SCCA Solo C Street Discussion

[Jawnathin]

For me, the biggest consideration of 245/40 and 235/40 is the change in gearing. 235 is shorter and will accelerate better but will have lower top speed than the 245/40. On a tighter course where you don't hit the rev limiter the 235 will be better. On faster courses a 245 may save you from a shift or riding the rev limiter as long.

Between the two, I would lean 235/40. Both the 235 and 245 will be pinched, so effective contact patch probably isn't all that different. With the 235s you get the benefit of shorter gears accelerating out of every corner while the 245s taller gearing only becomes a factor at the very top end. As noted earlier, the better response of a 235/40 is a nice plus too.

And this is coming from someone who just got 245/40 RS3s.

[Nm86]

Originally Posted by Shark_Bait88

Yeah, I was thinking about those but that $100 price jump is pretty ridiculous. Granted, I've never driven on really nice tires so anything is going to feel a lot better to me coming from Primacy HPs. haha

Go with 225/45/17 z2 or rivals, they are on close out right now. It's a killer deal. If you have never used high performance tires you will be amazed by how well they work. You can get a set of those for $422 right now. I have been running 225s and love them, a lot of people have mentioned they are 1 inch wider then the stock tires, yes I know it's supposed to be 1cm but trust me it's really like an inch.

[Jawnathin]

I wouldn't bother with 235/45. That's the worst of both worlds.

Following up on my last post, a 245/40 isn't a bad option. It has a distinct set of advantages and disadvantages over the 235/40. What is better will be very course and driver dependent. It's a toss up, though I think the 235 will have the advantage more often.

Side note, a set of Z2 SS in 245/40 on a 7" would make your tire installer hate you.

[Shark_Bait88]

Originally Posted by Jawnathin

I wouldn't bother with 235/45. That's the worst of both worlds.

Following up on my last post, a 245/40 isn't a bad option. It has a distinct set of advantages and disadvantages over the 235/40. What is better will be very course and driver dependent. It's a toss up, though I think the 235 will have the advantage more often.

Side note, a set of Z2 SS in 245/40 on a 7" would make your tire installer hate you.

Well I'm gonna be going down to South Bend to pick them up and have them installed at TireRack, so I imagine they've dealt with similar situations before. I'm kind of curious about the 235/40, might spring the extra $100 for them to see what it's like.

[qoncept]

The difference in diameter between a 235/40 and 245/40 is about 1%. I don't see how anyone can choose one or the other based on that. Likewise, weights are very difficult to find and questionable at best anyway. I'm looking at width and sidewall stiffness alone.

[renfield90]

Originally Posted by Jawnathin

For me, the biggest consideration of 245/40 and 235/40 is the change in gearing. 235 is shorter and will accelerate better but will have lower top speed than the 245/40. On a tighter course where you don't hit the rev limiter the 235 will be better. On faster courses a 245 may save you from a shift or riding the rev limiter as long.

Between the two, I would lean 235/40. Both the 235 and 245 will be pinched, so effective contact patch probably isn't all that different. With the 235s you get the benefit of shorter gears accelerating out of every corner while the 245s taller gearing only becomes a factor at the very top end. As noted earlier, the better response of a 235/40 is a nice plus too.

And this is coming from someone who just got 245/40 RS3s.

IMO most of these gearing changes are minimal. Check out this site: http://www.rimsntires.com/specspro.jsp

The difference between 235/40 and 245/40 at 60MPH indicated is less than 1MPH actual. With that in mind you should go for most amount of functional rubber on the ground, with an eye towards your preferred amount of sidewall compliance.

[Jawnathin]

It isn't about just the top speed.

A car with shorter gearing has a higher torque multiplier and will accelerate more quickly than a car with taller gearing at any given point in the powerband.

Additionally a car with shorter gearing is going to have a higher average RPM than the taller geared car. On a straight, out of a corner, everywhere. Given how these engines need RPM to make power, plus the desire to stay out of the torque dip, keeping RPMs up is an advantage.

A car running a higher RPM plus a higher torque multiplier will have an advantage. But with a lower top speed, you're going to find the rev limiter more often compared to the 245/40 car.

All these tiny factors can add up. It might not be much, but even if its just a tenth or two over the period of a 60 second course, that can be the difference between winning and losing.

[renfield90]

http://www.advanced-ev.com/Calculators/TireSize/

For the purposes of 2nd gear in a Twin, use 9.307 for the differential ratio. You can verify this by entering redline (7500 RPM), top speed in 2nd (59), and OEM tire diameter (24.6").

At 30mph, the 245/40 is at 3798 RPM and the 235/40 is at 3845 RPM. That is a difference of just 47 RPM. Ever had your tach jump a couple hundred RPM just cause you go over a bump or expansion joint? 47 RPM is a rounding error.

We are not talking tenths here, and maybe not even talking hundredths. An extra 1-2mph apex speed from putting more meat to the ground is a much bigger difference than this, assuming you can deal with the compromised sidewalls.

[Jawnathin]

There is no rounding errors for engine speed, mechanically the engine will be turning over at a higher RPM. That 50RPM at 3800rpm is almost 100rpm at 7000rpm. The car with the shorter tire will have a higher average RPM. The car makes more power at higher RPM resulting in higher average horsepower on course.

You haven't added the more advantageous torque multiplier the shorter tire gives you. Yes, that is small too, but compounded with the higher RPM/power output, the car will apply more power to the ground compared to a taller geared car. Even a little more power is still more power.

In an autocross you're on the throttle like 80+% of the time. Given the choice between a car that accelerates slower or faster, what would you take?

Thousandths add up to hundredths. Hundredths add up to tenths. This can be the difference between a win and a loss. No one who takes autocrossing seriously would turn down an opportunity to off some time.

I'm not saying 235/40 is better than 245/40, they both have their pros and cons, but to say that an acceleration advantage doesn't matter or is of no benefit is nonsensical.

[renfield90]

Hopefully you don't take me the wrong way. I'm not picking on you, but I disagree that the advantage is meaningful.

Originally Posted by Jawnathin

There is no rounding errors for engine speed, mechanically the engine will be turning over at a higher RPM. That 50RPM at 3800rpm is almost 100rpm at 7000rpm. The car with the shorter tire will have a higher average RPM. The car makes more power at higher RPM resulting in higher average horsepower on course.

Can you quantify this? Show me a dyno plot where there is a meaningful difference at any 100 RPM gap above our 3800 RPM baseline. That's pretty generous given that it's only a 100 RPM gap from 7000-7500 RPM. You should be able to, say, use an integral (or hell, even a Riemann sum) to get the area under the curve from 30-55 MPH for both a 24.4" tire and a 24.7" tire. Then make a statement along the lines of "Tire A delivers X% more horsepower from 30-55 MPH than Tire B." We can then perform a similar thought experiment assuming we installed a new air filter that provides a 2HP improvement across the entire dyno plot and see what percentage improvement that is. That will tell you something very good about the magnitude of your result.

Originally Posted by Jawnathin

You haven't added the more advantageous torque multiplier the shorter tire gives you. Yes, that is small too, but compounded with the higher RPM/power output, the car will apply more power to the ground compared to a taller geared car. Even a little more power is still more power.

Can you quantify how much extra torque multiplier you get? A number talks. Math talks.

Originally Posted by Jawnathin

In an autocross you're on the throttle like 80+% of the time. Given the choice between a car that accelerates slower or faster, what would you take?

We can go look at someone's datalog to get a real number instead of 80%. But whatever that means, it's now a multiplier against whatever result you got from your Riemann sums - i.e. you only get that advantage 80% (or whatever that multiplier is) of the time, which reduces its impact.

Originally Posted by Jawnathin

Thousandths add up to hundredths. Hundredths add up to tenths. This can be the difference between a win and a loss. No one who takes autocrossing seriously would turn down an opportunity to off some time.

I'm not saying 235/40 is better than 245/40, they both have their pros and cons, but to say that an acceleration advantage doesn't matter or is of no benefit is nonsensical.

No disagreement here on thousandths being important. I missed contingency last year by .069 one event. I missed a trophy by .006 another event. What I disagree on is that 0.3" of tire diameter is a more significant effect than extra grip from a wider tire. I can carry more speed through a turn (which I then carry down the straight), I can power out of the turn sooner, and I can brake later.

Even just carrying 1MPH more through a turn would put my 245/40 tire higher up on the revs than a 235/40. Ok, the torque multiplication is different - is it enough to make up the 1MPH gap I will carry for the whole straight plus the fact that my motor is pushing more HP at corner exit? You can calculate this all out. I'll eat my words if the math says I'm wrong.

[qoncept]

Originally Posted by renfield90

We can go look at someone's datalog to get a real number instead of 80%. But whatever that means, it's now a multiplier against whatever result you got from your Riemann sums - i.e. you only get that advantage 80% (or whatever that multiplier is) of the time, which reduces its impact.

I would also argue that unless you're at 100% throttle it doesn't even matter. If you're at 50% and want more power you just push the pedal. I don't have any logs here at work but I would imagine you're at 100% throttle very little. The gearing difference (and effect) between the two tires is obviously not zero, but IMO it's a much smaller performance factor than width and how the tire fits on the wheel.

And because you never know when you'll need another .5mph in 2nd gear you couldn't possibly make an informed decision. If you bounce off the rev limiter one time because your tire is 1% too short you've killed any advantage it gave you.

[simpleisbest]

Originally Posted by qoncept

If you bounce off the rev limiter one time because your tire is 1% too short you've killed any advantage it gave you.

^THIS!

No matter what calculations people make, the truth of the matter is that if you are on the rev limit too long you will hit fuel cut and will have a loss of power & speed. The moment that happens, you just left a ton of time on course.

[Jawnathin]

I do not feel like I'm being picked on, so no worries there. I do enjoy engaging discussions but I am not interested in endless forum debates.

Unfortunately I couldn't find a dyno chart online that plots out HP in static RPM increments like I've seen before, so I have to eyeball a gain and come up with my own using a dyno chart.

The biggest HP gains of additional RPM are during the mid range where we will be accelerating and heavy on the throttle coming out of an exit. This range is between 3500rpm to 6000rpm. According to that dyno chart, at 4500rpm the car makes 110whp and at 5000rpm it makes 130whp, a 20hp gain in 500rpm bracket. Slicing it up in 50rpm increments, it means every 50rpm is worth 2hp or each single RPM gains 0.04hp.

When a 235/40 is at 5000rpm, the 245/40 is at 4938, 62rpm difference. 0.04*62 = 2.48hp advantage to the 235/40.

5000rpm = 130whp
4938rpm = 127.52whp

This is just RPM, let's look at the gearing.

A 235/40 is 1.2% shorter than 245/40. That is 1.2% more power being sent to the ground at the same RPM. But since they aren't at the same RPM, we have some more math to do.

5000rpm = 130whp * 1.012 = 131.56hp.
4938rpm = 127.52whp * 1 = 127.52whp
= 4.04whp or 3% HP advantage to the 235/40

The actual HP advantage will vary depending on where you look at on the RPM bank, but this demonstrates there is a measurable HP gain using a shorter tire. This also doesn't take into consideration the faster accelerating car will travel through the rev band more quickly than the slower accelerating car, so it may be more than just a 62rpm difference by the time the shorter tire hits 5000rpm. The slower accelerating car may be at 4850 or 4900rpm for example. The best part about this is that you have this acceleration advantage on every single time you accelerate while on the course.

You might find these power gains to be insignificant, but I would bet that most autocrossers would take an additional 3% power to the wheels if they could. For a car that doesn't have much HP to begin with, I know what I would do.

So I've demonstrated that there is an increase in output to the ground. On the other end, there has been no analysis of how much more tire is actually being used on a 245/40 compared to a 235/40.

A 245/40 is 9.6" wide and a 235/40 is 9.2" wide. Both of these tires are 2" wider than the wheel and are severely pinched. Looking at my own 245/40s, the inside and outside shoulders aren't even touching the ground when the car is static. Here, take a look.




Now of course, the car won't be static on course, it will roll onto the shoulder when cornering, but it isn't using the whole face of the tire, so actual increase in tire touching the pavement is much smaller than the 10mm width increase would make you believe. I can see this even on my own set of tires as even after 300 runs on my 245/40 RS3s, the inside shoulder looked untouched. THe tread pattern goes all the way to the end of the tire, completely unused. On a 7" wheel, you'll never use up the entire 235 width on course, yet alone a 245mm. So how much benefit are you really getting from that extra tire while pinched on a 7" wheel?

And because you never know when you'll need another .5mph in 2nd gear you couldn't possibly make an informed decision. If you bounce off the rev limiter one time because your tire is 1% too short you've killed any advantage it gave you.

^THIS!

No matter what calculations people make, the truth of the matter is that if you are on the rev limit too long you will hit fuel cut and will have a loss of power & speed. The moment that happens, you just left a ton of time on course.

This is ironic because this is precisely the point I made earlier and that you had claimed didn't matter. So now you're saying gearing and the rev limiter does matter?

Let's see my original quote to verify this is what I said...

For me, the biggest consideration of 245/40 and 235/40 is the change in gearing. 235 is shorter and will accelerate better but will have lower top speed than the 245/40. On a tighter course where you don't hit the rev limiter the 235 will be better. On faster courses a 245 may save you from a shift or riding the rev limiter as long.

Yup! Exactly what I said earlier. Thanks for proving my point

[Briankbot]

Originally Posted by Jawnathin

I wouldn't bother with 235/45. That's the worst of both worlds.

Following up on my last post, a 245/40 isn't a bad option. It has a distinct set of advantages and disadvantages over the 235/40. What is better will be very course and driver dependent. It's a toss up, though I think the 235 will have the advantage more often.

Side note, a set of Z2 SS in 245/40 on a 7" would make your tire installer hate you.

I think the idea is really to see the advantages and disadvantages of each tire size and assess what types of courses you will be running. The course design will be the deciding factor in what the right tire size is for your car.

Being new to my region I have been at a bit of a disadvantage as I have only run one event at each of the two sites they use near me. If you have some idea what the courses you run on will be like, its just a matter of making compromises and picking your poison.

That said I went 235/40 17 which is pretty close to stock tire height. I also want to run nationals and realize that size may put me on the limiter as those courses tend to be faster (or so I hear). I may buy an extra pair of wheels and mount 235/45 17s or 245 40 17s for tall gearing as necessary. I believe the 235/45s increase gearing by about 3% and and will raise the ride height 1/4in. Any thoughts on raising the center of gravity that much? If you really want to go nuts with taller gearing I think there is a tire for an 18in wheel which will be even taller...