New user with two unrelated problems

[rbpeirce]

I finally got HLT working yesterday and it worked great except for two things.

Thing 1: I can see the shift points on the tac but the gear indicator doesn't increase. It gets to 3 and seems to stick, although I did see it get to 4 once. Why is that?

Thing 2: The probably has nothing to do with HLT but if I render a lap video and store it in Photos (iPhone) I can't upload it to FaceTime. I stored the video in iCloud when I got home because it wouldn't store at the track. When I tried to send it to FaceTime it said it was the wrong format, but it is mp4 and that is a valid format.

On a related note, I had previously asked if raw video required 40Mb/min, what would rendered video require. I thought more but a 2:28 video only took 56Mb. How is that possible?

[Harry]

[Moved to Using LapTimer]

#1 Please revisit all settings influencing the gear calculation. This are wheel sizes, drive axle, gear ratios and drive ratio.
#2 I cannot help here, but please keep in mind video formats are complex and mp4 is the container type only.

Video sizes differ depending on the bitrate used. This varies by encoder and format and quality settings. The standard values listed in the Video Documentation and in the Numbers, Numbers, Numbers FAQ here in the forum will vary.

- Harry

[rbpeirce]

[Moved to Using LapTimer]

#1 Please revisit all settings influencing the gear calculation. This are wheel sizes, drive axle, gear ratios and drive ratio.
#2 I cannot help here, but please keep in mind video formats are complex and mp4 is the container type only.

Video sizes differ depending on the bitrate used. This varies by encoder and format and quality settings. The standard values listed in the Video Documentation and in the Numbers, Numbers, Numbers FAQ here in the forum will vary.

- Harry

Very helpful. Everybody is pretty consistent about individual gear ratios but final drive is all over the place. I think my final drive number could be wrong. Published information puts it at 3.25 for the PDK Cayman R, but I have seen as high as 3.90, and my actual experience on the track causes me to think it is actually somewhere around 3.75 because in one particular location I know it shifts to fourth around 100 and to fifth around 120. A 3.75 final drive calculates out to 96 and 122 in your app, which is close enough. A 2->3 upshift around 69 is also about right. I am going to use 3.75 and see how that works in September.

For what it is worth, here are my current numbers. If anybody has a correction, let me know. BTW, I have 275/19s on the rear. Spec is 265/19 but I couldn't get that size. However, I have recorded this.

F 3.75 (est, instead of 3.25)
1 3.91 (40 mph)
2 2.29 (69 mph)
3 1.65 (96 mph)
4 1.30 (122 mph)
5 1.08 (146 mph) [Not on any track I've been on]

[gplracerx]

To calculate the tire size, you need to give the aspect ratio as well as the section width and rim diameter. That's the middle number, the 35 in 285/35/19, for example. You can calculate overall gearing from the speed and RPM. Use GPS for the speed as the speedometer is likely not accurate. Either run at constant speed and record the RPM's or constant RPM's and record the speed. The overall gearing is then tire circumference times RPM times 60 divided by speed. The length units for tire circumference and speed must be the same. More conveniently, use the inverse of the circumference, tire revolutions per mile or km and divide instead of multiply. There are on line calculators that will give you approximate tire specs from the tire size:
https://tiresize.com/calculator/

The OEM rear tires are 265/35/19 calculate to 767 revolutions/mile. Assuming the aspect ratio is the same, 275/35/19 tires would be similar at 759 revolutions/mile. If that's the case, then a 3.75 final drive ratio and a 1.30 fourth gear at 122 MPH gives 3.75*1.30*122*759/60 = 7523 RPM. My guess, though, is that the final drive ratio is 3.89 and you've specified the tire size incorrectly. If you have 275/40/19 tires, you get the same speed at 7500 RPM as you show in your note.

[rbpeirce]

My guess, though, is that the final drive ratio is 3.89 and you've specified the tire size incorrectly. If you have 275/40/19 tires, you get the same speed at 7500 RPM as you show in your note.

The tire size is correct -- 275/35/19 -- but I have wondered about the 3.89 final drive. That is the ratio they use in the 6MT and it is possible, I suppose, that it got into my PDK equipped car by mistake. It also produces reasonable estimated numbers.

Until I can nail this down, if I ever can, I think I will use 3.8. That implies a 3-4 shift at 95mph and a 4-5 shift at 120, which is darn close to what I actually experience.

[Harry]

This is from the manual:

R.jpg (43.33 KiB) Viewed 2509 times

This means you need to enter 3.25*1.114 = 3.62 as the drive ratio.

- Harry

[rbpeirce]

I didn't understand the significance of the constant ratio. I don't even think I know what that means. Nevertheless, 3.62 is still too low to explain my on track experience.

I know PDK is shifting from fourth to fifth at around 121-122 mph I noticed it before I started using HLT and I can see it in the video. Assuming everything else is correct, and I do seem to agree with all the other published numbers, the number that will produce the correct gear numbers in HLT, which isn't necessarily the correct number, is 3.78. That's with 275/35/19 tires in the rear and a gear ratio of 1.30 for fourth.

One thing I don't know a way to put in is PDK seems to shift between 6900 and 7300 RPM in sports+, depending I think on whether I am ascending or descending and the side load on the car. Power peak is at 7400. My tac is white at 7400 and red at 7600 and I have assumed red line is 7500. I'm not sure how this factors into anything and maybe I am just over-analyzing.

[Harry]

Try it, it will work. There is no special magic for the Cayman. In case you find different values in your manual, change them. The constant ratio is some ratio added by the PDK.

- Harry

[rbpeirce]

I had an amazing insight. If you use 3.62, the maximum speed in fourth is 126 at redline. However, PDK actually shifts between 6900 and 7300 which puts the actual speed at the 4-5 shift pretty close to 120! That's good enough for me.

Thanks for the tip on multiplying the final drive ratio by the constant. I had to dig into the csv file to figure out the rest.

[gplracerx]

I forgot about the .csv export. If you have a spreadsheet program do a scatter plot of GPS speed vs RPM in a single gear. Then do a linear trend calculation and show the equation. Now you have a really good estimate of the overall gearing ratio from the slope and can calculate either the revolutions per mile of the tire assuming a 3.62 drive ratio times the gear ratio or the final drive ratio for a given gear ratio and tire revolutions/mile. Usually the revolutions per mile will be somewhat different from the value calculated from the tire size. For example, my 285/35/19 Bridgestone RE-71R tires calculate to 751 revolutions/mile, but measure 760 revolutions/mile.