vinniemc

Hello CF, I'm in my second season of autocross with my 1992 coupe that is a 6-spd base model with Z07 shocks, sway bars, and front brakes. The course I run on is about 80 seconds.

My current tire is a square setup of 275 Nitto NT05s. I have an opportunity to get some scrubs in my size (BFG R1-S) and I'm looking for feedback as to how much of a difference they make. I understand the difference is pretty substantial, I'm just curious how many seconds I could knock off my time!

Solofast

Rick Ruth came up with a .975 factor for a street tire PAX index. That would be worth about 2 seconds on an 80 second course. I think that's probably pretty close.

naschmitz

Look at some of the results for the NE-SVT, SCMC, or BSCC events at Devens. They class by tire. You should be able to see what the average benefit of a race tire vs a street tire is on those courses.

vinniemc

Yeah I always race with SVT and BSCC at Devens but it's quite hard to tell because I don't really know where people's skill levels are. Looking at the last event, 2-3 seconds seems about right. I guess I'll have to get the tires and find out!

lefrog

There are not that many C4s running anymore with these clubs and the level of preps of cars are so vastly different that it is impossible to compare times in a meaningful way.
Between the slicks and the NT05 on the C5, there is between 3 and 5 seconds depending on the course.

vinniemc

Agreed, it's so hard to compare...

And that's a crazy difference on the Z06! 3 seconds is a lot!

naschmitz

LOL "It's impossible to tell, but the answer is 3-5 secs"

lefrog

I was talking about just my car with the 2 different setups. Comparing his car to any other car out there is going to be hard.
The NT05 is not the best street tire either so that might also explain why there is such a gap. But when you run with BSCC or NE-SVT, the courses are pretty fast and you can carry so much more speed with the slicks that it makes a huge difference in the end.

UrbanKnight

At my corvette club, on our 45 sec course, I gained two seconds on race tires over streets, but they were pilot super sports.

On an 80 second course, I would guess 3-4 seconds

Bill Dearborn

I can give you a little reference to work with. I used to autocross on a Go Kart track in upstate NY. The course was always the same so it removed course variance from the equation. Given the weather was similar between different days when I was running my C5 on R Compounds (Kumho 710s) Vs GY EMTs the R Comps were good for 4 seconds on runs that were in the 128 sec range.


Bill

Solofast

Bill's comparison is between R comps and a run flat. The newer generation of street tires are much closer than those two tires.

Understand how Rick Ruth develops the PAX index. It is an exhaustive process. He goes through reams of data from SCCA national tour, pro solo and SCCA national events. Then he compares times for each class winning car and driver and compares the variations within the class and between the classes. At these events the winning drivers are all exceptional and Rick's analysis allows him to compare the times of cars and then let him boil down the data and derive an index. Added to that is the fact that he has been doing this for years and has a huge amount of background data on each class.

What folks are doing here is trying to get one data point of comparison and then try to get an idea of what the difference is. Rick has reduced data from hundreds of classes running at top level events every year and built a database and factored out the perturbations of weather and surfaces to get something that is very very good at predicting the performance. Comparing similar cars running in different classes, some with street tires and some with R comps, has allowed him to derive an index for comparing street tires with R comps.

It has been my experience that the PAX index is an excellent way to compare your performance with how you will do at nationals, provided there are some other national level drivers running at the same event. If Rick published an index of .975 for the top of the line street tires as compared to R comps, I would believe it and just use that factor and not even think twice about it.

jamesNewman

Strano/Junior and I tested my A-Stock car on Rival's last season. The delta was about 3 seconds over the course of a 45 second course between A6's and street tires. Lane seems to have gotten his car working fairly well with them, so I'm sure there could be more time eeked out of it.

ltborg

Thanks Newman. The car is certainly working, although I just bolted on Hankooks, so we'll see if there is more to be gained from a different setup and/or different brand.

I've seen deltas mentioned between 1-3 seconds, but it depends on the length of the course, the car, which tires your comparing, etc. About 8 or so years ago I was told that race tires made a 2 second improvement on a 60 second course, but I was not given data to support that. It was just gossip in the paddock.

As Solofast said, Rick does a pretty good job on PAX. I am genuinely surprised how good it is. Using ASR and BS as an example (just because that's how my car is classed), on a 60 second course the difference would be 1.2 seconds. Just based on the subjective feel of my car, I'd say that's about right and based on using the Hankooks, most of that is given up laterally (sweepers, slaloms).

jamesNewman

On the Rivals we tested on, we had serious longitudinal issues (mainly braking). Car was pulling *near* the same peak G as A6's but didn't like to be shocked at all and couldn't hold sustained like an A6. Also, to note, we didn't change the setup on the car at all (was way too close to nationals to go fiddling with things), so I'm sure with a bit more rear toe, we could have solved some of the longitudinal problems (acceleration out of corners).

I watched your videos from the TX Tour, and it looks like the car is working pretty well!

ltborg

Thanks man! It definitely worked better than I expected, but there is still room for improvement. In my B-Street build thread I have a picture from the first event (the Texas Tour) and you can see how much dive I was getting. Jerry (my codriver) complimented my car on how well it stopped compared to his FRC. I'm sure there are some alignment tweaks I can make, but overall, I just think I need to increase the total capacity and make the entry oversteer a little more predictable.

Back to the original post, I haven't seen any comparison done between the NT-05's and the R1-S's so I'm not sure if the test data you want is out there. My guess is on an 80 second course, you'd get somewhere in the 2-3 second range from grip, but then have to adjust that for balance. If the BFG's give the car a better balance, they could make you a bunch faster even at the same grip levels. Likewise, the opposite could occur. That said, assuming it's rules legal, you can adjust the balance with the suspension to compensate.

I think that's part of the reason you see big differences in people's estimates. I don't think anyone (or at most, VERY few people) have set up a car on an ideal setup for one tire, done the same for a second tire, then tested those complete setups back to back. Most people just bolt on the new rubber and watch what happens.

FASTFATBOY

Most people start the NT-05 out with too much pressure.

I am on a course, not autoX.

I am 5.5 seconds quicker on BFG R1's vs NT-05 of the same size.

I added .6 more neg camber.

Race Prepared

autocross street tire = Proxies @ 140 tread wear, they work great!

troyguitar

Too bad they're already not allowed in some competition and next year won't be allowed in any of them that I know of... everyone is going to 200 treadwear.

AverageVetteNut

I know one of our local guys ran the R1s on his car ( Porsche 968 ) and he said it took allot more heat to get them to work like his A6s on the car but he liked them enough to switch from the A6 last year.

We have another local guy running the Dunlop ZII on his 93 coupe and they work really well.

Like other's have said, it's all going to come down to prep and pavement as well as tire choice to see the difference.

I say get them and try them out, but look what your local regions are going to do about people not in SSR that want to run R-comps. Might not be worth getting if you can't run them somewhere.

RallyeChris

I found the same thing. It seems they like very low pressures for a steet tire. More similar to R6's I used to run on my BMWs. Out of curiosity, what did you find we're optimal hot pressures for the nt05's? I am running 275/40/17 front, 295/35/18 rear on my 02 Z06.

Thanks

Chris