The "Great Tire" Dilemma (GTN, GTW, GTU)

The addition of tire sub classes to this year's Challenge has got me reevaluating my tire options and I think this could be a good discussion topic for competitors. In my case I was planning on staying with the 6yr old Hoosier S80 slicks pinched on to 6.5” wide wheels and swapping out the front wheels for some narrower all season tires for the drags, but with the new dimension of multiple classes I am considering spending money on new 200tw tires (I can’t believe I just said that!) and entering in the Great Tire Narrow class. My thought is that new more appropriately sized 200tw tires may actually be as fast in the autocross as the old slicks and since the sidewalls are softer than the Hoosiers they should be more forgiving on the drag launch than the Hoosier road race slicks. (I also wouldn’t be in the same class as guys running A7’s and wrinkle wall drag slicks.)

If I had wider wheels I would be leaning more towards running the Hoosiers and just doing a tire treatment to rejuvenate the tread, but for the past few years I have not been able to find any budget friendly 15x8 wheels that fit. So I am seriously considering the 200tw class instead. 

I currently have three choices for wheels: 15x6 NB Miata wheels, 15x6.5 no name wheels, and 17x7 Kosei K1-TS.

I have tabulated my tire and wheel combinations including the cost to budget per wheel and the out of pocket cost for the tires.(zoom in on the spreadsheet below) At this point I have not put drag tires into the mix because that would require making room in the budget for another pair of wheels and a pair of tires and drag tire run $225 - $275 each.(used drag tires seem hard to find in the right size)

Car Specs:

• Weight: 2150 lbs (w/o driver)
• 200+ HP
• LSA front suspension
• Solid axle with torque arm rear suspension
• 96" Wheelbase
• 52" Track width

What are your thoughts?

Do you ever street drive the car?

If I were spending your money (haha), I would probably go with the Miata wheels and 205 street tires. Would that get you into the "great tire narrow" class?

May also free up some budget for other shenanigans?

I do love the bronze of those 6 spoke wheels though. They would probably look the best imo. Maybe paint the Miata wheels bronze?

In your spot, this decision comes down to an out of pocket increase vs potential finish position. It is a tough choice with many factors involved. I agree with the validity in your criteria and I think this is an overlooked situation.

I just wanted to say I see you, and your car is berkeleying cool. I think you got a raw deal on the wear and tear paint decision and now dealing with this; I am glad you are still considering attending, no matter what tires you choose.

2 thoughts -

1. Like most Challenge cars, I doubt you will need drag tires. What ET do you honestly expect to run? A7's and 200TW are going to have similarly stiff sidewalls (bad for drag) but very grippy compounds (good for drag). With A7's on a 2500 lb RWD car with 260hp +75hp nitrous shot, Datsaniti was digging into 13.1 with a 60ft of 2.1. That's a pretty decent launch, and that car's power:weight exceeds yours. I came back to a drag strip a couple weeks later with drag radials and ran a 12.9 (1.9 60 ft), but I also sprayed way earlier, so I don't know if the drag tires actually helped me compared to the A7s. All that to say I don't think you need to worry about drag tires at all, regardless of which tires you choose. The drag strip is extremely sticky on its own.

2. After-Challenge life - do you drive this car on the street often, and if so, do you need new tires for it? 200TW are a great option for a "toy". No need to pack tools and swap wheels all the time (including at the Challenge). Just drive to the track or autocross, race, and drive home. If that sounds good to you, then just decide which wheels you like best. Like for like, the 17x7s should give you marginally more grip than the 15x6.5s. But 17s might look goofy on a car that small. 

As far as drag launches on 200tw: mine dont seem to hook that great for it. Lots of spin on my v6 miata with either the 205 federal rs-rr or the 245 rs4 

But the 200tw stuff is definitely faster around the cones tha the old spec miata slicks were.

This car is road legal and ideally I would drive it regularly, but in reality ever since I did my first autocross and time trial I am always finding just one more thing that I want to improve. So I take it apart to modify or upgrade something and it doesn't get back on the road for a long time.

I have a set of better looking 17x8 wheels for the street, but they were purchased new and not budget friendly.

In reply to maschinenbau :

I want to shoot for an ET in the low 13's to high 12's.  Previous to the 2019 Challenge I had never managed to get traction. It would go something like this: release the clutch and spin like crazy, shift to second and continue with excessive spinning, short shift to third and bog because actual vehicle speed is too low, then shift back to second and go.

In 2019 I warmed up the road race slicks with a burnout before pulling up to the line. With the warm slicks it dead hooked and snapped the front u-joint at the light tree. Very anticlimactic.

I don't want excessive wheel spin but I also don't want to repeat that 2019 launch.

In reply to bigben :

Man your car looks awesome. Nevermind what I said about 17s, they look perfect on there. I suppose my Datsun hooked better due to being automatic - less jerkiness and driveline shock to break traction (or U-joints...)

I feel like unlimited tire option puts you into needing 6 wheels and tires.

The 200tw is a much easier class because you can buy one set of new tires and run them all day. Also saves time by not needing to find used tires of unknown use. 

In reply to bigben :

Do you have a drag strip close to you where you could try out your tire options for launch potential? I think a little test and tune here would help you not only get more comfortable drag racing the car but also what combo/tire pressure/etc, gives you the best results.

Dusterbd13-michael said:

As far as drag launches on 200tw: mine dont seem to hook that great for it. Lots of spin on my v6 miata with either the 205 federal rs-rr or the 245 rs4 

But the 200tw stuff is definitely faster around the cones tha the old spec miata slicks were.

That sounds like a relevant data point. I think my S80 road race slicks are close to the same construction and compound as your old SM7's.  So, even your Federal 205's were faster around the cones than the SM7?

In reply to maschinenbau :

Thanks. I searched for a long time to find the right wheel with the right offset for the look.

In reply to Andy Neuman :

That's what I was thinking. To be competitive in GTU you'd need 6 to 8 tires and wheels, or bolt a harbor freight manual tire changer to your trailer and swap tires on the same set of wheels between the autocross and the drags. 

Dang it! I just gave away my secret plan for dominance.

hobiercr said:

In reply to bigben :

Do you have a drag strip close to you where you could try out your tire options for launch potential? I think a little test and tune here would help you not only get more comfortable drag racing the car but also what combo/tire pressure/etc, gives you the best results.

Yes there are a couple. I plan to do some launch practice at an 1/8 mile track and also use it as a safe place to datalog for road tuning the ECU. I'd have to buy some 200tw tires to be able to test that option.  If I went with 205/15 then I could mount them on the Miata wheels and still have the old Hoosiers mounted on the other 15s to compare against. 

There is one obstacle with using the Miata wheels. Their hub center is smaller than the pilot diameter on my rear hubs so I would need to find a cheap way to open up the bore on two of the Miata wheels. 

I think you should address the u joint issue.  The weakest link will always fail first.  I remember it happening but what was done to make it stronger?

In reply to Stampie :

Yes. That is a given. I'm still working out how to do that and keep the budget. The driveshaft is from a Nissan Z31 which have staked in "non-replaceable" joints. Which just means they're more difficult to replace and you have to use joints with clips on the inside of the yoke. U-joint caps have to be thicker than standard ones so in order to have a groove for the clip, which means the cross shafts are a lot smaller in diameter than the original and weaker.

The driveshaft I was running had a replacement joint in the front and the original Nissan joint in the back. Guess which one snapped.

I bought another used driveshaft on ebay that was supposed to move freely and smoothly, but it doesn't. There's no slop in it but the joints are notchy. So I've been trying to figure out a way to rejuvenate the grease in the joints.

In reply to bigben :

Remind me of your trans and diff plus length of driveshaft.  I love junkyard built driveshafts for $15.

In reply to Stampie :

The trans is a nissan FS5W71C from a 240sx. The output shaft should be the same splines as the older 71A and 71B transmissions. I think the only vehicles that may have the conventional style joints with outside clips are some of the nissan pickups. I think I read somewhere that some years of Frontier with a 4 banger came with 1310 joints. The rear axle is a Volvo 1031 (dana 30) from a 1990 Volvo 240. I carefully cut the diff yoke off of the Z31 driveshaft and had some adapter cups machined to slide over the Z31 u-joint caps to make the joint fit the volvo differential yoke.

From u-joint center to u-joint center the shaft is 26.25" long, but it could be up to an inch longer and still work. The spline count is 22/24 (2 splines are missing from the 24 pattern) and spline diameter of .995. Slip yoke OD is 1.375.

In reply to bigben :

Do you think the failure was the 1310s or just an old ujoint?  I'm wondering if a 1330 front and maybe a 1330/1310 rear would be needed.

A 1310 would be a significant improvement in strength. The stock Z31 joint has 0.947" diameter caps vs 1.062 for a 1310. The joint that broke was an aftermarket joint which is the same .947 diameter as the original Nissan, but the cross shaft diameter is much smaller due to the cap being thicker to accommodate the c clips. It was a joint like this that snapped. 430-11A

Something to think about........if you're picking tires for the Challenge, rather than general autocross use, the lot at Gainesville is very abrasive and will heat up race tires effectively. Normally, I see 200TW tires do as well, or better than race slicks because it's sometimes hard to get heat into the slicks, depending on the surface. Not so at the Challenge. I ran one year old SM7s on my Miata in Gainesville and they were getting too hot, the pro driver actually asked me if I happened to have a brought along a sprayer (no). The big advantage to the SM7s is that they can be had pretty cheap if your real out of pocket budget is a concern. Aside from that I'd go with the 200TW. Also, I have notes someplace from when I was trying to find an appropriate drive shaft for my Challenge car. I'll see if i can find them and see if there were any candidates in the 26-27" range, that is pretty short.

Just curious about what spec of wheels you are in need of?  In a perfect world...15 x 7"/8"/9"  and what offset would you need?

for example, what offset are the "out of budget" 17 x 8 wheels?

Catch22 said:

Just curious about what spec of wheels you are in need of?  In a perfect world...15 x 7"/8"/9"  and what offset would you need?

 

for example, what offset are the "out of budget" 17 x 8 wheels?

I think 15x8 with +35 offset would work well. 15x9 with around +45 might work but I'd need to take some measurements to check in board clearance. With a 15x7 or 7.5 I could probably do a +25 offset. (Even this would be better than the 15x6.5 +40 six spoke budget wheels.)

The non-challenge friendly 17x8 are a +35. With 235/40 tires the edge or the tire is pretty much even with the outside of the rear fender flare.