SCCA Solo C Street Discussion

[qoncept]

Originally Posted by Jawnathin

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?

I think you meant me. My point wasn't that gearing doesn't matter, and it wasn't necessarily directed at you.

I've seen people mention a handful of times that the taller tire can keep you in 2nd gear in some circumstances and weigh it (without quantifying because we really can't, and I'm about to tell you it's moot anyway) against the gearing-based advantages you're talking about.

My point is that you can't look at these tires at beginning of the year and decide which one is just the right diameter. ie, whether that extra ~.5mph is going to save you a gear change or not. You couldn't do it for a single event, let alone a full season.

I basically said the same thing about weight. One of my friends on Facebook just posted a big spreadsheet with all the different tire models and sizes available to him, with weights, size, subjective points, etc. What's he going to do when he chooses, orders and then says, "wait, this tire doesn't weigh 23.4lbs! I should have gone with the RS3s!"

And I something about your math doesn't quite jive to me. I think you're doubling it - the power advantage comes from engine speed, but then you're multiplying that difference because of what exactly? (Not a smartass question)

[qoncept]

Originally Posted by Briankbot

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?

The roll center will raise by 1/4" too. So uh.. I don't know what that means. But I feel like it won't really matter.

[renfield90]

Originally Posted by Jawnathin

.

Avoiding a mega quote here

The fundamental premise of your HP analysis is that both tires are capable of the same apex speed. If a 245 gets you even 1MPH more, the HP advantage you're describing is gone. If it's 2MPH, the HP advantage is reversed! Ignoring rev limiter discussions for a moment (don't ascribe points to me that I haven't made ), I don't think you can state with any certainty that a 245 doesn't provide more grip. From my experience our camber limited cars use roughly 2/3 of the tire or more; 2/3 of a wider tire is still more rubber on the ground than you.

I'm going to exit this conversation for now, and go focus on winning a jacket on 245s.

[Cueman]

Originally Posted by qoncept

The roll center will raise by 1/4" too. So uh.. I don't know what that means. But I feel like it won't really matter.

It will raise the instant center by 1/4"... which in our case (and most cases) will move the roll center a lot less. The result is less roll stiffness. But I agree with your feeling that it won't really matter.

[Jawnathin]

Originally Posted by Briankbot

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.

I agree 100% with this. Once you get used to a certain club and site, you understand what kind of courses are made and how fast they can get. If your local courses are always having you hit the rev limiter on each course, a slightly taller tire would help. On a site whose course's rarely gets that high, a shorter tire would be be ok. On a high speed runway course where you're deep into 3rd, I'd probably go for the shorter tire since you're going to shift anyway. Tire selection can be course dependent and what works for me in my region might be different from yours in your region.

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...

235/40/17 is a good choice, but yes, national level courses will be faster and you may get into 3rd. But with that said, if the courses are so fast that a 245/40 gets into 3rd too, then you haven't really lost anything either. I haven't run a 235/45 on these cars but I think it is just too tall. Whatever you gain up top by not having to shift, you're going to lose everywhere else on the course from the taller gearing. On courses that wouldn't need a shift to 3rd, you've got all the disadvantage with no benefit.

It is unfortunate our car's top speed in 2nd is JUST at the point where its a problem. My old RX8 had a 68mph 2nd gear, a shorter tire would help put more power to the pavement without any meaningful sacrifice in banging off the rev limiter or a shift to 3rd. That would be nice...

Originally Posted by qoncept

My point is that you can't look at these tires at beginning of the year and decide which one is just the right diameter. ie, whether that extra ~.5mph is going to save you a gear change or not. You couldn't do it for a single event, let alone a full season.

Well, you kind of can. As talked about above, if you know your region or club can build certain types of courses, you can build around it. There is no one size fits all solution, but if you know your courses tend to be tighter, a shorter tire may be better. It works the other way for faster courses too.

Also, just as a reminder, it isn't just the .5mph, it is also the effective gearing as a result of a shorter/taller tire.

And I something about your math doesn't quite jive to me. I think you're doubling it - the power advantage comes from engine speed, but then you're multiplying that difference because of what exactly? (Not a smartass question)

The tire, final drive, and gearing all equate to a ratio. There is a lot of other factors such as actual gear ratios, final drive, etc that would calculate actual power output to the ground, but since they're the same between the two cars and we're talking about just 2nd gear, I left it alone to keep it simple.

Anyway, the multiplier is due to the shorter gear ratio you get from the shorter tire. A shorter tire will have a shorter ratio than a taller tire. In this case, a 235/40 is 1.2% shorter than a 245/40. This means the effective gear ratio is 1.2% different. I multiplied the power output of each setup by that ratio. The 245/40 was the baseline. I could use the 235/40 as the baseline, but I would just multiply the 245/40 by 0.988 (1.2%) to measure output. You could use any whp number, the ratio/multiplier would still be applicable since the gearing is a fixed ratio.

If you have a correction for the math, I'd gladly accept any corrections.

Originally Posted by renfield90

Avoiding a mega quote here

The fundamental premise of your HP analysis is that both tires are capable of the same apex speed. If a 245 gets you even 1MPH more, the HP advantage you're describing is gone. If it's 2MPH, the HP advantage is reversed! Ignoring rev limiter discussions for a moment (don't ascribe points to me that I haven't made ), I don't think you can state with any certainty that a 245 doesn't provide more grip. From my experience our camber limited cars use roughly 2/3 of the tire or more; 2/3 of a wider tire is still more rubber on the ground than you.

Yes, you're right, a different corner exit speed would influence acceleration rates, and could be faster, but the shorter tire car would always have an acceleration advantage.

With that said, I'm not entirely convinced a 245/40 has any more usable rubber than a 235/40 when pinched on a 7" wheel, so I'm not sure you'd carry any more speed on the wider tire. Look at how my shoulders curl up, there is at least a half inch on the inside and outside maybe more. The tires were flipped, but you can see from the wear what was inside and out. The insides were hardly touched, and this is after 300 runs with the outsides cooked

Just for some background, generally I am of the belief that wider is better for tires. On my other car, I'm displeased that I can't run a 315/345 because the sizes available to me in the tire I want would throw off my car's ABS/TC system. I had to settle on a 295/335 because that was a proper ratio.

But this is on an 11" wide wheel up front and a 13" wheel on the back, they can properly utilize those sizes. I'm not sure a 245 or 235 on a 7" wheel has any real difference.

I'm going to exit this conversation for now, and go focus on winning a jacket on 245s.

Understood. I've spent far too much time than I intended, but it was a good discussion. Good luck on the quest for a jacket, I'm right there with you and ironically, I'm on 245s too

Though I may go 335 if I can get these to fit...

[7thgear]

Originally Posted by qoncept

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.

here's a log... WOT quite a bit (the red line on the top graph), I would say 85-90% of my throttle use during this run is WOT.

[7thgear]

Originally Posted by Jawnathin

[bimmer635csi]

Originally Posted by 7thgear

here's a log... WOT quite a bit (the red line on the top graph), I would say 85-90% of my throttle use during this run is WOT.

600 - 500 = 100 (The biggest area of non-WOT)

820 - 100 = 720 (Full run minus above area)

720/820 = 87.8% and that's assuming WOT every other part of the run, which is clearly not the case. Care to re-estimate?

[7thgear]

Originally Posted by bimmer635csi

600 - 500 = 100 (The biggest area of non-WOT)

820 - 100 = 720 (Full run minus above area)

720/820 = 87.8% and that's assuming WOT every other part of the run, which is clearly not the case. Care to re-estimate?

not sure what your numbers represent..

the argument is that when you apply the gas, that it's rarely WOT. If you take my run and remove all the instances of where I'm braking, all the instances where I'm lifting, and all the ramp up rates, coasting, etc, then you'll see that
the % of me being WOT is high when I actually want to gun it.

also note the green line, that is steering.

[bimmer635csi]

Originally Posted by 7thgear

not sure what your numbers represent..

the argument is that when you apply the gas, that it's rarely WOT. If you take my run and remove all the instances of where I'm braking, all the instances where I'm lifting, and all the ramp up rates, coasting, etc, then you'll see that
the % of me being WOT is high when I actually want to gun it.

also note the green line, that is steering.

The red line is throttle, as you stated in a previous post.

I looked at one segment of the run that was not WOT, on the x-axis from approximately 500 to 600. This is a segment of approximately length 100 that is not WOT.

Assuming every other part of this run is at WOT (which is clearly not so), you would have been at WOT 720/820, which is 87% of the run.

I think your estimate of being at WOT is much lower than the 85 to 90% you estimated previously based on this one simple calculation. I didn't estimate it, but I'm very confident it is much lower than 85% of the run.

[jdrxb9]

I should’ve done this a long time ago because it would’ve saved me a bunch of second guessing and angst, but I finally made some detailed calculations on the gearing effects of different tire sizes. This also addresses some of the recent discussions on tire size. Rather than just use a simple constant acceleration estimate, I put together a spreadsheet that uses an input torque curve, gearing, mass and aero+rolling resistance to calculate the acceleration, speed and distance vs time in small increments. Of course it is an approximation, but I checked it against ‘Car Test 2000’, reported test results and my own data and added a correction factor to achieve reasonable correlation. I’m satisfied with it for comparative purposes.

This is likely to be a long post, so if you just want the summary: The time differences across the range of sizes I looked at are <0.1 sec. Unless you are already that close to winning, I suggest using whatever tire you think will give you the best cornering performance.

Rather than just compute random or guesstimated conditions, I went over some of last year’s data and based the calculations on actual speeds and distances. I only looked at acceleration zones where I was at full throttle – if you aren’t using all the torque the engine can give you, it doesn’t matter what gear ratio you are putting it to the ground with.

With that filter, my Wilmington Pro data shows about 13.5 sec of FT for each ~30 sec run. The FT zones consist of the start, 2 pulls of ~40m from ~15m/s, 1 pull of ~40m from ~11m/s and 2 pulls of ~40m from ~20m/s.

The calculated results for the sum of these pulls are:
Rev/mi tire time notes

856	235/40		13.889 sec	 total accel time
845	245/40		+0.035	 	less than half a tenth slower
873	225/45-16	-0.029	 	less than half a tenth quicker
833	225/45-17	0.059	 	just over half a tenth slower
856	+2% exits	-0.152

Aside from the generally small differences, this result was as expected because none of those pulls result in hitting the limiter. Also note that improving exit speed by 2% helps by a tenth and a half which is 5X the improvement of a 2% smaller tire. In other words – just drive better.

Looking at my Toledo Pro data where I was solidly on the limiter once, the ‘pulls’ are: Start, 1x116m from 15m/s and 2x60m from 16m/s for a total FT time of ~15.5 sec according to the data.

The calculated results for the sum of the ‘Toledo-representative’ pulls are:

Rev/mi	tire	time	notes
856	235/40		15.332 sec	 total accel time
845	245/40		+0.032	 	less than half a tenth slower
873	225/45-16	-0.009	 	less than half a tenth quicker
833	225/45-17	0.051	 	just over half a tenth slower

Now this is more interesting. The overall differences are smaller due to the specific conditions, but the smaller sizes are still quicker even with significant limiter time. For the long pull, every size is on the limiter and the calcs for just that pull are:

Rev/mi	time		on limit
856	5.312		1.312 s	(34m)
845	5.310	-0.002	1.110 s	
873	5.321	0.009	1.571 s	(40m)
833	5.312	0.000	0.912 s

Despite the differences in the amount of time on the limiter, the change in overall ET over the 116m distance is very small.

Which size is quicker in a ‘on limiter’ situation will obviously depend on the initial speed and the length of the FT zone. Doing some additional comparisons for just the 235/40-17 and 225/45-16 shows:

Vo		time on limit at equal distance
mph	m/s	856 sec	873 sec
24.6	11	1.500	1.800
33.6	15	0.800	1.050
42.5	19	0.050	0.300

And this confirms what you would expect. If you are starting from a relatively slow corner, the smaller tire is better until you are on the limiter for a fairly long time. If you are starting from a fairly fast corner, the smaller tire is hurting you almost as soon as you hit the limiter. But as shown above, those differences are going to be small.

So my overall conclusion is the gearing effect should not be a significant factor in deciding what tire to use. That said, I don’t see why I’d choose 225/45-17 over 235/40-17 or 225/45-16. For 245/40-17, I think it will come down to dynamic factors and ‘feel’ with a big driver-dependent component.

I also wanted to check the implications on slalom spacing. For any given lateral acceleration and car width, you can calculate the time between cones. From that time and the cone spacing, you can calculate speed. So flipping that around, I wanted to see at what spacing you’d run out of 2nd gear. For the BRZ at 1.14 g:

Tire		max 2nd		slalom spacing
235/40-17 ZII	57.8 mph	80.8’
245/40-17 ZII	58.6 mph	81.8’
225/45-16 ZII	56.7 mph	79.2’
225/45-17 Rival 59.4 mph	83.0’

Again, the differences are small. What is the likelihood of running into a course where one tire is under the limiter and another isn’t? Also keep in mind that for the BRZ in a slalom, ‘missing’ one cone by just 1’ is 0.068 sec slower than being on it. Again, my conclusion is to make a reasonable choice and just drive.

Congrats if you read this far – I really did this for my own piece of mind but took the time to type it up in the hopes it will be useful.

[Jawnathin]

Thanks for sharing your data. It is very early (5am~) over here, so I'll have to read it with a better state of mind later on, but I have a quick question...

How did you derive that a 245 is .035 slower than a 235 in the FT zones from your Wilmington data using revs per mile? Do you have any data from a longer and more traditional course like a National Tour?

I had an event this weekend and with the amount of time I was sitting hovering and exceeding the high 50s, I feel that the 245/40 was a faster tire for that day. A 235/40 would have been on the rev limiter even more so while I could hold on just a little more on a 245/40 before the shift to 3rd. Speaking of which, the rev limiter on these cars really hurt. My logger says that while on on the rev limiter the car jumps around 6900-7300RPM at 56-57mph before settling at 6900rpm if I keep on the throttle. Despite being able to rev higher, does anyone why does the car doesn't stay at fuel cut and just kinda sorta bounces around?

[jdrxb9]

Originally Posted by Jawnathin

Thanks for sharing your data. It is very early (5am~) over here, so I'll have to read it with a better state of mind later on, but I have a quick question...

How did you derive that a 245 is .035 slower than a 235 in the FT zones from your Wilmington data using revs per mile? Do you have any data from a longer and more traditional course like a National Tour?

For my wilmington-representative calcs, it is no surprise that the smaller tires are quicker since none of them hit the limiter. For the toledo-representative calcs, the smaller tires are still quicker (though less so) because they 'make up for' being on the limiter sooner by quicker accel prior to hitting the limiter.

I don't have any data from a National Tour or Nats since I wasn't taking data prior to this year. I have data from some longer/faster runway-based events, but I don't think they are really NT-representative courses so I didn't look at them closely.

Originally Posted by Jawnathin

I had an event this weekend and with the amount of time I was sitting hovering and exceeding the high 50s, I feel that the 245/40 was a faster tire for that day. A 235/40 would have been on the rev limiter even more so while I could hold on just a little more on a 245/40 before the shift to 3rd. Speaking of which, the rev limiter on these cars really hurt. My logger says that while on on the rev limiter the car jumps around 6900-7300RPM at 56-57mph before settling at 6900rpm if I keep on the throttle. Despite being able to rev higher, does anyone why does the car doesn't stay at fuel cut and just kinda sorta bounces around?

My data shows the ECU cuts throttle and holds 7200-7100 rpm at the limiter. If you give the ECU control long enough, it will re-accelerate back up to 7300+ and start the cycle over. If I know after the first run or two I'll definitely be on it but don't want to shift, I try to modulate the throttle myself to stay close to 7400 without hitting it. However, 40m at 7400rpm (26m/s) vs 7150rpm (25.1m/s) is 1.538 sec vs 1.592 sec (-0.054 sec) so I try not to beat myself up about hitting it. Here is the throtle/rpm/distance Toledo data that shows my limiter data around the 13.7-15.3 sec timeframe.



I'm pretty sure there is a course where the corner speeds and full throttle lengths combine to give the advantage to a larger tire, but I'm not going to play 'what if' trying to come up with an example. If you (or others) have data from a NT or Nats, I'd be happy to parse it down and run representative calcs.

[Jawnathin]

Originally Posted by jdrxb9

For my wilmington-representative calcs, it is no surprise that the smaller tires are quicker since none of them hit the limiter. For the toledo-representative calcs, the smaller tires are still quicker (though less so) because they 'make up for' being on the limiter sooner by quicker accel prior to hitting the limiter.

Thanks and that is what I figured would happen with a shorter tire. I am still looking to understand how you landed at your calculated numbers. The numbers I see is distance and revs per mile, and I'd like to know your methodology and how you used those numbers to determine the difference in time between tire sizes.

I don't have any data from a National Tour or Nats since I wasn't taking data prior to this year. I have data from some longer/faster runway-based events, but I don't think they are really NT-representative courses so I didn't look at them closely.

Okay, I know Pro courses tend to have a different course layout and run shorter than something like a national tour or even a local event. I figured a longer (50-60 seconds) and more traditional course would better represent the real world differences between tire height.

I was a volunteer worker at the Speedway CAM event this weekend. It was co-hosted by the National SCCA office and considered to be a national event, though limited to only CAM classes for competition. We host the local NT and Pro at this site. Course wasn't overly technical, but it was a good mix of speed maintenance through a long series of of high speed slaloms, a very fast long sweeper, into the back half consisting of a series of 'Z's, which were heavy braking into medium sized acceleration zones until the finish.

Because I was a volunteer worker, I was able to run it and I might have some data to share on it later on. I'll need to see what my data logger was able to record.

My data shows the ECU cuts throttle and holds 7200-7100 rpm at the limiter. If you give the ECU control long enough, it will re-accelerate back up to 7300+ and start the cycle over.

My data comes from OBD2, so I think it is a readout of what the ECU's commanded throttle, not the actual throttle pedal position. It's hard to remember exactly my foot position in that run I recorded, but I think I was flat out of the slalom into a chicane and the throttle/RPM goes all over the place before eventually settling at 6900rpm before I shifted. I have the mic near the exhaust and it sounds pretty crazy going on and off.

I run 245/40s now and I think the 235/40 meant I would have hit the rev limiter sooner with lower MPH until that shift.

I wasn't staring at the speedometer, but in the corner of my eye some yellow light (not the shift light) tended to flash occasionally despite the TC being off. I know it isn't 100% off until I do the pedal dance, but I wonder if the ECU is playing with the throttle when it does flash.

Anyway, I'll see what my data logger actually captures (I use Solo Storm) and I'd like to know a little more about your calculations from your previous post.