (probabally) What sizes are the two sets of tyres you have? hi Tony, the tyres are 225/45/17 front and 255/40/17 rear. i've spoken to a couple of people who do geo set-up, and they thought it would be "ok". But i want to be as sure as i can. however, my basic plan is to use the original wheels (with toyo 888's) for track, and have the car set-up accordingly (and it wont be an agressive set-up by a long way). Then use the replicas for road. Thus, as long as the set-up wasnt affected too much, i'm thinking i could live with a sub-optimum set-up for road use, provided it was only by a small margin. if that makes sense.. ! Quote Link to comment Share on other sites More sharing options...
jon Posted January 30, 2008 Report Share Posted January 30, 2008 Nick, That sounds like a sound plan, the car doesn't see many road miles does it? As long as you're not trying to road rally I can't see the handling being so far off optimum it makes a huge difference. My bigger concern is whether the replica wheels with the decreased offsets will fit under the arches. If i remember from Saturday the stock wheels seem to fit quite well. The front pair of replicas are especially worrying as they will sit around a CM further out each side. Quote Link to comment Share on other sites More sharing options...
nickb Posted January 30, 2008 Author Report Share Posted January 30, 2008 Nick, That sounds like a sound plan, the car doesn't see many road miles does it? As long as you're not trying to road rally I can't see the handling being so far off optimum it makes a huge difference. My bigger concern is whether the replica wheels with the decreased offsets will fit under the arches. If i remember from Saturday the stock wheels seem to fit quite well. The front pair of replicas are especially worrying as they will sit around a CM further out each side. hi Jon, thx for that. i always want to use it on the road, but i dont have to, so I'd like to do about 3-5k miles pa on the raod (if i ever get the time!). Oddly, the only aestetic point about the car i think could be better is the front wheels, which altho they sit close to the top of the arch (and next geo i will probably lower the car again), they do sit some way inside. In other words, if i just wanted it to look better, i'd consider spacers on the front wheels. I've also been told by the guy that did the geo for me recently, that if the tyre does threaten the arch, then he can "bend the arch back" with a tool, which involves no pain (supposedly....), ie u keep the panel on and gently bend it, was my u/standing. Quote Link to comment Share on other sites More sharing options...
jon Posted January 30, 2008 Report Share Posted January 30, 2008 Yes, you can roll the arches. I had a tyre connecting with an arch at Donington, you can hear it on this video at the end of crainer curves in the depression: (by the way, this video is a great example of the first time out on a new setup and I'd not got the dampers or the pressures right, check out the oversteer!) At lunch time in the pits I got a nylon hammer and heat gun and hammered out the arches, job done Can't see any difference on the outside, only the inside. One option you do have is to space the original wheels to the same offset as the replicas, have the geo done then when you change wheels it won't matter. I certainly advise you get it on Tony's new Hunter Geo Machine (what's the ETA Tony?), the data that comes off it is mind boggling! Quote Link to comment Share on other sites More sharing options...
Tony Posted January 30, 2008 Report Share Posted January 30, 2008 nickb i'm sorry but i have to disagree with your Geo guy... Small changes in off-set 4-5-6mm my be less detectable but your is quite a transition... Since the point that intersects the ground is established by the SAI, and this is a mechanical system free from distortion and remains in the same place relative to the cars body, problem is this moves relative to the tyre as the tyre moves, ie turning, braking etc. On a plus note fitting lower profile tyres would move the SR points inboard Quote Link to comment Share on other sites More sharing options...
nickb Posted January 30, 2008 Author Report Share Posted January 30, 2008 Yes, you can roll the arches. I had a tyre connecting with an arch at Donington, you can hear it on this video at the end of crainer curves in the depression: (by the way, this video is a great example of the first time out on a new setup and I'd not got the dampers or the pressures right, check out the oversteer!) At lunch time in the pits I got a nylon hammer and heat gun and hammered out the arches, job done Can't see any difference on the outside, only the inside. One option you do have is to space the original wheels to the same offset as the replicas, have the geo done then when you change wheels it won't matter. I certainly advise you get it on Tony's new Hunter Geo Machine (what's the ETA Tony?), the data that comes off it is mind boggling! ok.. been speaking with JZM. THey'll do the track set up, apparently need unibore top mounts in order to get 3 camber. also they say replicas weigh about 2-4kg more, which (and i',m a bit thick when i come to physics..) he says means equivalent 100kg on the body. Quote Link to comment Share on other sites More sharing options...
Tony Posted January 30, 2008 Report Share Posted January 30, 2008 You should only need one kit per corner..... Jon.... Mid to late Feb Quote Link to comment Share on other sites More sharing options...
jon Posted January 30, 2008 Report Share Posted January 30, 2008 Rotational inertia is normally multiplied by 4 isn't it, so an average of 3kg a wheel extra would mean 48kgs, not 100kgs. Another option is to get a good set of forged wheels, a set of volk te37s would be ideal if they came in your sizes as they're one of the lightest forged wheels on the market (however they cost about £1600 a set) Where did 3 degrees of camber come from? I'd learn to walk before running and opt for something less aggressive while you're getting used to the car. Quote Link to comment Share on other sites More sharing options...
Tony Posted January 30, 2008 Report Share Posted January 30, 2008 Rotational inertia is normally multiplied by 4 isn't it, so an average of 3kg a wheel extra would mean 48kgs, not 100kgs. Another option is to get a good set of forged wheels, a set of volk te37s would be ideal if they came in your sizes as they're one of the lightest forged wheels on the market (however they cost about £1600 a set) Where did 3 degrees of camber come from? I'd learn to walk before running and opt for something less aggressive while you're getting used to the car. I concur 3 degrees is a hell of a lot (bye bye tyres) if used on the road... Anyway the car would need around two on the rear to boarder a reasonable corner-out transition, is the car capable of this type of adjustment at the rear? Quote Link to comment Share on other sites More sharing options...
Rich Posted January 31, 2008 Report Share Posted January 31, 2008 Would this only become a major problem because of the difference in rim width and tyre size, as well as the different offset? My last set of wheels had different offsets, I bought them off ebay! Before anyone says it, yes I did check the offset before buying them and was told they were all ET35. However, the rears were ET20, one front was ET35 and the other ET29!! Had them on for a good year and it was driveable, however, to be honest I was never happy with how the car handled, although in part that was down to the tyres as well. My current wheels have the same offset and after having them fitted the difference between them and the others was very noticeable! Quote Link to comment Share on other sites More sharing options...
nickb Posted January 31, 2008 Author Report Share Posted January 31, 2008 ok... according to these guys http://www.dymag.com the ratio is something like 6:1 (tho of course they're trying to flog wheels to you), but is similar to JZM figure. I suggested a progression in terms of tightenting up the handling. JZM think they know best . Have even been tod by someone that after setting up the car for him, they refused to tell him excavlty what settings they used, because it was their "intellectual property". yeah... When Calum Lockie drove my car, he said it could do with 2 to 2.5 deg of camber. So i'm going to tell JZM to add a deg and take it to two. Steel top mounts not a bad idea, so i go along with that. net net, i agree, i want to keep the car relatively tame. So will get jzm to set-up, run it this year, then when i have a better sense of what i need//want, i'll take it elsewhere. i think he has a point about the wheels, and i'll just run the 888's on road, and see how it goes. and be patient to find an orignal set. and one of the above coments aobut the o/set changing the handling is what Don Palmer told me. I had a day with him last week. I'm technically challenged but i thought he said that the pivot point of the wheel is fixed, so a different o/set means you get a different contact patch on the road. his adivce was stick to originals. all your advice and help is very very appreciated. Nick Quote Link to comment Share on other sites More sharing options...
jon Posted January 31, 2008 Report Share Posted January 31, 2008 Dymag use carbon for their outer wheel meaning they save more weight further from the centre hence the bigger ratio. I don't think this can be applied to the usual forged vs cast debate as they have similar weight distribution. Where are JZM based? Close to us? Quote Link to comment Share on other sites More sharing options...
nickb Posted January 31, 2008 Author Report Share Posted January 31, 2008 Dymag use carbon for their outer wheel meaning they save more weight further from the centre hence the bigger ratio. I don't think this can be applied to the usual forged vs cast debate as they have similar weight distribution. Where are JZM based? Close to us? yes, and to be entireyl frank, it saves me takign a day off work (which is going bananas), as they are in kings langely and i can drop off in mrng and get train to Ldn form there. i spoke with Calum who thot 3 was fine for front. and many of the 968 chapsrun 3 at front. everyone i spoken to say jzm know what they're doing with gt3's and 968's etc, and are v good. So for now i'll use them for initisl set-up, run it thru the summer and take to your man at hemel in the autumn for reviewing. Quote Link to comment Share on other sites More sharing options...
nickb Posted January 31, 2008 Author Report Share Posted January 31, 2008 fwiw, found a solution to one of my problems... found a breakers breaking a 968 and the original wheels are intact (most of the car's intact for that matter!) So i'm going for those. cheers nb Quote Link to comment Share on other sites More sharing options...
Tony Posted January 31, 2008 Report Share Posted January 31, 2008 fwiw, found a solution to one of my problems... found a breakers breaking a 968 and the original wheels are intact (most of the car's intact for that matter!) So i'm going for those. cheers nb Result Quote Link to comment Share on other sites More sharing options...
jon Posted January 31, 2008 Report Share Posted January 31, 2008 i spoke with Calum who thot 3 was fine for front. and many of the 968 chapsrun 3 at front. I'm sure -3 at the front is fine, however my point was as you're, by your own admission quite new to trackdays I'd go slightly less aggressive until you understand the car under you then make your choice. Geometry isn't going to find you seconds of pace, it's there to give you a car your comfortable driving hard at the limit and a balance that suits your style. For instance my setup works for me, but everyone that drives my car is scared by the rear end being so loose. fwiw, found a solution to one of my problems... found a breakers breaking a 968 and the original wheels are intact (most of the car's intact for that matter!) So i'm going for those. cheers nb Now that is a result! Quote Link to comment Share on other sites More sharing options...
nickb Posted January 31, 2008 Author Report Share Posted January 31, 2008 i spoke with Calum who thot 3 was fine for front. and many of the 968 chapsrun 3 at front. I'm sure -3 at the front is fine, however my point was as you're, by your own admission quite new to trackdays I'd go slightly less aggressive until you understand the car under you then make your choice. Geometry isn't going to find you seconds of pace, it's there to give you a car your comfortable driving hard at the limit and a balance that suits your style. For instance my setup works for me, but everyone that drives my car is scared by the rear end being so loose. fwiw, found a solution to one of my problems... found a breakers breaking a 968 and the original wheels are intact (most of the car's intact for that matter!) So i'm going for those. cheers nb Now that is a result! yes! had to pay up for em a bit, but i thot best to buy them while they were there. they seem rarer than rocking horse sh*t! tho they do come with mich pilot sports , so can use them for an airfiled tuiition day, as a set i can destroy! (unless they're in good nick, which i doubt). spoke to donnington, they souhld be posting their calendar for march soon, so will mail u if there's a date that loks suitable. am going down to see amate of mine who used to rally (international), and we'll go for blast round castle comb which is near him (i "think" i trust him with my car). ...anyway, he sent me this clip of CC from utube, to make me at ease.... oh, and i ordered those books form amazon u suggested, thx for that. cheers Nick Quote Link to comment Share on other sites More sharing options...
Tony Posted February 2, 2008 Report Share Posted February 2, 2008 i spoke with Calum who thot 3 was fine for front. and many of the 968 chapsrun 3 at front. I'm sure -3 at the front is fine, however my point was as you're, by your own admission quite new to trackdays I'd go slightly less aggressive until you understand the car under you then make your choice. Geometry isn't going to find you seconds of pace, it's there to give you a car your comfortable driving hard at the limit and a balance that suits your style. For instance my setup works for me, but everyone that drives my car is scared by the rear end being so loose. fwiw, found a solution to one of my problems... found a breakers breaking a 968 and the original wheels are intact (most of the car's intact for that matter!) So i'm going for those. cheers nb Now that is a result! Jon your rear Geometry goes against the grain for most people.... Personally i like it but it takes a particular type of driver to handle it Quote Link to comment Share on other sites More sharing options...
jon Posted February 3, 2008 Report Share Posted February 3, 2008 Jon your rear Geometry goes against the grain for most people.... Personally i like it but it takes a particular type of driver to handle it Works for me Quote Link to comment Share on other sites More sharing options...
Tony Posted February 3, 2008 Report Share Posted February 3, 2008 Jon your rear Geometry goes against the grain for most people.... Personally i like it but it takes a particular type of driver to handle it Works for me Me too... I liked the evolution, although i think we have loads of work to do on the new imaging equipment.... The information gathering is, well it's unbelievable. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.