SCCA Autocross setup thread

Slowclimb said:

Wheels and tires...my thought was to move to a 16 inch wheel - especially with the new rules for Street Tire where you can use a wheel an inch shorter with the width of the size above, probably go with the Star Specs 225 50 16 if they will fit

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Give me til this weekend and I will know if the 225/50-16 will fit. Fingers crossed, it will be a tight fit, especially in the back.

From another thread we have good data that the 225/45-16 Direzza Z2 fits well

I would like to know if the 225-50r16 BFG Rival fits. It might be better than the Direzza z2 but if it won't fit, the Direzza seems like a good option too.

I think that'll be course dependent. The z2s were the clear favorite at nationals on a concrete surface. I've heard the rivals may be better on other surfaces ie sealed asphalt.

I know one guy here running the Rival on a mazda 2 in SMF switched back to the dunlop - stating that the rival did not like to rotate on the small front wheel drive.

Would love to see some pictures of a ST with the dunlop 225, 50, 15

Slowclimb said:

I know one guy here running the Rival on a mazda 2 in SMF switched back to the dunlop - stating that the rival did not like to rotate on the small front wheel drive.

Would love to see some pictures of a ST with the dunlop 225, 50, 15

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If only they made that size :grumpy:

Ooops meant 16

Slowclimb said:

Ooops meant 16

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Ahhh thats fine i had to go back and look see if i missed something new oh well i was hoping

Sorry to get your hopes up

Why 225/50/16? Z2's come in 225/45/16 , I've posted a pic of mine on here a couple times (also see profile pic).

Guess I could go either way, but was thinking that the 50 aspect ratio would offset the smaller wheels impact to gearing. maybe that is not valid?

The 225/45 is actually closer to the stock diameter than 225/50.

Still the taller ratio could improve top speed in 2nd....waiting to see how people's set up shake out anyway. So have time to make a less hypothetical decision.

Slowclimb said:

Still the taller ratio could improve top speed in 2nd....waiting to see how people's set up shake out anyway. So have time to make a less hypothetical decision.

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225/50R16 rivals show 60.4mph at the top of 2nd. Stock is 58mph

Slowclimb said:

Still the taller ratio could improve top speed in 2nd....waiting to see how people's set up shake out anyway. So have time to make a less hypothetical decision.

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Remember you have 17 more horsepower in 3rd vs 2nd. This has made me wonder if shorter tires might be better on fast courses.

AlanBDahl said:

Remember you have 17 more horsepower in 3rd vs 2nd. This has made me wonder if shorter tires might be better on fast courses.

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17hp at peak, which is 6000rpm; to get to that engine speed in third would have you at 83mph (on stock tires). I know that every region and course are different, but not many runs will have you above 65mph. Changing at 55mph would have you at 4000rpm in third, which is still in the meat of the powerband, but if you drop down to 45, thats ~3400rpm, and falling out of the sweet spot.

This is not the slickest shifting car I've ever driven, so a bunch of shifts on course don't seem like the answer, but maybe the shorter gearing would keep you in the power better. Have you decided what tires you are running yet? Are you planning on running multiple sets?

Right....there is no way I will be using 3rd very much. There is only one course that I have run that I have been tempted to shift up in, and after some tries it turned out that bouncing the limiter was the quickest way to get around. The time to shift up and shift back down negated any extra mph you got. This is one reason some Miata owners take the 6 speed out in put in a 5 speed, to get the higher top end in 2nd.

Perhaps I should instead have indicated that you have more boost in 3rd instead. It would be interesting to compare 50-60 times in 2nd using the D2s vs 3rd using the Toyos. Sure you will loose time to shifting but on faster courses (I've driven half of one of our local sites in 3rd before in the Solstice) it might pan out. In addition on some Pro Solo courses I've barely hit 50 and once again the short tires could be the ticket.

BTW I am not advocating for running short tires all the time but a dual-prong approach with two sets of wheels and tires could be an interesting one if you're running near the top.

Point taken...I am sure there are courses where there could be benefit and it may be interesting to do a comparison. Unfortunately I don't have the funds in my autocross budget for that right now. Most have been allocated to rebuilding my 64 beetle.

Peak horsepower is less important, I think, than the stronger torque numbers throughout the meat of third gear. The dyno torque numbers out there are more impressive than the already impressive horsepower numbers. The idea of going small is enticing. Reduced rotating mass and lower center of gravity would be nice side effects as well.