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It's Friday evening and I feel like expounding on a topic that doesn't come up often but can be quite significant to endurance racing. :)

 

singletiredragline.jpg?512

Let's accept the illustration as presented. Note the small amount of induced drag that occurs as the grip builds. At about 85% of peak grip, the induced drag starts to climb quickly, and by peak grip is about three times what it was at 85%. As the slip angle continues to increase, the induced drag also continues to increase. When the grip level is back at about 85% of peak, the induced drag is now about 12 times what it was at the low slip angle. One might assume that the tire wear is also about 12 times more!

 

This graph might make it easier to see that relationship:

alignment_sacurve.jpg

 

So we can see that there can be a significant difference in induced drag for a given cornering force, and by extension for a given lap time. The conservative and efficient driver stays on the upslope of the curve and has low induced drag and wear, while the aggressive driver uses the tires at excessive slip angles, trashes the tires and uses more fuel but ends up with the same lap time.

 

Okay, now for some questions. Let's say that the peak grip so far is 1.0 g, so the grip at the low induced drag slip angle is about 0.85 g. We want to go as fast as the team that is running the corners at 1.0 g without giving up our low induced drag and subsequent low tire and fuel usage.

1. Is it possible to achieve 1.0 g at the low slip angle with a softer compound? If so, what will happen to the induced drag and tire wear? Will it be less than the original tire drag and wear at 1.0 g? Any downside?

2. Is it possible to achieve 1.0 g at the low slip angle with a wider tire? If so, what will happen to the induced drag and tire wear? Will it be less than the original tire drag and wear at 1.0 g? Any downside?

 

 

Edited by mender
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Beyond me, all I know is sliding em is going no where fast.   

 

And does anyone have a comparison on the new Maxxis tires compared to others that have been run in the series ?    Don't want to start a tire bashing here, but would like to know if they feel better ? produce better lap times ? last as long or longer ?  etc.....

 

Thanks for any info.   We wouldn't mind trying them but not at the expense of lap times and longevity.

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38 minutes ago, mender said:

It's Friday evening and I feel like expounding on a topic that doesn't come up often but can be quite significant to endurance racing. :)

 

singletiredragline.jpg?512

Let's accept the illustration as presented. Note the small amount of induced drag that occurs as the grip builds. At about 85% of peak grip, the induced drag starts to climb quickly, and by peak grip is about three times what it was at 85%. As the slip angle continues to increase, the induced drag also continues to increase. When the grip level is back at about 85% of peak, the induced drag is now about 12 times what it was at the low slip angle. One might assume that the tire wear is also about 12 times more!

 

This graph might make it easier to see that relationship:

alignment_sacurve.jpg

 

So we can see that there can be a significant difference in induced drag for a given cornering force, and by extension for a given lap time. The conservative and efficient driver stays on the upslope of the curve and has low induced drag and wear, while the aggressive driver uses the tires at excessive slip angles, trashes the tires and uses more fuel but ends up with the same lap time.

 

Okay, now for some questions. Let's say that the peak grip so far is 1.0 g, so the grip at the low induced drag slip angle is about 0.85 g. We want to go as fast as the team that is running the corners at 1.0 g without giving up our low induced drag and subsequent low tire and fuel usage.

1. Is it possible to achieve 1.0 g at the low slip angle with a softer compound? If so, what will happen to the induced drag and tire wear? Will it be less than the original tire drag and wear at 1.0 g? Any downside?

2. Is it possible to achieve 1.0 g at the low slip angle with a wider tire? If so, what will happen to the induced drag and tire wear? Will it be less than the original tire drag and wear at 1.0 g? Any downside?

 

 

 

I'll bite...  so friction doesn't have anything to do with area; other than perhaps the small amount of mechanical friction you get from the rough surface of the track ..  the tire material has a coefficient of friction which is fixed, just like brakes it has an optimum operating temperature..  so tire width is all about controlling that heat...  so you need to find a tire which can sustain the grip required at the lower temperature, this you can do with potentially a softer compound...  but then you need to be able to control the temperature of that tire; a softer compound generally doesn't dissipate heat as well as a harder thinner tire..

 

In general I suspect want a harder tire, which you need to get "hotter" to work effectively and drive it into the maximum slip angle to retain the heat and friction desired.

 

Further the wider tire...  will have generally a larger rotating mass, and potentially have a harder time generating the initial heat to come up to temp, too low of a surface pressure will limit the ability to drive into the required slip angles to build the heat required;  in practice, in endurance racing, its not that hard to get the tires up to temp.

 

Further in your supposition, you are using the concept of a G, where in the G actually equates to a real amount of work, IE 1g being 2700lbs of force...   if you make your 2700 lbs into 2295lbs then .85g is all you need :) 

 

Or at least thats my random thoughts...

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Some good thoughts. :)

 

But first, let's dispel some of the physics: tires don't conform to the traditional friction formula (F=mu*r) that states that area doesn't matter. Besides friction, tires use mechanical keying, as you mentioned, and also adhesion. All three need to be considered.


Next: regarding the benefit of wider tires, here's a significant set of images

bhvrcpslip.gif

 

 

And for reduced car weight and also the idea of a fixed coefficient of friction; some tire load graphs:

TYRE%20LOAD%20GRAPH.jpg

Edited by mender
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15 hours ago, mender said:

Some good thoughts. :)

 

But first, let's dispel some of the physics: tires don't conform to the traditional friction formula (F=mu*r) that states that area doesn't matter. Besides friction, tires use mechanical keying, as you mentioned, and also adhesion. All three need to be considered.


Next: regarding the benefit of wider tires, here's a significant set of images

bhvrcpslip.gif

 

 

And for reduced car weight and also the idea of a fixed coefficient of friction; some tire load graphs:

TYRE%20LOAD%20GRAPH.jpg

 

Again, nothing with temperature though...  so in this case its a bit like looking at the max power output of my supercharger...   its all great until you see 300-400 deg intake temps, well then so much less...  same as brakes...  gotta get the temp in the zone, and have to keep it there...  in addition to the mechanical grip, you have the internal tire structure, which is doing the deforming creating the max friction inside those slip angles, its that molecular bond which is the one that is so heat sensitive, and passing the limits results in you seeing the belts too soon.  All I see in that slip angle set of images, is heat..  more work being done by less tire, more need for larger diameter tires and larger widths to heat-sync and increase the cooling time before the contact patch comes around again.

 

but as for the lower graph, I see the 150kg able to hold almost 2g lateral grip (in this case) where the 600 kg can barely hold 1.5g...  and of course  the 150kg weight gets to maybe 90 percent of max grip by 3-4 deg, getting you your hypothesized lower tire wear..  assuming of course your 150kg mass can get your tires into the proper temp zone and keep them there..

 

This is one of several places that stupid light results in better...  just less heat, less work to do to move, most notably on the corner here which we are focused on, as opposed to top end, where we want to reduce friction, aero and rotating mass. (everyone knows you win races in the corners though)

 

Finally as we talk theory, we have to bring into this discussion just a little reality of chump...  we dont have the suspensions to run extreme slip angles, the tracks are just not smooth enough and our suspensions not ideal enough to allow for the tire to be in the correct relationship to the road... the more slip angle, the longer it takes to get the tire back to that spot after a suspension geometry altering incident (read bump).   and of course also in some cases, where we have camber or toe limiting issues...    This is one of many reasons we drive radials rather than bias ply (lol slip angles).

 

I suspect in the end it would be much like brakes...  if you can get the right amount of grip, the life will be keeping the tire in the right heat range, if you could "cool" your tire you would find it will last longer...  Kart tires I used to run for instance, 135deg was where they would come in...   maybe to 150, but after that, bleh...  you have a greasy mess, have to switch them out or use a harder compound...  

 

I guess I stress heat so much because at a place like road america, we have a long straight, if I overheat my tires during the brakeing section, I have to wait to turn in its only a second or so...  but enough to loose my line...  I have to build in that second between braking and turning to hit my apex.  and in that case I feel I am getting all I can out of that particular tire.   move to a 245 from a 205, and I loose some speed on the front straight, just not enough power to spin them, extra aero etc.., car noticeably scrubs more speed in the turns, due to increased friction in the non ideal angles generated by caster, but its much harder for me to overheat those tires (same compound)...  and as long as I am nice to the tires (no flat spots, right camber, etc) the 245 last longer..

Edited by Xph
brake not break (thanks frankrehnelt)
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9 hours ago, Xph said:

I guess I stress heat so much because at a place like road america, we have a long straight, if I overheat my tires during the breaking section, I have to wait to turn in its only a second or so...  but enough to loose my line...  I have to build in that second between breaking and turning to hit my apex.  and in that case I feel I am getting all I can out of that particular tire.   move to a 245 from a 205, and I loose some speed on the front straight, just not enough power to spin them, extra aero etc.., car noticeably scrubs more speed in the turns, due to increased friction in the non ideal angles generated by caster, but its much harder for me to overheat those tires (same compound)...  and as long as I am nice to the tires (no flat spots, right camber, etc) the 245 last longer..

 

Did your lap times drop when going wider?

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10 hours ago, Xph said:

 All I see in that slip angle set of images, is heat..  more work being done by less tire, more need for larger diameter tires and larger widths to heat-sync and increase the cooling time before the contact patch comes around again.

The point of the slip angles is to show that the front part of the tire is doing the gripping and the rear has progressively more sliding. Change the shape of the contact patch from narrow and long to wide and short by using wider tires and a higher percentage of the tire is in the grip area for a given slip angle.

 

Yes, heat generated is important. Assuming that the contact area hasn't increased (same tire pressures), less of the contact patch will be in the sliding phase, which generates a lot more heat than the gripping phase, so the tire will be cooler for the same grip level. Less sliding area also means less wear.

 

And as you mentioned, control of the contact patch becomes more critical as the tire gets wider and the sidewall gets shorter. A car with good suspension geometry can go further in that direction than one with poor geometry before getting to the cross-over point and starting to slow down.

 

Light is always right; not sure who came up with the idea of "road-hugging weight" but it probably wasn't a racer. :D

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3 hours ago, mender said:

Light is always right; not sure who came up with the idea of "road-hugging weight" but it probably wasn't a racer. :D

 

So only place thats not always right now is in Power to Weight...   as it slides now between light for tracks with lower top speeds and the trade-off the HP affords you in reaching the top speeds...   enough seconds on the track at 100+ and the aero load not curb weight becomes a limiting factor...

 

3 hours ago, karman1970 said:

 

Did your lap times drop when going wider?

 

Not enough data to tell yet...   made to many other changes...   I hope to be at Road America for the drive for babies event in early October to do some more testing.

 

What I know from my testing so far... is that I have started to look into electric power steering...  its amazing how much harder it is to brake and turn all of a sudden..

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3 hours ago, mender said:

The point of the slip angles is to show that the front part of the tire is doing the gripping and the rear has progressively more sliding. Change the shape of the contact patch from narrow and long to wide and short by using wider tires and a higher percentage of the tire is in the grip area for a given slip angle.

 

Yes, heat generated is important. Assuming that the contact area hasn't increased (same tire pressures), less of the contact patch will be in the sliding phase, which generates a lot more heat than the gripping phase, so the tire will be cooler for the same grip level. Less sliding area also means less wear.

 

And as you mentioned, control of the contact patch becomes more critical as the tire gets wider and the sidewall gets shorter. A car with good suspension geometry can go further in that direction than one with poor geometry before getting to the cross-over point and starting to slow down.

 

So I wonder what would happen if we could look at the temperature of the contact patch in your slip angle map...   I wonder if we would see that the boundary temp between the slip and slid portions is at a maximum threshold temp causing the tire construction to give way, or if that region is instead defined by mechanical geometry...  the latter suggesting tire pressure, rim width and sidewall height (well and of course suspension geometry) would have more to do with the size and shape..

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5 minutes ago, Xph said:

 

So I wonder what would happen if we could look at the temperature of the contact patch in your slip angle map...   I wonder if we would see that the boundary temp between the slip and slid portions is at a maximum threshold temp causing the tire construction to give way, or if that region is instead defined by mechanical geometry...  the latter suggesting tire pressure, rim width and sidewall height (well and of course suspension geometry) would have more to do with the size and shape..

Like this?

mich.jpg

 

From here:

http://mccabism.blogspot.ca/2012/05/

Edited by mender
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6 minutes ago, mender said:

Like this?

mich.jpg

 

Sort of, so we know point b is sliding, but the lower chart here seems to indicate that the rear edge of the "adherent" section is the coolest..  not the hottest...  and that the heat generated in the sliding section raises quickly...   more of a presentation picture than I would like to use as a basis to reject my theory, but the dipping red line sure seems to indicate the opposite of my suggestion above.

 

The friction coefficient chart does clearly show optimum friction is achieved in a very specific temperature range , about 50 deg wide...  for me karting on race tires, it was only about 25 degrees in width, and very obvious...

Edited by Xph
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Oval trackers set up our cars differently.  There is Ackerman and weight jacking etc. In other words if the tires are squealing get the wrenches out.

 

 There is another way to race in our series.  Have A LOT of motor. That will solve many handling issues.

 

Regards, 

 

Cam " duration is a good thing" Benty

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1 hour ago, mender said:

 

so interesting...  so from this graph we see the cooling of the contact patch is due to contact with the road surface,  now the question still remains, the spike before it exits the contact patch, does that start before the tire starts sliding...

 

Both sources seem to indicate no, the tire "frictional heating" is likely due to sliding...

 

With that assumption, and given the friction vs heat map, could it be that too much contact with the surface of the road is actually over cooling the tire dropping it out of the correct friction range..  seems absurd..  but, keep in mind that most of these tires make more "grip" in those high slip angles, where a large portion of the tire is in sliding friction...  making heat..  hmmm

 

So if we try to connect those dots for a moment, at 1 degree slip angle, we assume the tire isnt generating enough heat for a "quality" contact patch...  not generating enough grip, so even with the smaller contact patch at say 6 degrees, the tire is now creating the optimum balance between heat and contact to do the most "work"...

 

In this theory then, increasing the diameter of the tire, or width of the tire, or geometry to affect the contact patch size would only yield more grip if the correct ratio of sliding friction to contact patch being cooled by the track surface could be maintained.

 

Working back to your initial query...  could you make the tire last longer with less slip angle...  maybe but not really...  the data would seem to indicate to me... the tire compound has a desired heat range, only works in that heat range, and you need just as much slip angle as required to get it into that heat range...  of course this statement somewhat ignores the dynamics of the tire deformation which contribute significantly to why the tire works best with some slip angle.  but in many cases those can be adjusted with tire pressure, allowing the tire to slip with more or less side load....  the other tangential item is rim width...  this is what we have been playing with lately, following the guidance of those before and trying our 205's on 8" wheels instead of 7's and running the 245 on a 10 incher...  I suspect this also has to do with that latter deformation of the tread, running the tire a bit stretched makes it behave more like the mechanical forces of the slip angle are applied without the time and forces needing to be applied...  IE you dont have to load the tire and twist it into its slipped state, when you have the tire stretched on the rim, its already closer to that state; you waste less time transitioning and or recovering after a bump.

 

So it would seem to indicate, if you want your tire to last longer, you either run a different compound, or run a different tire size such that the weight (which should have had an area dimension) in that graph, would lower into a more desirable ground pressure to G load ratio... and or somewhat in the diameter...  intuitively enough, both of those options increase the amount of rubber you are carrying, so even if you consume it at the same rate, you have more to consume...  with the physical limitations obviously being rotating mass, and the ability to get the tire hot enough to work.

 

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20 hours ago, mender said:

Okay, now for some questions. Let's say that the peak grip so far is 1.0 g, so the grip at the low induced drag slip angle is about 0.85 g. We want to go as fast as the team that is running the corners at 1.0 g without giving up our low induced drag and subsequent low tire and fuel usage.

1. Is it possible to achieve 1.0 g at the low slip angle with a softer compound? If so, what will happen to the induced drag and tire wear? Will it be less than the original tire drag and wear at 1.0 g? Any downside?

2. Is it possible to achieve 1.0 g at the low slip angle with a wider tire? If so, what will happen to the induced drag and tire wear? Will it be less than the original tire drag and wear at 1.0 g? Any downside?

 

 

 

In an effort to reign myself in LOL

 

so based on where I went..

 

1) yes I would assume a softer compound (or rather different compound) could generate more grip at lower slip angles...  I would assume however you would overheat and over wear the tire, sort of like running the Hoosier AS03 compound for a race (qualifying compound) rather than the RS03...  which takes a warm-up lap or two, but will last the race...  qualifies faster, has more grip, tire cant take the heat of a race, and goes away...     as for drag, 1.0g of drag might be fractionally less because you dont require enough slip, but in reality its probably all going to be used up between things like scrub and suspension geometry and actually work against you unless you can have ideal suspension geometry for it.   as for wear, I dont think you have a chance with the softer compound...  AS03 to RS03 real world examples to demonstrate...

 

2... wider tire, this is harder...  if we assume you switch compound, then maybe, but just going to 10" AS03 cant compete with an 8" RS03...  why you dont see SCCA racers doing that...  further the rotating mass is significant...   if we assume you dont switch compound and just run a wider tire, well thats what we are doing with the 245 over the 205...  the larger tire is more of a heat sync, has more mechanical grip and carries more rubber to wear off...   what we are finding is   more rotating mass...  and more sensitive to suspension geometry (ie if you wear out the inside inch, doesn't matter how wide your tire was)  our camber numbers are much more sensitive...  we can see that there is a sweet spot where we get enough heat in the inner edge to cause it to grip and pull down the outer edge, too much camber, and we wear out the inner edge, not enough and we never get the tire to really work.

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In my answers to the questions, I am depending on being able to match the initial tire's grip at a lower slip angle with the new tire, and getting a usable increase in tire life because of less sliding of the contact patch. In both cases, my answer was yes but I wanted to see what others might have to say!

 

1. As noted in the graphs, it's the slipping part of the tread that generates the excess heat and wear. My contention is that a softer compound tire can last longer as long as the driver maintains the same g loading and lap time. Not many drivers can resist the temptation to go as fast as the new compound is capable of, so not always a sure thing.  :P

 

In F1 the trend is to use the softest tire for the drag race start, then switch to a harder compound at the first pitstop. The balance between a two stop and a three stop strategy depends on whether using the softer tire is fast enough to compensate for the extra pit stop. Already this season a few teams have upset the apple cart by doing long stints on the softer compound by not running them as hard, and the tires seem to be able to last as long as the harder compound when used that way. I had been wondering about using that strategy for a few years and was gratified to see it play out at the F1 level. Again, a delicate balance and one that requires restraint by the driver, but I feel the real world evidence is enough to call it a yes to question #1.

 

2. Actually, I think this is the easier question. I personally have always gone faster with more tire. And, to address my question, I have also noted that the tire life is extended enough to easily justify the extra expense. Yes, there is additional rubber to use up but I also feel that the better grip again allows the driver the option to match the former lap times with lower tire temps, and slip angles, reducing the friction portion of the contact patch area for less wear.

 

Again, using F1 to demonstrate a trend:

https://www.formula1.com/en/latest/headlines/2016/5/pirelli-reveal-wider--faster-2017-tyres.html

The essence is that tire width will be increasing by about 25% in order to reduce lap times by about four seconds/lap. Looking forward to next season to see how the new tires affect pit stop strategy, and whether my answers are borne out or thrown out. :)

 

 

In any case, thank you for indulging me, and I really enjoyed the discussion!
 

 

Edited by mender
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10 hours ago, Cam Benty said:

Oval trackers set up our cars differently.  There is Ackerman and weight jacking etc. In other words if the tires are squealing get the wrenches out.

 

 There is another way to race in our series.  Have A LOT of motor. That will solve many handling issues.

 

Regards, 

 

Cam " duration is a good thing" Benty

A lot of motor is my angle right now while making sure the suspension is right.

 

If it's off bad from the start I will be very upset with myself however.

 

315's will help. Will need to play with spacers to get the dewedging I want though. This turning both ways...

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As a follow-up here's an interesting article that is worth reading by Mark Ortiz:

http://www.auto-ware.com/ortiz/ChassisNewsletter--September-November2004.htm

 

An excerpt from the wider tire part as a teaser:

"One theory, advanced by the late Chuck Hallum and evidently picked up by Paul Van Valkenburgh in his recent column, is that a tire is primarily limited by thermodynamics. It generates drag when running at a slip angle. The drag times the speed equals a power consumption, or rate of energy flow. This energy is converted into heat. For the system to be in equilibrium, the heat must be dissipated as fast as it is generated. Even short of the point of true equilibrium, the tread compound needs to be kept below a temperature where it softens to the point of being greasy rather than tacky. If the contact patch is shorter, that means that each square inch of tread surface spends less time getting heated and more time getting cooled.

 

Also, when a tire is operating near its lateral force limit, the front portion of the contact patch is “stuck” to the road and the rear portion is a “slip zone” in which the tread moves across the pavement in a series of slip-and-grip cycles. The slip zone grows as we approach the point of breakaway. Beyond the point of breakaway, the entire contact patch is slip zone. The slip zone generates less force and more heat than the adhering zone. A shorter, wider contact patch is thought to have a larger adhering zone and a smaller slip zone at a given slip angle, and wider tires are also known to reach peak force at smaller slip angles. Therefore, a wider tire is not only better able to manage heat, but also generates less heat at a given lateral force.

 

This all makes sense, but it fails to explain why wide tires give more grip even when stone cold.

 

There is little doubt that they do. If you have a street car with four identical tires, and you replace the rear tires and wheels with ones an inch wider, using the same make and model of tire, with no other changes, the handling balance will shift markedly toward understeer. You will see this effect at all times, from the first turn in a journey to the last. Surely this effect is not coming from heat management."

 

From a page that is worth bookmarking:

http://www.auto-ware.com/techref/lib_index.htm

 

Don't be afraid to scroll all the way to the bottom. And yes, quite a few articles include math! :)

Edited by mender
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Reducing slip angle on the rears while maintaining a loose or neutral car has been part of my success racing the VWs.   MY "Dynamic rear steer" actually allows /promotes the rear axle to shift, adding toe out under lateral pressure . This reduces the dynamic slip angle a couple of degrees , reducing drag, resulting in a higher mid turn  and exit speed. 

 The rear tire tracking outside of the front tire, also reduces the dynamc front loading . goes faster.

Edited by flyinglizard
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10 hours ago, mender said:

This all makes sense, but it fails to explain why wide tires give more grip even when stone cold.

 

There is little doubt that they do. If you have a street car with four identical tires, and you replace the rear tires and wheels with ones an inch wider, using the same make and model of tire, with no other changes, the handling balance will shift markedly toward understeer. You will see this effect at all times, from the first turn in a journey to the last. Surely this effect is not coming from heat management."

 

From a page that is worth bookmarking:

http://www.auto-ware.com/techref/lib_index.htm

 

Don't be afraid to scroll all the way to the bottom. And yes, quite a few articles include math! :)

 

All makes perfect sense to me...  right along the lines of much of my thinking...

 

So to me its all about getting the tire to work...  because the wider tire acts like a heat sync, you have a harder time getting a localized spot "hot enough" to start working...  further because initially you are depending more on the mechanical grip of the tire, you are lowering your ground pressure..  if you could dynamically adjust your tire pressure, starting with something stupid high and bleeding down as the tire warmed up, you could significantly reduce this..  the full tire would have a smaller contact patch and exert more ground pressure, heating the tread faster and leaving less of it in contact with the ground for cooling effects...   its all about getting the rubber compound into the optimal temperature range and keeping it there.

 

Also the lower slip angles of the wider tire are working against you, as you are doing less sliding friction (and when sliding at lower ground pressures generating less heat in the inner compound of the tire).. so it will come up to temp slower.

 

There are those spring loaded valve stems that allow you to pick and set a pressure and auto bleed off, they would while not giving the full effect, allow you to start your cold tire more full, and not require a pit stop to bleed down...  now I am not sure I would trust them not to leak down and ruin my tire...  but they are fairly cheap and might help some.

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I think that's where my ice racing experience comes in handy, can't get any heat in the tires but have to go hard anyway! :P

 

But seriously; I've learned how to drive cold tires and also how to get the tires up to temp quickly for sprint races. Trophy dashes on the oval are four laps, just a little bit more time than a decent autocross course. Not much time to get temp so every little bit counted. I usually managed to make up a few places on the first lap because I did a lot of accelerating and braking during the pace laps while they were weaving back and forth. I know the other drivers figured I was a bit of a wild man as I slid the tires under braking and spun the right rear as far as I could right after!

 

For the bigger stock cars, the trick was to under-inflate the tires to generate heat more quickly and have them stabilize close to the ideal temp and pressure. I didn't like doing that because of the risk for the first couple of laps and also it isn't good for longer runs. I haven't tried the bleeders.

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