Ok, I know I'm Captain Slow, but...

... on an average 45-55 sec auto-x course, how much time difference would there be between a 195/50 340tw on a 15x7, and a 15x8/205 or 15x9 225 200tw tire, all other things being equal (i.e. same car, same driver, etc...)?

I mean, I know they're going to be faster, but how much?  1-2 sec a size up?  So if I run a 50, would the 15x8/205 run a 48, and a 15x9/225 run a 46 (again, all other things being equal)?

Trying to determine just how much I suck at auto-x.  Even though I have a blast doing it for it's own sake, and trying to improve against just myself does keep it fun, I'd like to think if (when...) I step up I'll get a little closer to "competitive". 

OK two things going on;

First, on a 50 seconds course I'd expect 1.5-2 seconds with sticky tires. 

Second if the 320i is what your autocrossing then you need to be mindful of the weight of those larger wheels and tires. Unless the car is sporting 300hp it's going to have a noticeable effect.  You may end up giving back .5 or more of the gain due to compound. If the motor is near stock in the 320i I'd go with the same size tire but in the 200TW. My Datsun vintage race car uses little 185/60-13s and going up 20mm on tire width adds 4lbs per tire. Meaning 16lbs of flywheel effect.

In reply to OldGray320i :

What's the weight and prep level of your car? Generally I'd recommend the 225, but if the combination of body roll + camber gain means you're only riding on the outside edge of the tire, going wider likely won't gain you much. 

Locally we has a class for the older cars that now have limited preparation as most cars ar no longer "as delivered by the factory".  

If the BMW 320 is the car you are using for your events you have a number of front end challenges, the key is embracing the body roll.  Your car has basically "zero" camber as built.  The wheel off set will limits your options , 15x7 with a 13mm offset fits those stock front fenders but your choice of tires is going to be limited by how you fix or increase the camber.

If you are running stock sized springs, spend some time on "real OEM" and look up the part number for the top hats for the euro 323.  They are flat, as apposed to the US spec hats that raze the front  end of your cars for the bumper height regulations of the day.  Now with the addition of your "ST springs", 30mm front bar and Bilstein Sports you can think about 205/50 15's.

Spending the money on a "Ground Control" front end will provide more camber, but again the architecture of the front end limits the width of every thing you can run under those front fenders. 

To answer the original question....  I'm thinking less than .5 seconds.

Even on a roadcourse that is 2+ minutes long, you might only see a couple of tenths.

Compound is going to be more important than that slight change in contact patch shape.

In reply to jr02518 :

Thanks for confirming what I suspected. In that case I'll change my answer to dial in as much negative camber as you can get in the front, do your best to limit body roll(and thus camber gain(, then stuff the widest/stickiest 200tw tirs2 that will fit in the remaining space. 

That should gain you a second or more pretty easily.

Tom1200 said:

OK two things going on;

First, on a 50 seconds course I'd expect 1.5-2 seconds with sticky tires. 

Second if the 320i is what your autocrossing then you need to be mindful of the weight of those larger wheels and tires. Unless the car is sporting 300hp it's going to have a noticeable effect.  You may end up giving back .5 or more of the gain due to compound. If the motor is near stock in the 320i I'd go with the same size tire but in the 200TW. My Datsun vintage race car uses little 185/60-13s and going up 20mm on tire width adds 4lbs per tire. Meaning 16lbs of flywheel effect.

Ummmmm, didn't GRM do a test a few years that showed heavier wheels and tires on a Miata fell into the "statistical noise" category as far as impact on times? 

And didn't they also show something on a BMW where larger heavier wheels were actually faster?

Sorry, I should've clarified it's the Miata, not the 320i.  Though it is shameful that I do not have the 320i running and autocrossing.

I class myself in STR, but it's based on what I thought I read in the rules two years ago. 

"Cold air" intake, f/swaybar (FM's - took the rear bar off), coilovers, and the car has lsd. 

Relative to tire, .5 seems a little light between a bigger sticky tire and a smaller non-sticky tire, but I'm Captain Slow and looking for excuses.

That said, it sounds like I'm in the ball park for what I'm thinking.  When I looked at results, Miatae times were everywhere, and there were CSP cars that were only a second or so faster (clearly they're as bad a driver as I am...).  "Consistent/ good" CSP Miatas were about 7-8 sec faster, and good STR guys a second or two behind that. 

Ok, I'll keep working at it until I'm consistently 3-4 seconds behind good STR guys and then see what wider, stickier tires do to me.

z31maniac said:

Tom1200 said:

OK two things going on;

First, on a 50 seconds course I'd expect 1.5-2 seconds with sticky tires. 

Second if the 320i is what your autocrossing then you need to be mindful of the weight of those larger wheels and tires. Unless the car is sporting 300hp it's going to have a noticeable effect.  You may end up giving back .5 or more of the gain due to compound. If the motor is near stock in the 320i I'd go with the same size tire but in the 200TW. My Datsun vintage race car uses little 185/60-13s and going up 20mm on tire width adds 4lbs per tire. Meaning 16lbs of flywheel effect.

Ummmmm, didn't GRM do a test a few years that showed heavier wheels and tires on a Miata fell into the "statistical noise" category as far as impact on times? 

And didn't they also show something on a BMW where larger heavier wheels were actually faster?

Yeah.  Unsprung weight is important, but it mainly shows up when the suspension needs to react to a lot of bumps -- think mountain roads, not autocross in a flat parking lot.  Wider tires add drag which can be a factor on a big tracks, but autox is generally too slow for the aero to matter that much.  Rule of thumb for autox, IMHO, is to go as big as possible, try to keep it light, but don't worry about a few pounds.

OldGray320i said:

Sorry, I should've clarified it's the Miata, not the 320i.  Though it is shameful that I do not have the 320i running and autocrossing.

I class myself in STR, but it's based on what I thought I read in the rules two years ago. 

"Cold air" intake, f/swaybar (FM's - took the rear bar off), coilovers, and the car has lsd. 

Relative to tire, .5 seems a little light between a bigger sticky tire and a smaller non-sticky tire, but I'm Captain Slow and looking for excuses.

That said, it sounds like I'm in the ball park for what I'm thinking.  When I looked at results, Miatae times were everywhere, and there were CSP cars that were only a second or so faster (clearly they're as bad a driver as I am...).  "Consistent/ good" CSP Miatas were about 7-8 sec faster, and good STR guys a second or two behind that. 

Ok, I'll keep working at it until I'm consistently 3-4 seconds behind good STR guys and then see what wider, stickier tires do to me.

 

I somehow missed the different TW ratings on teh tires.

Let me re state my comment.

Wider tires that are an inch wider and are the same compound will NOT make much difference.  You probably cant even statistically tell the difference if you did 10 back to back runs.  The difference would be "noise" in the data.

Compound is everything when you are talking about tires this close in size.

The stickier compound could get you 1+ second, I would think.

Ok guys I did say "if" it was the 320i and there is a reason for that. I actually have practical experience with this exact situation on a  79 320i. Back in the early 90s a friend autocrossed one and long story short it was faster with the narrower tires. On a 45 second-ish course it was something like .4-.6 seconds faster. This was the same day with the same driver.

The 30lbs of extra wheels and tires might not be a lot for most cars but in a 2500lb+ car sporting all of 90 horsepower at the wheels, it's the equivalent of a 75whp Miata.  Couple that with the fact that 320s have a huuuuuge gap between first & second, a 3:45 final drive and the car just will not pull the larger tires well. The suspension is such, that as mentioned in this earlier thread, the outide edge of the tire is doing most of the work. 

 If you have to slow down for a 18 mph hairpin and then the next corner is a 45 mph turn but the car arrives at said corner going 40 mph, there's kind of no need for wider tires. 

I think the 70s 320s are an overlooked gem but in comparison a Miata (even a 1.6) feels like a rocket. Naturally a modified 320 would be a different story as the M10 engine is capable of way more than the stock tune delivers and the suspension can easily be improved.

For the record my son's  Miata has the wider tires on it.............

Tom1200 said:

Ok guys I did say "if" it was the 320i and there is a reason for that. I actually have practical experience with this exact situation on a  79 320i. Back in the early 90s a friend autocrossed one and long story short it was faster with the narrower tires. On a 45 second-ish course it was something like .4-.6 seconds faster. This was the same day with the same driver.

The 30lbs of extra wheels and tires might not be a lot for most cars but in a 2500lb+ car sporting all of 90 horsepower at the wheels, it's the equivalent of a 75whp Miata.  Couple that with the fact that 320s have a huuuuuge gap between first & second, a 3:45 final drive and the car just will not pull the larger tires well. The suspension is such, that as mentioned in this earlier thread, the outide edge of the tire is doing most of the work. 

 If you have to slow down for a 18 mph hairpin and then the next corner is a 45 mph turn but the car arrives at said corner going 40 mph, there's kind of no need for wider tires. 

I think the 70s 320s are an overlooked gem but in comparison a Miata (even a 1.6) feels like a rocket. Naturally a modified 320 would be a different story as the M10 engine is capable of way more than the stock tune delivers and the suspension can easily be improved.

For the record my son's  Miata has the wider tires on it.............

Some of this is gear ratio effect aka height.  I use this to my advantage on the Rabbit.  255/40/13 on 10.5 very light wheels for autox and 205/50/15 streets for the road to get the taller gearing and reduced rpm.  The shortened ratio with the race tires has a magical effect on it's raging 100whp....

I think from your hard 195's to a set of good 225's on 9's, you will pickup 2 seconds or more. Tires are everything in autox. And the more tire you get for the weight of the vehicle, the faster you are going to go. In autox, wider tires are much more important than light weight tires. 

There are situations where adding with and better tires won't help, but your Miata isn't one of them  

mw said:

I think from your hard 195's to a set of good 225's on 9's, you will pickup 2 seconds or more. Tires are everything in autox. And the more tire you get for the weight of the vehicle, the faster you are going to go. In autox, wider tires are much more important than light weight tires. 

There are situations where adding with and better tires won't help, but your Miata isn't one of them  

 

 

This fits with my assessment that I still have more work as a driver.

Tom1200 said:

Ok guys I did say "if" it was the 320i and there is a reason for that. I actually have practical experience with this exact situation on a  79 320i. Back in the early 90s a friend autocrossed one and long story short it was faster with the narrower tires. On a 45 second-ish course it was something like .4-.6 seconds faster. This was the same day with the same driver.

The 30lbs of extra wheels and tires might not be a lot for most cars but in a 2500lb+ car sporting all of 90 horsepower at the wheels, it's the equivalent of a 75whp Miata.  Couple that with the fact that 320s have a huuuuuge gap between first & second, a 3:45 final drive and the car just will not pull the larger tires well. The suspension is such, that as mentioned in this earlier thread, the outide edge of the tire is doing most of the work. 

 If you have to slow down for a 18 mph hairpin and then the next corner is a 45 mph turn but the car arrives at said corner going 40 mph, there's kind of no need for wider tires. 

I think the 70s 320s are an overlooked gem but in comparison a Miata (even a 1.6) feels like a rocket. Naturally a modified 320 would be a different story as the M10 engine is capable of way more than the stock tune delivers and the suspension can easily be improved.

For the record my son's  Miata has the wider tires on it.............

 This fits with my assessment that I really need to get off my arse and put the 320i together!