pmcrespo

I just spent several hours searching but outside of many, many discussions, I really could not find a definitive answer.

I want to use 18 x 10 wheels with 285 or 295 all the way around (R6 mainly). 968 Race car, stiffly sprung with Motons. Obviously lowered/cambered. MO30 brakes. Will roll fenders.

What is the IDEAL offset? I presume the answer will be an "IDEAL" offset to fit the front, with a small spacer for the back?

Thanks in advance as always...

RedlineMan

Hmmm...

I can't answer your question, but I also can't help commenting. Sorry. That is simply too much tire, especially in the front. Anything less than 400RWHP and you will be losing speed.

IcemanG17

I was thinking the same thing......

Is there some type of formula that equates tire size (width) to the weight-HP of a racecar? Miatas handle VERY well on tiny 205 tires...& my heavy 928 at nearly 3100lbs wet with driver handles VERY well on 245's all around....but its also under 200whp too

IcemanG17

I've been researching this concept a bit..... On my current 200whp 3100lb racer I run 16x8 wheels with 245/45-16 BFG R1's.. the car handles GREAT lots of grip and a touch of understeer at the limit.... Each wheel-tire weighs 42lbs

Now if I were to switch to an 18x10 65mm offset wheel made by OZ that weighs 19lbs and use a 285/30-18 BFG R1 that weighs 24 lbs my total wheel-tire weight is only 43lbs..or just 1lb heavier than before.....and the rolling diameter is nearly identical too..... So how much would 4lbs unsprung weight really hurt you?

I understand that at my power level I don't really need more tire...and having too "wide" tires could cause the tires not to reach operating temps and end up having less grip than a narrower tire would....

Just trying to figure this out

mikew968

Unless you are running a 968/944s2 and comparing tires/times I not really sure you would know the right tire combination.

Every track record at every track NASA Great lakes runs in has been set on 285-30-18's. This would include Mid Ohio club and pro course, BeaveRun, Putnam Park. I'm not sure about Grattan. I see some lost speed at the end of the back straight at MO but faster everywhere else. On top of the times is the tire life. I have run within 1 sec of my pb on the 26th hc on R-6's.

RedlineMan

Hmmm...

I'm sure there are guys that are way smarter than me... well... I KNOW that... that can do some fancy math to calculate what tire is best for any given vehicle. What I know from days past is that the top GT3S 911 campaigners were all running 245 fronts. Some of the guys tried 275 and thought it worked, but more tire is not always the answer, especially if there was no question being asked. The question would be real -vs perceived need. If your tires are suffering for some reason - too much weight, too much HP, etc. - then a change in tire might be indicated. Otherwise, you want the smallest tire you can get away with. The losses are likely rolling resistance, aerodynamic inefficiency, and weight. Better gains might come from a change in chassis setup or driving style than just slapping bigger tires on with their aforementioned negatives.

IcemanG17

Mike
What are the specs of your 968 racer....like weight & weight balance....spring rates and WHP-torque?

Rich Sandor

My 968 has hankook z214 275's all around (track car) and my buddy's 951 (which I co-drive for autox) has 285/295 hoosier A6s.

For autocross and tight tracks, there is a definite, empirical gain with the wider slicks. For track days, I find 275's enough on the 968, however, there are times when I feel I could use more tire even then.

I have driven all the combinations possible on these cars, and I still prefer to run as much tire as possible all around.

Lemming

I moved to running 275/40-17's on all four corners at the beginning of the 2009 race season. At the same time I also added a front splitter and rear wing, thus it if difficult for me to say which of these changes is the main factor in my setting my new lap records for every track that I visited in 2009 (of course one could also argue that I was bcoming a better driver; lap times decreased nearly 1.5 seconds at BMP and RA). What I do know is:

1. Top speed at the end of long straights was decreased significantly (5 mph breaking for 10A at Road Atlanta).

2. Cornering speed has increased.

3. My tires are still good at the end of a 90 minute enduro (R6's). During my last PBOC enduro race I set my fastest laps in the last 15 minutes of the race.

4. My tires do not come up to temperature as quickly, this is a problem when the track and air temperatures are below 70F and I am clearly at a disadvantage for the first lap or two (on R6's).

5. On cold days (<70F) I can easily run A6's and keep them going for up to 45 min and they are great from lap 1 to lap xx.

Just to clarify, this was on Franky I, 227 rwhp with a race weight of 2,506 lbs in NASA GTS3 trim. Franky II will continue to run 275's on all 4 corners with an anticipated rwhp of 300 and race weight of 2,550 lbs.

disasterman

The original question was also about offsets. I think that most guys running in GTS Great Lakes are running a 60 offset with a space used in the front so they can use four equal wheels. Mike could clarify.

pmcrespo

Thanks! That is the confirmation I needed. Seems like 60-61 offset is what works best.

Thanks for all the other comments and insights. Philosophically, we can rationalize and discuss what should or should not work based on opinions, math, thoughts, theories, etc... At the end of the day, based on my own personal set of parameters; lap times is what matters most. Within that, is the ability to out-brake and carry more speed through the entire turn at the majority of turn types I will encounter in the tracks I visit. Lastly I also look for the benefit in tire wear/life as a result of having the ability to rotate tires freely among other things.

Based on this, and that all 3 above are met with 285s all around, it is an easy decision. Worst case scenario is that it does not work at all tracks. In these cases, I can still go to the 8.5 fronts as I will have them on the tire rack. I also have much respect and trust in those that are running the setup already. These guys are at the cutting edge of developing the setups for these cars (968 and 944S2) and they show it every weekend with dropping lap times (and records).

With that said, the Aero Package makes a big difference when done properly which will have an impact on the front 285s' effectiveness.

Cheers!

RedlineMan

Hey;

There are certainly many different parameters involved here, and you have to winnow it down and choose a few to concentrate on or you'll go nuts. I will stick with my statement that you only need what you need, and anything more is a detriment. One thing to remember regarding racing; there are lap times, and then there is actually racing someone. If you find yourself actually racing wheel-to-wheel often (-vs just out there "racing" around by yourself most of the time), and you are giving up a chunk of straightaway speed, you'll get eaten alive. If the track is about straights (WGI, R/AT., R/Amer, etc.) then go small, light, and aero. If it's all corners (M/O, Barber, etc.) or tire chewing pavement, then fat boys might be best.

It all depends on how fine you want to cut it.

Lemming

I'm just going to add HP/TQ to overcome the rolling resistance and aero drag

John, can you fab me up a rear wing stall device for the straights?


It would be interesting to know what width tires they ran in the Trans Am series.

mikew968

I'm not sure of my off sets but Dave at Forgeline would know as he made mine. I use the f/r interchangably and am getting great tire life...like 20-25 good cycles unless I run at Putnam Park!!!!!

pmcrespo

Thanks Mike. I did reach out to Dave already after Jim C's recommendation...

Cheers!