Tire correction factor.. What is this??

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Tire correction factor.. What is this??

Post by tek1229 » Fri Sep 11, 2015 8:11 pm

I have a little problem.. I asked TAC and was told I shouldn't have to mess with it, but if some of you are bored and wondering why the pico is showing no vibration when there is a vibration check this out, see if I'm a little nuts, lol.. Well we already know that answer so maybe just try this to amuse me..

On the second screen of NVH, Vehicle Information..
There is a tire correction factor.. I have seen the trucks I'm working on at either 1.030 or 1.040.. I swear I didn't touch the setting to make it that value, lol!!

Anyways.. I have a truck that has a bad vibration, showing some T1 but not as high as it should be if that's the vibration.. Center console shaking side to side pretty bad..

So if I turn on unknown vibrations and every other vibration t1-t4, p1-p4,E1-E4,etc and see what's high..
I have an unknown vibration pretty close to T1 in Hz but not in the range.. Also another unknown as well sort of around tire 2 ,all known vibrations on the left are under 10mg... so why is the center console shaking so bad? If I had a coffee in it I would be wearing it!!!

T1 listed below, same snapshot of data, just changing the correction factor on the 2nd screen and going back to thr 3rd and reading..

T1 9.14mg at 1.040
T1 17.5mg at 1.030
T1 28.0mg at 1.020
T1 39.0mg at 1.010
T1 55.5mg at 1.000
T1 58.0mg at 0.990
T1 55.9mg at 0.980
T1 49.6mg at 0.970
T1 40.2mg at 0.960

It also makes my T2 vibration go from being non-existant at 1.6mg at 1.040 go up to 14.5mg at 0.990.. Now if I go by that I have a T1 and a T2 vibration bad enough to chase..

See how it maxes out at just about 1.000, actually 9.990 is the highest, if that value is a multiplier for tire correction it would make sense that a tire with 10k miles on it would be slightly smaller, hence a value of 0.990 would be the most accurate? Where or why was it at 1.040? Who decides that value? I don't remember changing it and I swear the last truck I used the pico on had it set at 1.030

Anyone confused yet? I've asked this every time I call TAC for a vibration about this setting, I can hear the silence as I'm explaining this.. They don't actually use the software, for the most part they have no idea what I'm talking about,hopefully someone here does?

Have fun guys, enjoy your weekend!!!

Steve Smith
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Re: Tire correction factor.. What is this??

Post by Steve Smith » Mon Sep 14, 2015 10:23 am

Hello and thank you for the post.

Could I ask “What is TAC”? (Would this be GM Technical Support?)

Regarding the Tyre Correction Factor, this is a feature included within the GM and Generic NVH software and is applied to the Wheel Speed frequency calculation to allow for a number of variables.

Both the GM and Generic NVH software obtain road speed using OBD data and calculate Wheel Frequency using road speed and the tyre circumference.

Tyre circumference is subjected to many variables including pressure, wear and deformation and so the VM engineers have arrived at a default value of 1.030 to provide an average “one size fits all” setting that will enable users to capture a realistic overview of all vibrations, taking into account the numerous variables mentioned.

What is interesting in the case above “Tyre Correction Factor” may require amending subject to tyre condition and arrangement on the vehicle. The good news here is the Tyre Correction Factor can be adjusted post capture when reviewing test results allowing for deviations during the road test as we are dealing with “real world” scenarios.

We must also be aware when obtaining road speed data via OBD; does the OBD road speed data match the Speedometer data? If so this is highly unlikely to be the true road speed as the vehicle manufacturer will build in a deliberate error into the speedometer reading to ensure the drivers road speed is always compliant with the law.

For Example: Check your speedometer reading against any aftermarket GPS navigation device displaying road speed. 30 mph on your speedometer is in reality 27 mph via GPS calculation.

This can generate a concern during wheel frequency calculation via the ODB/Tyre Circumference method if ODB speed matches Speedometer speed. Tyre correction factor can help here.

An alternative wheel frequency calculation method (and more accurate) is to use engine speed with Transmission/Final drive ratios.

For Example:
Wheel Frequency calculated from OBD road speed and tyre circumference.
Road speed 47.8 mph = 76.92 Km/h
Tyre size 205/55/16
Wheel Frequency 11.34 Hz Calculated using tyre circumference
Engine speed 1476 rpm = 24.6 Hz
6th gear ratio 0.638:1
Final drive 6th gear 3.238:1
NVH tyre correction factor 1.030:1

Wheel Frequency calculated from engine speed and transmission ratios:

Engine speed (input shaft speed) 24.6 Hz, now via 6th gear (0.638:1) = 24.6 Hz / 0.638 = 38.55 Hz (Output faster than input due to over drive style gear ratio 0.638:1)
38.55 Hz output shaft now acting upon differential ratio 3.238:1 = 38.55 / 3.238 = 11.90 Hz drive shaft speed and so Wheel Frequency.

ODB/Tyre Circumference method 11.34 Hz
Engine RPM/Transmission Ratio method 11.90 Hz

Apply Tyre correction factor to the OBD/Tyre circumference method 11.43 Hz X 1.030 = 11.77 Hz

This will now bring both calculation methods closer into line whilst remaining conscious of the numerous other factors of machine tolerances, wear, backlash, thrust and differential action that all have an effect on wheel frequency.

I hope this helps, take care……..Steve

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Re: Tire correction factor.. What is this??

Post by eric989 » Fri May 20, 2016 7:16 am

Thank you Steve.

I asked the question before, and only knew the Tyre correction factor related to pressure,ware etc.. Now, I know how to calculate it. :D

Did you have a typo here:
ODB/Tyre Circumference method 11.34 Hz
Engine RPM/Transmission Ratio method 11.90 Hz

Apply Tyre correction factor to the OBD/Tyre circumference method 11.43 Hz X 1.030 = 11.77 Hz
I think in this situation the Tyre correction is 1.049. (11.34 Hz X 1.049 = 11.90 Hz)


Steve Smith
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Re: Tire correction factor.. What is this??

Post by Steve Smith » Wed May 25, 2016 10:28 pm

Hi Eric thank you for the post.

Good catch with the figures and theoretically this is correct

Whilst the engine RPM/Transmission ratio method should be considered more accurate it is still subject to the variables mentioned above.

The good news is that we are all in the ball park and the Tyre correction factor is there to get us out of jail

Take care.......Steve

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Re: Tire correction factor.. What is this??

Post by FioranoCars » Wed Nov 09, 2016 10:41 am

Hi Steve
Just to follow up some musings on the NVH and tyre sizes, as we have found some issues, that affect all of our cars and many normal cars:
1. we have different front and rear tyre/wheel sizes, even on 4wheel drive vehicles, so need tyre size by axle
2. Our tyre sizes rarely mathematically follow the formulae "(width x profile%)+(Diameter(in mm)/2) * 2 * π", and the size will need manual correction (for both axles), the difference between manufacture spec and the formulae is often large.
3. Correction for a stationary, or very slow moving car, is very different than a moving one, there is a "rule of thumb" that the contact patch created with the tyre on the vehicle, on the floor, changes the circumference by a factor of 0.96, clearly type of vehicle (weight) and profile of tyre and wheel diameter all will affect this ... there may be some more accurate data to be had ...
4. a Fast moving car (yes, that's ours!) will have a different circumference correction than a slow moving car, so the formulae is needing a speed based adjustment from very slow to normal city (25-40mph) to normal roads (55-60mph) and motorway/fast (lets not forget track use or Germany, just to save any "speeding" allegations get discussed) :D ) of say 75-80mph or even higher, much higher ... thankfully we already have a speed input from the OBD :P ... but a dynamic adjustment option (for vibrations under acceleration conditions or braking) verse steady speed is something I think needs to be planned for future releases, as these issue become more complex,this sort of correct will be critical ... best start the process/thinking/planning now? Yes, I've had F40 owners complaining of vibrations at specific speeds in the 150mph + range, and did try dyno testing, but was track testing that found the issue.
5. The GM study you have used for your 1.03 adjustment/correction is for American 4x4 which somewhat different from average cars,let alone ours. So this one size fits all is a poor compromise, maybe a good start, but time to allow a more complex correction. In fact using this alone, could lead to over 15% error, just letting you know :!:
6. Tyre pressure (increasing it above normal operating settings) might be a way of reducing some some of these factors, but may in itself cause other problems including masking the fault or creating more unrelated noise hiding/,asking the real problem, let alone potentially causing issues with TPMS etc, but I think it would be good to start to understand how tyre pressure might be used to help the NVH process ... I guess user feedback and back to back runs is the only way, trial and error?
7. Logging other OBD data? While RPM and Road speed are already a key element, what about steering angle? YAW (Lateral and transverse) from the cars sensors, Even ABS parameters (ok, harder due to ABS module ceasing comms at circa 40mph) ... Pump etc might have a direct and very hard to trace relationship? Ok, the last is a bit blue sky, I'll admit that! :oops:

Anyway, I think that covers what we've discussed about tyres etc, and some too

Richard Lukins

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