Size does matter - sometimes it's too big...

[widebody911]

In an election year-style quest to assign blame for my lethargic lap times, I've come to the conclusion that 17" wheels may not be for me, at least with stock 915 gearing and a 8:31 R&P. I just don't have the 'ooomph' out of turns that I used to, and the cars that I used to smoke on the track are leaving me behind.

From the tire rack site, I see the diameter of the 245/45 V700 I was using is 24.3" and the 315/35 A032 is 25.8" - a 1.5" difference.

Using a GPS, I found that @ an indicated 60MPH, with the 245/45 rears, my speedo is 5MPH optimistic. With the 315/35's, @ an indicated 60MPH, it's 3MPH (actually almost 4MPH) pessimistic.

I don't have the HP I need to get out of the turns as fast as I used to, or at least not with these tires. With the wider tires but a less-sticky A032R (vs V700's), I'm not taking full advantage of the extra width.

Of course, I could learn to drive better, and carry more speed thru the turns, what Jim (AddictionMS - great instructor, BTW) likes to call a "momentum driver"

We also haven't put suspension settings on the table, ie sway bar settings, shocks (I'm using Bilstein greens all 'round right now), fancy bumper-steer tie rods, etc. I look at those as icing to get those last few tenths or hundreths. Right now I'm watching 914's fade into the horizon in front of me.

Another option could be to go with a 7:31 R&P, but I've heard they're not as strong.

Opinions? Comments? Smartass remarks?

[emcon5]

I thought this was going to be another thread on the girl that likes rice cars.

Damn the bad luck.

Tom

[Kevin Stewart]

Thom loosen the car up off the corners, your probably to tight, Kevin

[KTL]

I'm no racer by any means, so take this for what it's worth (a squirt of piss maybe? ).

Everybody says the driving is the key. Well, you seem to do plenty of that, so maybe you just need to fine tune your skills. Your car has all sorts of added goodies (brakes, motor, sway bars...........) to get you where you want to be. Tires could be a good reason though. Gear ratios make a noticeable difference.

What about the cabrio. job? Did that soften up the chassis and lose you some time there?

914s are a tough pill to swallow. Those wacky little cars are so light and nimble, it's an uphill climb to keep up with those little buggers.

Keep us posted.

[Chuck Moreland]

Yeah, what about the rice girl?

Back on topic -

Your results are not surprising. Going to the taller tires reduces your gear ratio 6.65% in everygear. Your wheel torque (and acceleration) are reduced the same amount!

Not to mention the extra rotating and unsprung weight you added with the setup. Acceleration and braking are impacted since they now need to overcome the extra rotating weight.

Plus the ride height is now .75 inches taller introducing more roll, dive and sqat.

I realize that you lowered the suspension recently, presumably with the 17 inchers? Depending on how agressive you lowered the, the suspension may have funky geometry since the hubs are sitting up so high. The wheels trace out the wrong arc as they move through the range of motion.

If you want to keep the big tires, you can fix some of the problems by going to a 7:31 R&P. But as Jack will tell you from experience, that is a short term solution on the 3.6.

The better solution is short gears in the tranny but that is an expensive proposition.

Fixing the suspension geometry issues is best dealt with using raised spindle struts and an adjustable bump steer setup.

You could also go to a skinnier 17 inch tire that has a shorter diameter.

You could fix everything by using 16 inch tires. You could do even better by 15 inchers but you may run into tire availability issues.

I am a big supporter of using shorter tires/wheels at the track than on the street (herecy, I know). I've got 15 inch track wheels and 17 inch road wheels.

All of the negatives listed up top turn into positives when you drop on the shorter rubber.

[Kevin Stewart]

actually decide wether you are pushing in the center of the corner, a lot of times you push from the center out, you may be able to do a tire pressure thing, any way more front weight will loosen the car up you have to much wedge or to much load on the sway bar so loosen the sway bar, I dont think your 17's are hurting you, but will be better when you get the car set up for the tire wheel combo, so work with the set up instead of changing tires, Kevin

[Jack Olsen]

I think it's just right the way it is, Thom. When are we racing next?

It can't hurt to try some smaller tires. Your car has great grip, so you might be able to give up width and not get penalized for it very much at all. 16x7x9's will allow you to run 245's or 255's in back.

Tuning that suspension would certainly also help. What are your camber settings right now?

[jpachard]

Thom, How wide are your rims?? I have 17x9's in the back with 255 Pilot Sport Cups and a stock 3.2 and I have no problem flying around the tracks out here. I think you might have hit the nail on the head by stating that you need more seat time. Carrying more momentum through turns is definitely the key. I just did 9 days at Watkins Glen out here over the course of two months. I was able to drop 16 sec. off my times over the course of these DE's. The last time I was there I was really able to concentrate on what I needed to work on since I knew every bit of the track by heart. Don't get too caught up with feeling like you are slower. Get out on the track, have someone time you and work on only two corners per session. That way, when you come in you can see the times and corelate that with your lines etc. Remember, the clock never lies. Best of luck and keep us posted.

Cheers, James

ps. Where did you get your 930 brakes??

[Bill Verburg]

I think that the shorter stiffer sidewlls of the 17" 40 series tire is better for lateral grip and stopping than even 245/45x16 but worse for acceleration. You have the most exagerated problem of all of us because of the extra height of your tires(.1" taller than even a 275/40) this will certainly hurt acceleration. The only compensation available to you is to follow Jack's lead and re-gear at least the top 3 ratios in your trans. You were going too rebuild it anyway, right? This has its own negatives as well, for street use you will have much higher revs at all time(great on a track) even when you would rather a bit more relaxed cruising as on an interstate. You have to decide what's most important to you and make the corresponding compromises which may include shorter tires, shorter trans gears, and or shorter final gears.

[widebody911]

Well, everything is a compromise.

This is primarily a street car, like 99%. It's a cab, for cryin' out loud!

So the track-oriented trans is out. Some day I'll put together a no-holds-barred BB killer with a short-geared G50 6-speed and all the toys, but until then...

Seat time would definitely help, but the path of least resistance would be to dust off the 7" & 9" track Fuchs w/V700's. My best time @ T-Hill with the 'old' setup running CCW was a 2:15. My best running CW with the new setup for the first time was 2:20 My best time with the new setup running CCW was 2:18 - although that was with the broken steering rack. Want consistency? I ran *5* consecutive laps of 2:24.xx where each .xx within a 3-tenth spread.

There's not a lot of options out there in terms of DOT-R tires. It would be nice to go to a shorter-sidewall 17" tire, but I'm just not seeing anything out there.

And to answer the other question, the rims are 16x9 & 17x11

[Jack Olsen]

Confession: cruising on the freeway at 4000-4500 rpms does lose its allure, after a few hours. Great on the track, but not so good for mileage or conversation.

Admonishment: you're complaining about lack of acceleration in a 4.7-seconds-to-60 car? I'm surprised the SC and Carrera guys haven't strung you up to a pole.

To get an idea of what the bigger tires are costing you, sprint-wise, you might want to get the g-tech out and see what you're giving up, exactly, in straight line speed. Since you're rebuilding the transmission anyway, it's a good time to do it.

[widebody911]

Originally posted by JackOlsen
Admonishment: you're complaining about lack of acceleration in a 4.7-seconds-to-60 car? I'm surprised the SC and Carrera guys haven't strung you up to a pole.
Apples and Antelopes - the 0-60 runs were done with 16" wheels.
To get an idea of what the bigger tires are costing you, sprint-wise, you might want to get the g-tech out and see what you're giving up, exactly, in straight line speed. Since you're rebuilding the transmission anyway, it's a good time to do it.
Not a bad idea, although I do need to leave something of a transmission for them to rebuild! OTOH, it can't be any worse than the first rebuild, when I lost enough dog teeth that the thing grenaded entirely. The car would buck at idle! One of the brake bands had come out and was floating around in the transmission.

[scca_ita]

Right now I'm watching 914's fade into the horizon in front of me

Hummmmmm, Well then you have answered your question. Focus on handling and driving. Try to work with what you have. You have plenty of power.

Actually, most everyone I know are moving to 18s. At most, I would add turbo tie rods and put stiffer bushings in on both front and rear.

Then determine if you need to go to yellow bilsteins. Dont touch the sway bars. I think a lot of people go way over board with sway bars. Most of the fast cars I know still have stock bars on the car. It allows the suspension to work............

[Chuck Moreland]

Originally posted by Bill Verburg
I think that the shorter stiffer sidewlls of the 17" 40 series tire is better for lateral grip

Actually sidewall height doesn't have much impact on grip. It does have an impact on slip angle and spring rate of the tire.

All else being equal, tires with shorter sidewalls operate at a lower slip angle making them feel quicker and more responsive. But they don't actually grip better, the limit of adhesion hasn't changed. And they begin to slide more abruptly with less warning.

[Kevin Stewart]

Thom, check and see if you are driving to hard, well maybe not hard enough because the rears should get loose, i think i would fill the cell and get some more front weight or loose rear weight witch ever also try adding a couple pounds of air, this will make the corners feel that they have stiffer springs, Kevin