SCCA Solo C Street Discussion

[jdrxb9]

Originally Posted by Jawnathin

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.

All you are seeing in my original posts are results, not the calcs. To expand on my brief introduction:

• The calcs start with a torque curve - I chose this one as reasonably representative: http://www.ddperformanceresearch.com...CU%20reset.jpg
• The torque curve in the spreadsheet is represented by data points at 500 rpm intervals.
• The initial conditions are used to find the current rpm and the torque for that rpm is interpolated.
• That torque is multiplied by the selected gear ratio, final drive ratio and tire size to calculate the force applied to the ground.
• Drag forces for aero and rolling resistance are subtracted from the accelerative force to calculate a net force.
• The net force is divided by the total mass to calculate acceleration.
• A correction factor is applied to the calculated acceleration to achieve correlation with reported and measured data.
• The currently calculated acceleration is used with traditional linear acceleration equations to calculate the change in speed and distance for the current time increment. The linear equations are reasonable here due to the small time increment (typ. 0.02 sec)
• Those results set the conditions for the next time increment and all of the above is performed iteratively
• For any given set of initial conditions, the full time/speed/distance history is solved and can be interrogated.
• Of most interest is time to cover a known distance.
• To generate the 'representative' results, I just input a few different sets of conditions and manually sum the results.

Originally Posted by Jawnathin

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'd say most Pro courses are pretty representative of national courses, just shorter. Obviously, if you have 2x as many FT zones, the reported differences could be multiplied 2x, but will still be <~0.1 sec.

Originally Posted by Jawnathin

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. ... I use Solo Storm

All my data is OBDII data as recorded by solostorm as well. The throttle reported is ECU commanded. I can guarantee that is not me dropping the throttle to ~18% in the 14-15 sec timeframe in the plot above.

[Jawnathin]

All you are seeing in my original posts are results, not the calcs. To expand on my brief introduction:

Ok, that sounds really complicated, I'm not sure how you could accurately measure the effects of some of these factors...

• Example 1 - Rolling resistances: different tires have different rolling resistance, they get softer in the heat, harder after getting worn, etc
  
• Example 2 - Drag: Exhausts, wheel design/offset, tire width, FRS vs BRZ bumpers/spoilers all have influence on the aero of the car.
  
• Example 3 - Torque curve per RPM without the CSV or spreadsheet detailing the exact value at those RPMs. (Maybe you have that somewhere? Maybe you went pixel by pixel for close enough approximation?)

...and without understanding your formula and how each factor is weighted, I'm naturally a little skeptical. It isn't skepticism of you or your effort, I appreciate the contribute and discussion, its just this calculation is unknown and without validation of any type to ensure accuracy. Hopefully you understand where I'm coming from, but let's go with it and see where we end up.

BTW, did you also factor in the gravitational pull of Mars?

I'd say most Pro courses are pretty representative of national courses, just shorter. Obviously, if you have 2x as many FT zones, the reported differences could be multiplied 2x, but will still be <~0.1 sec.

Well I was thinking of the drag strip start being a little different from your typical NT event. But yes, the primary difference is that they're shorter, 35~ seconds vs 60~ seconds or so, so there may be double the number FT zones. Can also x2 for each side (on a pro) or each day (for a tour/Nationals).

All my data is OBDII data as recorded by solostorm as well. The throttle reported is ECU commanded. I can guarantee that is not me dropping the throttle to ~18% in the 14-15 sec timeframe in the plot above.

What do you think about looking at 90%+ or something for commanded throttle for acceleration runs? I think the ECU does something funny with its commanded throttle calculation. I took a quick look at one of the logs and in different parts of the run I was able to have several duplicate 98.5294117647058 and 97.0588235294117 commanded throttle position entries. Not sure if it blends it with a previous sample or if I was really able to nail town 98.5% instead of 100%. I have a few other entries of very high throttle % where I'm sure the intent was WOT and the car is being fed all the air it can take anyway.

If you wanna take a stab at my logs I can send them over. Let me know and I'll PM you with a way to get them. This might be going a little far, but if it isn't much to do three different events, I'd like to see how they turn out. One is from the long runway course (the course I described above) and the other is a 'square' site that is decently open but not quite as fast as the runway is. I should have some data from the 'fast' course on that site where I end up on the rev limiter a few times and data from a pretty slow course where I'm still a few hundred RPM off. I'd be curious to see what if any differences between the three different course/runs are.

[jdrxb9]

Originally Posted by Jawnathin

Ok, that sounds really complicated. I'm not sure how you could accurately measure the effects of some of these factors << snip >>

Well, good thing I'm not trying to accurately account for those factors, or others - non-level pavement probably being the biggest. Seriously, as I mentioned in the first post, it is an approximation to be used for comparison purposes. The objective is to answer the question: if all else is the same, what is the effect of gearing on time to distance for representative starting speeds and distances? I don't think the rolling resistance or aero differences between the tires considered will be significant and I think a torque curve approximated at 500 rpm intervals with linear interpolation between is also ok since it is the same for each case. It really isn't that complicated for straight line acceleration with no tire slip - see here for a more detailed discussion of the principals and equations.

That said, I did compare the results to my data and also C&D's 5 mph - 60 mph data (which, btw, shows 0.5 sec variance for the same car) and added a correction factor to get it to generally match. I say generally, because the factor required to match 7.5s 5-60 is different than matching 8.0s 5-60 is different than all the different factors required to match all my different 'pulls'. In the end, I just had to pick one factor and let it be.

Originally Posted by Jawnathin

Well I was thinking of the drag strip start being a little different from your typical NT event.

Since the general technique is to spin (gearing irrelevant) through a good part of 1st gear, I'm only considering from 4500rpm to redline in 1st at the 'start'. How that compares to a non-pro course will be wildly variable - could be a lot different if the course forces a 'short shift' near the line or could be similar if the course allows carrying 1st across the line and reaching redline before grabbing 2nd.

Originally Posted by Jawnathin

What do you think about looking at 90%+ or something for commanded throttle for acceleration runs? I think the ECU does something funny with its commanded throttle calculation. I took a quick look at one of the logs and in different parts of the run I was able to have several duplicate 98.5294117647058 and 97.0588235294117 commanded throttle position entries.

Other than to verify you've done solostorm's throttle calibration, not sure what to tell you here. My csv outputs show consistent 100%'s aside from the odd leading or trailing values. If you have csv outputs from SCCA courses at Crows Landing, I'll take a look - see my PM for an email address. Keep in mind, I'm just going to look at them get representative corner exit speeds and 'straight' lengths which can then be used to generate comparisons.

[Shark_Bait88]

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.

Yeah, this probably made things the clearest for me. haha I'm trying to be competitive locally, but this year my driver mod is going to make the most difference.

Decided to stop stressing over the details/data, and ordered a set of 235/40/17 Star Specs. Gonna go pick them up in South Bend on the 21st and hope for the best!

[G_Ride]

Originally Posted by Shark_Bait88

Yeah, this probably made things the clearest for me. haha I'm trying to be competitive locally, but this year my driver mod is going to make the most difference.

Decided to stop stressing over the details/data, and ordered a set of 235/40/17 Star Specs. Gonna go pick them up in South Bend on the 21st and hope for the best!

What a coincidence! I'm picking up my RE71Rs from Tire Rack on the 21st as well!

[Shark_Bait88]

Originally Posted by G_Ride

What a coincidence! I'm picking up my RE71Rs from Tire Rack on the 21st as well!

Woah! No way!

If you had a Outback wagon winter beater we could totally carpool together and fit both sets of wheels and tires, maybe even bring along some dude named Karl too...

[G_Ride]

Originally Posted by Shark_Bait88

Woah! No way!

If you had a Outback wagon winter beater we could totally carpool together and fit both sets of wheels and tires, maybe even bring along some dude named Karl too...

You read my mind! It's a good thing I do in fact have that winter beater

[Jawnathin]

Originally Posted by jdrxb9

If you have csv outputs from SCCA courses at Crows Landing, I'll take a look - see my PM for an email address. Keep in mind, I'm just going to look at them get representative corner exit speeds and 'straight' lengths which can then be used to generate comparisons.

I'll get you some logs in a bit.

Originally Posted by Shark_Bait88

Yeah, this probably made things the clearest for me. haha I'm trying to be competitive locally, but this year my driver mod is going to make the most difference.

Decided to stop stressing over the details/data, and ordered a set of 235/40/17 Star Specs. Gonna go pick them up in South Bend on the 21st and hope for the best!

Congrats, that is a great tire. I thought long and hard about them, but ended up going a different route due to contingency. With those tires you have a great chance at being competitive locally.

Originally Posted by G_Ride

What a coincidence! I'm picking up my RE71Rs from Tire Rack on the 21st as well!

Nice. I've got a set of RE71Rs on the way too. Curious to see what you think of them once you get some runs on them. So far I've heard some great things. Did you get 245/40s?

[Jawnathin]

I sent over my logs. Car is on 245/40 RS3s.

Here are the videos of those runs.

This is our runway site, Crow's Landing. This was the Speedway CAM course on 3/7. I ran a little faster on the 2nd day (same course) but since I couldn't get a video, I just sent the logs of this video. Tires have about 300 runs at this point.

[ame="https://www.youtube.com/watch?v=_T8V7zIS3co"]BRZ AutoX - 3/07/08 CAM - Jonathan - YouTube[/ame]


This is our local 'paddock'/parking lot site. This course was a little faster than your typical course here, I hit the rev limiter three times. Fun course though. Tires had around 180-200 runs at this point.
[ame="https://www.youtube.com/watch?v=jkE99yRSgXg"]Subaru BRZ AutoX - 10/26/14 AAS @ Marina- Jonathan - YouTube[/ame]

[Shark_Bait88]

Originally Posted by Jawnathin

Congrats, that is a great tire. I thought long and hard about them, but ended up going a different route due to contingency. With those tires you have a great chance at being competitive locally.

Thanks! Yeah, I've heard good things so far and figured I'd give them a try. G_Ride lives near me and we hang out a lot, so I will get a chance to experience the RE71Rs too. My main concern was improving my driving and doing well within my region. Next year though, I'll likely get a set of 245/40s and try to make more national events.

[G_Ride]

Originally Posted by Jawnathin

Nice. I've got a set of RE71Rs on the way too. Curious to see what you think of them once you get some runs on them. So far I've heard some great things. Did you get 245/40s?

I did

[jdrxb9]

Originally Posted by Jawnathin

I sent over my logs. << snip >>

Just based on the video, I took a look at the CAM event data first because it looked like it would have a lot of interesting analysis opportunities. Looking at the video and through the data, I pulled 9 distinct full throttle zones from the run. The first plot below (should be a clickable thumb) shows all the data as recorded by solostorm and the second plot shows just the 9 FT zones. I analyzed zones #2 and #4 as constant FT even though there are very momentary ‘lifts’ of the throttle – even if the data is good, we’ll say these could’ve been 100% on the later, quicker run

One thing I noticed right away is that you have a lot of ‘repeats’ in your RPM data and a fair amount in your speed data as well. I think your OBD dongle is reading at a lower rate than what solostorm is recording at. The data required a little more interpretation, but it still provided a good sampling of initial speeds and distances.

The full results are in the table/image at the bottom of the post. The various columns show the results from each pull and the total FT acceleration time is in the next to last column. The rows show the baseline 245/40-17 R-S3’s with the shifting as driven as well as various other comparisons. To summarize:

gears used	tire	
1-2-3-2-3	245/40-17 R-S3		18.317 sec
1-2-3-2-3	235/40-17 ZII*		-0.001 sec
1-2-3-2-3	225/45-16 ZII*		+0.004 sec
1-2-3-2-3	225/45-17 Rival		+0.004 sec
1-2		245/40-17 R-S3		-0.026 sec
1-2		225/45-16 ZII*		+0.121 sec
1-2		235/45-17 RE-71R	-0.061 sec

These results turned out a bit differently and even closer than I would’ve guessed. Over 9 zones the closeness of the different sizes using the shifting pattern driven is remarkably close. Seeing that you slowed down after shifting to 3rd at the 14 sec mark, the second shift to 3rd is quite close to the line and in general not a lot of the run is at speeds above the 2nd gear limit speed, I wondered what the calcs would show for just holding 2nd throughout the run.

Those 2nd-gear-only results are shown in the bottom 3 rows. I decided to go with the baseline tire and two extremes in the 225/45-16 and 235/45-17. To my surprise, holding 2nd was a bit quicker for the baseline size and also the tall 235/45-17, which I wouldn’t have guessed. Even more surprising is the 225/45-16 is only 0.121 slower despite being on the limiter for a total of 5.4+ sec!

So this is indeed a case where taller is better. But for a fairly fast course with ~18 sec of full throttle acceleration over an ~60 sec run, the differences are small. My conclusion is still to make a reasonable choice and drive! I’m really suspicious that the gearing advantage is not worth the compromise in overall cg height and sidewall stiffness (though you could mitigate the latter with 235/40-18’s ).

Definitely an interesting data set to look at. I do plan to also look at the AAS data as well, but I'm not sure when I'll get to it.

[Jawnathin]

Very interesting, thanks a bunch for taking the time and analyzing it. If it is too much effort, no worries about the AAS event, just wanted to provide two different data sets of your typical runway and 'parking lot' course and see how they pan out.

Doesn't get much closer than 0.001 I suppose. Interesting as I thought it would be favorable to the 245 than most other courses. I wonder what this means for a course that I thought was more favorable to a 235. Maybe 235/40 has a bigger gearing advantage than I thought? That AAS course is pretty 'neutral' I would think, as it did give three instances of briefly hitting the rev limiter even on a 245. Some other courses here it's still a few hundred RPM short of the rev limiter so the 235/40 has no downside on the FT zones.

You are right, I did slow down a bit once I got into the slalom and may technically have not needed a shift into 3rd, but I figure the car was more settled down @ 5500rpm in 3rd vs 7200~ rpm in 2nd while in a high speed slalom. Also upon the exit of the slalom I'm already in the right gear in the acceleration zone, rather than spending time shifting when I got there.

You do bring up a good point about leaving it in 2nd. I think I tried that for one of my earlier runs and I felt I was on the limiter too much, so I did the upshift to 3rd.

Which brings up a quick question. You touched it a bit with analyzing what happened if I left it in 2nd, but I'm under the impression this data only represent FT zones. So with that in mind, how does your analysis account for the times I'm above 59mph and into 3rd, but not FT? I was 65mph at the exit of the slalom entering that turn around. There was a few moments of partial throttle/lift before it drops below 59mph. I would have liked to save the time on the shifts, but I'd hate to be giving up 6mph at any point on the course.

[jdrxb9]

Originally Posted by Jawnathin

Doesn't get much closer than 0.001 I suppose. Interesting as I thought it would be favorable to the 245 than most other courses. I wonder what this means for a course that I thought was more favorable to a 235. Maybe 235/40 has a bigger gearing advantage than I thought?

I don't think it's that any size has a big gearing advantage - more like the actual time over distance differences are small when you on the limiter with one tire vs another and those small differences are offset by the improved accelerations elsewhere.

Originally Posted by Jawnathin

You are right, I did slow down a bit once I got into the slalom and may technically have not needed a shift into 3rd, but I figure the car was more settled down @ 5500rpm in 3rd vs 7200~ rpm in 2nd while in a high speed slalom. Also upon the exit of the slalom I'm already in the right gear in the acceleration zone, rather than spending time shifting when I got there.

It is definitely difficult to slalom at max g's when you are right up against the limiter and I understand wanting to be in 3rd before the highest speed pull. I actually thought the 225/45-16 was going to show better since it would pull 3rd harder, but that's not what the calcs showed. I also wasn't trying to show 'should've stayed in 2nd' - just how big the differences would be.

Originally Posted by Jawnathin

You touched it a bit with analyzing what happened if I left it in 2nd, but I'm under the impression this data only represent FT zones. So with that in mind, how does your analysis account for the times I'm above 59mph and into 3rd, but not FT? I was 65mph at the exit of the slalom entering that turn around. There was a few moments of partial throttle/lift before it drops below 59mph. I would have liked to save the time on the shifts, but I'd hate to be giving up 6mph at any point on the course.

It is a bit hard to see in the plot, but note the '2nd gear limit speed' line. You just touch it at the start - reflected in the calcs with the 'on limit' time. You just touch it briefly (0.2) just after the shift to 3rd (neglected in calcs). You go over through FT zone 2 (20.3-22.9) - in the calcs. You go over briefly at the end of FT zone 3 - in the calcs. And you go over at the end - again in the calcs. However, looking more closely at the data following FT zone 2 (23.0-23.6), there is some data with part throttle and mixed longitudinal accelerations - hard to tell if it is 'coasting' or braking. This wasn't included in the calcs. It is 64.3 mph over 16m, which would be 0.043 sec slower at the 59.7 limit - enough to wipe out the -0.026 advantage of the FT-only 'stay in 2nd' calcs. Championships have certainly been won or lost by less than 0.017, but overall I still say I'm not going to be too worried about the gearing effects in tire choice. And this may all be moot - if the current RE-71R hype holds up (not to mention the upcoming Rival S), I think the 235/40 ZII (with no contingency) will be really unpopular and almost everyone will be on 245/40-17's anyway