[Max Well]

A discussion in our F97/98 ¼ mile thread provided a prompt to research the relationship between psi and contact patch area [‘CPA’] of our Michelin Pilot Sport 4S 265/40 ZR 21 (105Y) rear tires. The comment started around Post #127 on page 6 (https://x3.xbimmers.com/forums/showthread.php?t=1444613).

I had indicated noticing significant improvement in my ¼ times in Feb’20 c/w Nov’19 at GNV Raceway, and although the DA was improved, nothing else had really changed in my setup - except I decreased psi from BMW's rec'd 33F/38R to 32F/35R for my Feb runs. https://x3.xbimmers.com/forums/showthread.php?t=1672345

It has been suggested by some that lower tire psi may be needed at some tracks to allow improved traction and better 0-60 foot times. Others have offered that minor tire psi changes may not play a major role improving CPA or traction to account for significantly improved times.

So it seemed reasonable to try to find objective parameters for our X3MC’s tires. Although some sources online have indicated calculations which may produce a theoretical number for CPA in general settings, at least one study showed a large discrepancy between actual imprint-based CPA numbers vs theoretical derivations (paper #68 at: http://ijste.org/index.php?p=Archieve&v=3&i=10 ).

As I didn’t want to have to lift our X3MC or use carbon paper, it seemed another option might be to just cut two 90 angle sections of thin poster board and slide firmly until they stop against the tire surface. I figured I could just mark with different color lines for each psi, then trace the footprint on another poster sheet. It was really easy to do and only took about an hour start to finish. I’m including images to show how I did it in case anyone else wants to try on their own tires (since some have changed out for 295s on the rear).

You just start at whatever lowest psi you’re interested in, as that will yield the largest CPA, make your marks, then fill up to the next psi your want to check, move the 90 degree pieces tight against the tire again to make another mark, and repeat. Although I only measured the right rear incrementally I did release and add air to the left consecutively for each measurement as well to insure I wasn’t falsely changing the single tire load and to more closely approximate what I’d do for a day at the strip. I also checked the front right 255 tire at OEM 33 psi just as a reference point.

Results - it would appear in this roughly estimated analysis, decreasing tire inflation by just 3 psi can induce ~ 2 to 2-1/2 square inches CPA increase. As I don’t have a Physics or Engineering background, I’m not about to comment on whether that is or isn’t significant. But I do know from working on the Simulation for acceleration that there are equations which those knowledgeable in the field could use to sort out that answer.

On roads or tracks with a high coefficient of friction, a larger CPA may not be as important, but intuitively, it would seem it should be at smooth tracks, or with tires having less tread. No doubt there is a trade-off with lower psi and increased CPA - higher rolling resistance as described in the study I mentioned, so the answer isn’t just to keep going lower and lower. But every tire and vehicle will have a sweet spot which works best in different track settings (strip vs long track, smooth vs poorly prepped, etc). A psi set too low can lead to sidewall/instability/handling/tire failure … issues . But set too high, especially on smooth strips, it’ll be tough to hit good 0-60 foot times consistently. No doubt those experienced with their own cars at tracks or strips have a good sense where their psi needs to be for the tires they use.

Next time I plan on trying 32F/32R as kingmonkeyjr indicated in an earlier post ( https://x3.xbimmers.com/forums/showp...0&postcount=30 ). Each time out is another chance to learn more about this strip and my X3MC’s settings and capabilities. If anyone has a chance to estimate 295 rears on their F97/98 would be interested in reading.

[AlexFL]

Wow, this is very interesting! Max, you must be a scientist! We should write a research paper together

[AlexFL]

Seriously, very clever experimental setup. Being a scientist myself, I really like seeing creative stuff like this.

[Max Well]

Originally Posted by AlexFL

Seriously, very clever experimental setup. Being a scientist myself, I really like seeing creative stuff like this.

Thanks, Alex. It seems a fair number of Forum Members have an element of STEM background - just an observation seeing garages arranged in orderly fashion with perfectly hung banners neatly spaced and impeccably maintained cars. I do appreciate the knowledge and ideas that many here contribute and enjoy the shared interest.

[AlexFL]

Max, based on your findings, wouldn’t putting wider tires result in greatly improved drag strip times for our car? I haven’t had a chance to test this.

[Max Well]

Originally Posted by AlexFL

Max, based on your findings, wouldn’t putting wider tires result in greatly improved drag strip times for our car? I haven’t had a chance to test this.

I've seen that question debated at length in a number of different Forums, and from what I can gather, there is no easy yes or no answer. Not trying to dodge your question, but the Physics of tire/vehicle/surface interactions is remarkably complex. My uneducated understanding is that the actual area of the contact patch may not be different with all else being the same (vehicle corner loading, tire rubber specs such as sidewall height, psi ...) - just a different shape. But that's where the discussions go beyond my pay grade, in that the behavior of the two patches might differ.

If you get the chance to obtain some measurements on your 295s, I can add your points to the same chart and spreadsheet as mine to see how that area changes with psi. That would be interesting to compare. It literally only takes an hour, and the poster paper was only $2.99 at HL (big box craft store). No special tools required, except for an air compressor and tire gauge, some colored fine-point markers, the poster paper, a mm ruler, and a reasonably smooth floor like a garage.

There is an excellent thread discussing this topic on a forum for Engineering Professionals (eng-tips.com) in which some of the participants appear to have an actual background in tire design and development. Their Forum rules prohibit linking so if you use their 'search posts' feature with the phrase 'contact patch' you should find the thread started back in Aug 2004. Highly rec as it provides a perspective as to how complex that question really is.

[Ironwood]

It isn't just the size of the exterior of the contact patch or flat tires would rule the drag strip. The pressure in the tire keeps the interior of the contact patch pressing against the pavement. A more conclusive test might be to see what force is required to drag the vehicle with the brakes stopping any wheel motion. See what combo of tire pressures yields the highest resistance.

[onfireX5]

Pressure reduction at the drag strip has another advantage ...

You are measuring the static contact patch.

The dynamic contact patch is increased (to a point) with less tire pressure. Weight transfer will plant the rear tire and the contact patch will increase .

A couple of tips we use at the track....you can "read" the edge contact over time and see the shadow. Less pressure will bring the shadow out towards the edge. Keeping the rim width to match the tread width (not section width) will plant the tread better at the hit. 12" tread plants better with a 12" rim.

For a quick and dirt test, just use a water hose on pavement and slowly drive through it. Inspect the wet patch on the tire and on the dry pavement after you drive through it.

Cheers

[Max Well]

Originally Posted by Ironwood

It isn't just the size of the exterior of the contact patch or flat tires would rule the drag strip. The pressure in the tire keeps the interior of the contact patch pressing against the pavement. A more conclusive test might be to see what force is required to drag the vehicle with the brakes stopping any wheel motion. See what combo of tire pressures yields the highest resistance.

That'd be another variable worth knowing but seems a tough test to execute for the average owner. As each tire-vehicle-track surface combination will be unique to a single vehicle, and distinct on any given day with differing temperatures, as well as accounting for treadwear of newer vs older tires, not sure how you'd check that. But your point is valid.

But wouldn't that assess only the tire's static/passive traction?? I mean, as the Engineers discussed in the thread mentioned, dynamic rotational forces under load deform the contact patch in complex ways, so even with your study, not sure that would explain the dynamic phase of what the patch looks like when it's 4,000 lb. vehicle goes WOT with LC. But a good idea, nonetheless.

Originally Posted by onfireX5

Pressure reduction at the drag strip has another advantage ...

You are measuring the static contact patch.

The dynamic contact patch is increased (to a point) with less tire pressure. Weight transfer will plant the rear tire and the contact patch will increase .

A couple of tips we use at the track....you can "read" the edge contact over time and see the shadow. Less pressure will bring the shadow out towards the edge. Keeping the rim width to match the tread width (not section width) will plant the tread better at the hit. 12" tread plants better with a 12" rim. ...

And in the Engineering thread, they expand that further by implying (if I understand correctly), a wider tire having the same area of contact patch as a thinner one but with same specs such as sidewall height, psi, and tire model, that the longer longitudinal surface of the 265's patch will handle the dynamic forces differently than the shorter longitudinal surface (although having the same total patch area) of the 295's.

Curious, even though you've run with completely different vehicles, what tire psi's did you find worked best on your X5M and your M5C for your average days at the Strip, and what was your mindset for making changes?

[GT500R]

Good analysis overall. Your best bet to test at the track is start with a higher psi and run then reduce then run and reduce to see if 60’ changes. Probably best to run twice at the same psi to try and avoid anomalies. But from what I see about 4-5% increase in contact patch if my math is correct from 38 to 35. I still believe that isn’t enough to drop .2 off a 60’ time as that is almost half a second isn’t he 1/4. The DA swing will help and assuming you launched the same for the first set of runs vs the latter set is another thing to consider.

[onfireX5]

Originally Posted by Max Well

That'd be another variable worth knowing but seems a tough test to execute for the average owner. As each tire-vehicle-track surface combination will be unique to a single vehicle, and distinct on any given day with differing temperatures, as well as accounting for treadwear of newer vs older tires, not sure how you'd check that. But your point is valid.

But wouldn't that assess only the tire's static/passive traction?? I mean, as the Engineers discussed in the thread mentioned, dynamic rotational forces under load deform the contact patch in complex ways, so even with your study, not sure that would explain the dynamic phase of what the patch looks like when it's 4,000 lb. vehicle goes WOT with LC. But a good idea, nonetheless.



And in the Engineering thread, they expand that further by implying (if I understand correctly), a wider tire having the same area of contact patch as a thinner one but with same specs such as sidewall height, psi, and tire model, that the longer longitudinal surface of the 265's patch will handle the dynamic forces differently than the shorter longitudinal surface (although having the same total patch area) of the 295's.

Curious, even though you've run with completely different vehicles, what tire psi's did you find worked best on your X5M and your M5C for your average days at the Strip, and what was your mindset for making changes?

I honestly do not remember the exact numbers on those two. The rear is MUCH more important than the front, even with AWD. It take a bigger part of the hit. On our race stuff it’s much more complicated since we manually set rebound and compression on each shock, but the goal is the same. 60ft. The most air with the quickest 60. I don’t go under 15psi on my blue car since it starts to sway at 180mph under that.
My 911 came with 4x psi front and rear. 1.61 sixty. It consistently runs 1.57 with a best of 1.54 on no prep asphalt with 31/33cold or 33/36 hot.

But it took a lot of hits to find that combo.

Tire temp, hot psi, DA, etc etc has to be taken into consideration when tuning a tire.

Cheers

[Max Well]

Originally Posted by GT500R

Good analysis overall. Your best bet to test at the track is start with a higher psi and run then reduce then run and reduce to see if 60’ changes. Probably best to run twice at the same psi to try and avoid anomalies. But from what I see about 4-5% increase in contact patch if my math is correct from 38 to 35. I still believe that isn’t enough to drop .2 off a 60’ time as that is almost half a second isn’t he 1/4. The DA swing will help and assuming you launched the same for the first set of runs vs the latter set is another thing to consider.

Thanks. I'll see what results I get with 32/32 next time out. Gonna be a while before I can make it back out there as the next Saturday morning Test-n-Tune isn't until December, but the DA will surely be different than my last two runs so I should be able to better assess the importance of DA and psi to the end result.

Originally Posted by onfireX5

I honestly do not remember the exact numbers on those two. The rear is MUCH more important than the front, even with AWD. It take a bigger part of the hit. On our race stuff it’s much more complicated since we manually set rebound and compression on each shock, but the goal is the same. 60ft. The most air with the quickest 60. I don’t go under 15psi on my blue car since it starts to sway at 180mph under that.
My 911 came with 4x psi front and rear. 1.61 sixty. It consistently runs 1.57 with a best of 1.54 on no prep asphalt with 31/33cold or 33/36 hot.

But it took a lot of hits to find that combo.

Tire temp, hot psi, DA, etc etc has to be taken into consideration when tuning a tire.

Cheers

Thanks for sharing. Good point about checking the psi before each run, as the tires' temp and psi will go up during the day. This shows the Michelins on my X3MC at HWY speeds (cold psi vs actual at speed):

[ONLYA6]

This is very common for drag racing. I typically run in the low 20's on a street tire and as low as high teens on a drag radial to allow not only a better contact patch but as much flex as possible in the tire.

Also increase the fronts up to max tire PSI (usually 50ish) to decrease rolling resistance.

I got down to 22 on my M2C and started clicking off 1.8 60's on stock tires. Don't waste your time at 32/32. I'd start at 22/50 in 4wd Sport then dial back the front if you are noticing slip.

[Max Well]

Originally Posted by ONLYA6

This is very common for drag racing. I typically run in the low 20's on a street tire and as low as high teens on a drag radial to allow not only a better contact patch but as much flex as possible in the tire.

Also increase the fronts up to max tire PSI (usually 50ish) to decrease rolling resistance.

I got down to 22 on my M2C and started clicking off 1.8 60's on stock tires. Don't waste your time at 32/32. I'd start at 22/50 in 4wd Sport then dial back the front if you are noticing slip.

Interesting - thanks for the additional perspective. I may still decrease by increments of 3 so I get a feel for where the sweet spot is. That way I should avoid a big overshoot, but its definitely helpful to read the pressures you found worked with your M2C. And makes sense to keep the fronts at a higher pressure as one backs down on the rears (with psi inversely related to rolling resistance).

[AlexFL]

Max, I think your latest drag strip times are already at the limit of what the stock F97 can produce (by comparing your times with what else has been posted here and the C&D results). For any further significant improvement, you may want to consider a flash tune or a piggyback.

[Max Well]

Originally Posted by AlexFL

Max, I think your latest drag strip times are already at the limit of what the stock F97 can produce (by comparing your times with what else has been posted here and the C&D results). For any further significant improvement, you may want to consider a flash tune or a piggyback.

Perhaps. But it seems the rabbit hole goes deeper the more I research this. Although I respect C&D's findings, I've never viewed their numbers as definitive. I wonder if we're not missing something by not addressing this psi-CPA issue as well. Much is placed on tunes (and rightly so for logical reasons) as those provide instant results, but as some have already shared - what could those run if psi-CPA had also been tweaked?

Seems many experienced Drivers really focus on the tire/track interface, so I'll be interested to see how decreases of my rear tire psi numbers alter results at the strip. I'll keep a log and share.

[Max Well]

Originally Posted by ONLYA6

This is very common for drag racing. I typically run in the low 20's on a street tire and as low as high teens on a drag radial to allow not only a better contact patch but as much flex as possible in the tire...

After input by ONLYA6, I've added two more psi values (21 and 24). Certainly a big difference in CPA size as you go low.