Longer Wheelbase vs Narrower Tires

[DaveW]

Seems like this is "reaching" a bit.. lot of 'ifs' and speculation.. and the temperature effect is surprisingly difficult to sort out with any amount of confidence.

In any event, in my experience a car that has a noticeable balance problem will typically have higher temps at the axle lacking grip. Understeer works the fronts harder than the rears by itself. Tight cars push front temps up and roast them, loose cars push rear temps up.

Beside that.. whatever temp change you get purely from the extra load of shifting the CG forward a couple inches... I don't think you'd even notice it.

Bravely put, JT. If I may put one of your statements the other way around (kind of, anyway), a car that has roasted front tyres will be tight. Of course it will. I recall that NASCAR's have (or used to have) an air blower installed in the right front wheel well, allowing a driver to cool an over-heated tyre & recover, or at least reduce, an imbalance (push) whilst on-track.

I'm afraid it is a fact that c.g. position affects the way tyres are worked. I could reel off many examples but, to keep it short, I will just say there is a reason for installing the fuel cell of an F1 car at its centre of gravity. I believe Peter Wright discussed the topic in "Formula 1 Technology".

[bill shoe]

DaveW,

I just finished reading some of your 2009 postings about the rear/front ratio of vertical tire stiffness. Great posts. Don’t know how I missed them at the time.

My assumption was that the key rear/front tire ratio was the grip. You are saying, or at least implying, that vertical stiffness distribution has similar importance. You have significant correlation evidence that backs this up.

Why is it so important to have a vertical stiffness distribution that roughly matches weight distribution? Is it the stiffness itself and how it relates to driver feel during transients? Is it because this influences how much energy/heat the tires pick up during use? Is it to reduce pitch or control pitch centers?

Why would Bridgestone continue to make F1 tires with such odd splits? Does it have something to do with durability? This would explain some of your subtle suggestions and Ross Brawn’s comment about Bridgestone not having the balls to do softer front sidewalls.

If current F1 tires have a rear biased grip distribution and a front biased stiffness distribution then how do you approach the setup? You are implying the optimum setup is to move weight distribution away from grip distribution and toward stiffness distribution (not completely, but in that direction).

It seems like you could put a “corrector” spring in series between the tire and the normal suspension at each front corner. This would make the stiffness of the tire/corrector system lower than the rear tire stiffness. This would fix the overall rear/front stiffness ratio (although the unsprung mass at front would still be downstream of the corrector spring, whereas unsprung mass is mostly upstream of the tire). This “corrector” idea is not an actual suggestion, just a concept I am using to explain why I don’t understand why vertical stiffness ratio is such a big deal.

[Jersey Tom]

Seems like this is "reaching" a bit.. lot of 'ifs' and speculation.. and the temperature effect is surprisingly difficult to sort out with any amount of confidence.

In any event, in my experience a car that has a noticeable balance problem will typically have higher temps at the axle lacking grip. Understeer works the fronts harder than the rears by itself. Tight cars push front temps up and roast them, loose cars push rear temps up.

Beside that.. whatever temp change you get purely from the extra load of shifting the CG forward a couple inches... I don't think you'd even notice it.

Bravely put, JT. If I may put one of your statements the other way around (kind of, anyway), a car that has roasted front tyres will be tight. Of course it will. I recall that NASCAR's have (or used to have) an air blower installed in the right front wheel well, allowing a driver to cool an over-heated tyre & recover, or at least reduce, an imbalance (push) whilst on-track.

I'm afraid it is a fact that c.g. position affects the way tyres are worked. I could reel off many examples but, to keep it short, I will just say there is a reason for installing the fuel cell of an F1 car at its centre of gravity. I believe Peter Wright discussed the topic in "Formula 1 Technology".

Yes, it's critical to have the fuel cell near the initial CG so that the balance doesn't change as fuel burns off and the CG moves. But.. that's gonna be more a function of purely the load sensitivity of the tires rather than any temperature change in the tread. The change in lateral usage of the tires is going to have way more impact on tire temps than the vertical load.

Moving the CG changes balance, but as far as setup parameters go, it's a hell of a lot easier to change springs and bars and air pressure than to change out an entire suspension! That's the point I'm trying to get at.

And in the vast, vast majority of cases.. if not all.. shifting the CG forward by itself is going to result in more understeer. In the hundreds and hundreds of race tires and conditions I've seen or personally tested, I've never seen a case or seen any data that would indicate the opposite.

[marcush.]

Here´s my two cent for a phenomena I personally experienced:

the car :Lotus Elise (in fact ,Opel speedster )weight Bias at around 40/60 (i do not remember exactly ,if any one cares about the real facts ,I´d look into the data).

The Owner of the car needed big fronts ,simply because he wanted them ,so no chance .
The fronts were something like 235 ,the rears if i remember correctly 265 mm tread width...
(it is surprisingly hard to find a wide front tyre with a usable diameter for the elise type cars so normally you are stuck with 205 mm ).

I did work on the setup on this car and do what you want ,the thing had :
Low speed understeer terminal ,uncurable.
As soon as you pushed it the tyres grained so badly on the front that it made you
and the owner cry ,as the front rubber was shedding so quick that you almost expected to see black black marbles raining ...
the tyres were COLD ,stone cold.the understeer went from bad to outright undrivable as the rears gained grip with temps...

The cure was also surprising: I reduced front track width as much as possible and added as much arb as possible (obviously you will not have a full portfolio of things you can change).The thinking behind this was to shift as much weight as possible towards the outside front to make it work .
Tel you what ...it worked ..demon ..the car was unbelievably good suddenly ,the front tyres came alive and the guy smashed the track record in Anneau du Rhin the following day for road registered cars ,prompting the circuit owner to convert his
own speedster to this unlikely setup...it also worked phenomenally on autocourse btw.

The speedster is a surprisingly good example for this wheight forward front grip increase thing:
On track days the car will perform a lot better with two guys on board ,lap times
surprisingly dropping by half a second on a 70second lap with 60 kilos added on the passenger seat ....I tried this several times with different(and differently setup cars..it always worked..

[autogyro]

Thanks Marcush, that reminds me of a few years back setting up cars like Mk 1 and 2 Escorts Lotus Cortinas and a super saloon Capri. Wider tyres on the front resulted in understeer and the answer was always a heavier front ARB and weight where possible towards the front.
Funnily enough on my racing Mini's it was the reverse, fit rear ARB and move what weight you could to the rear, this changed inherent understeer closer to oversteer.
I think on a single seater like in F1 the CofG is so much lower and roll stiffness so different that these cars work in a much finer set up envelope.
Which is why the extreems are seldom noticed.
Downforce is also the main input above very low speeds as well and this masks a lot of the high speed mechanical effects.
There are many more variables and in the case of the Mercedese F1, I still do not believe we have enough data to fully judge the issue.

[Jersey Tom]

The cure was also surprising: I reduced front track width as much as possible and added as much arb as possible (obviously you will not have a full portfolio of things you can change).The thinking behind this was to shift as much weight as possible towards the outside front to make it work .

Reducing front track width results in less load transfer to the outside tire...

[autogyro]

The cure was also surprising: I reduced front track width as much as possible and added as much arb as possible (obviously you will not have a full portfolio of things you can change).The thinking behind this was to shift as much weight as possible towards the outside front to make it work .

Reducing front track width results in less load transfer to the outside tire...

Yes but lateral moment from traction around the front axle center is increased for driver provoked oversteer.

[marcush.]

Still a reduction in track will also reduce the cornering capabilities of that axle ,so even with less load transferred you still end up working the fronts harder...right?

Did I claim an increased load transfer by reducing the track ,looking back at my post it could be read into it ...apologies there.

[Jersey Tom]

I'm not seeing why a reduction in track width at one axle would reduce the cornering capacity.

Keeping the average track width the same, narrowing the front and widening the rear will increase the cornering capacity of the front (less load transfer), decrease the rear (more load transfer), and free the car up.

[marcush.]

I'm not seeing why a reduction in track width at one axle would reduce the cornering capacity.

Keeping the average track width the same, narrowing the front and widening the rear will increase the cornering capacity of the front (less load transfer), decrease the rear (more load transfer), and free the car up.

your numbers do not stack up with me , do we talk the same loads ?

my formula is :

lateral sprung load (geometric) Transfer= (latacc(g)*CGHeight )/track width

or have I missed the boat somewhere...

[Jersey Tom]

That is true, for determining the total lateral load transfer the vehicle sees. However, changing the front track width relative to the rear changes the TLLTD, which will directly change balance.

Shortening the front track, and/or widening the rear track, will result in a higher percentage of lateral load transfer at the rear axle.. and less understeer as a result. Same as putting on a stiffer rear bar, etc.

[DaveW]

Yes, it's critical to have the fuel cell near the initial CG so that the balance doesn't change as fuel burns off and the CG moves. But.. that's gonna be more a function of purely the load sensitivity of the tires rather than any temperature change in the tread. The change in lateral usage of the tires is going to have way more impact on tire temps than the vertical load.

Moving the CG changes balance, but as far as setup parameters go, it's a hell of a lot easier to change springs and bars and air pressure than to change out an entire suspension! That's the point I'm trying to get at.

And in the vast, vast majority of cases.. if not all.. shifting the CG forward by itself is going to result in more understeer. In the hundreds and hundreds of race tires and conditions I've seen or personally tested, I've never seen a case or seen any data that would indicate the opposite.

Mmm.. perhaps my example was not the best I might have chosen.

I have had the opportunity of "playing" with several vehicles fitted with inerters over the last couple of years. One characteristic of inerters, observable & quantifiable on my rig, is to increase the "work" input to tyres at frequencies greater than 10 Hz. The effect is also observable on track, seen as an increase in tyre temperatures after an outing and as an improvement in lap time, at least during winter testing. In this respect, adding inerters is rather like adding sprung mass, so my observation suggests (quite strongly) that rolling a tyre in a straight line will lead to an increase in tyre temperature proportional, no doubt, to the mean load supported by the tyre.

Many moons, ago, in 1987, an active F1 vehicle with which I was connected was required to "cold start" the Mexican GP along with the rest of the field. The vehicle was reasonably competitive (was the first active vehicle to win a GP) &, whilst it might not have had the best aero on the grid, it did hold ride height & attitude better than most, it didn't pitch or roll during manoeuvres & it had controlled neutral lateral stability. Senna, the driver, fell from 2nd on the grid to (I recall dimly) 14th before starting to claw his way back up the field. Why? because the suspension halved the normal vertical load variations caused by road inputs, & it took 12 laps for the tyres to reach working temperature compared with 2(ish) laps for the field. The "cost" during that embarrassing warm up time was up to 2 seconds per lap. Slick tyre temperature effects over-power almost all else.

Again, a distant colleague of yours has compared tyre temperatures of two vehicles during an "out" lap, one of which weaved whilst the other drove straight & fast. If I interpret his post correctly, the "straight & fast" vehicle ended the lap with higher tyre temperatures than the "weaving" vehicle.

Apologies for rabbiting on like this (don't say I didn't warn you) & I don't wish to dismiss the impact of manoeuvring loads on tyre heat accumulation. I merely suggest that you may have underestimated the contribution of a rolling tyre supporting a vertical load & the effect on grip of working slick tyres into their "operating window". The position of the c.g. (& the related aero centre of pressure) can play a big part in manipulating the distribution of heat input to otherwise mis-matched tyres.

BTW, I agree with you that mechanical set-up can also compensate for mis-matched tyres, at least to an extent. The particular problem of F1 is that the wind benders, who cost the most & are therefore all-powerful, are not inclined to compromise.

[marcush.]

thanks for putting milk on the table there,Dave.It is appreciated .

[DaveW]

bill shoe: Thanks for your questions/comments. I took a while to post a reply because I tried several times to construct something that wasn't impossibly turgid, & failed. I hope you will forgive the following "cop out".

DaveW,

I just finished reading some of your 2009 postings about the rear/front ratio of vertical tire stiffness. Great posts. Don’t know how I missed them at the time. Thank you. I appreciate that.

My assumption was that the key rear/front tire ratio was the grip. You are saying, or at least implying, that vertical stiffness distribution has similar importance. You have significant correlation evidence that backs this up. I believe the two are related fairly closely.

Why is it so important to have a vertical stiffness distribution that roughly matches weight distribution? Is it the stiffness itself and how it relates to driver feel during transients? Is it because this influences how much energy/heat the tires pick up during use? Is it to reduce pitch or control pitch centers? Big question(s) that are difficult to answer succinctly. To summarize non-technically, a mixture of sometimes struggling to find a good set-up on a wide variety of race vehicles, logic, modelling, statistics (some of which you may have seen), & driver/team feedback.

Why would Bridgestone continue to make F1 tires with such odd splits? Does it have something to do with durability? This would explain some of your subtle suggestions and Ross Brawn’s comment about Bridgestone not having the balls to do softer front sidewalls. I'm afraid I have no ideas, none that I care to publish, anyway.

If current F1 tires have a rear biased grip distribution and a front biased stiffness distribution then how do you approach the setup? You are implying the optimum setup is to move weight distribution away from grip distribution and toward stiffness distribution (not completely, but in that direction). I'm no tyre expert, but I understand Bridgestone believes that 2008/9 problems were caused by front (slick) tyres that had over-sized contact patch areas. My reply to that idea is that driver & vehicle design problems started in 2007 with the pre-slick grooved tyres.

It seems like you could put a “corrector” spring in series between the tire and the normal suspension at each front corner. This would make the stiffness of the tire/corrector system lower than the rear tire stiffness. This would fix the overall rear/front stiffness ratio (although the unsprung mass at front would still be downstream of the corrector spring, whereas unsprung mass is mostly upstream of the tire). This “corrector” idea is not an actual suggestion, just a concept I am using to explain why I don’t understand why vertical stiffness ratio is such a big deal. Nice idea Bill but, with apologies, I don't think it would work.

[bill shoe]

DaveW,

No apologies needed. After I did my previous post I felt slightly bad that I was just dumping questions on you. I appreciate the response. Half the time my writing is so turgid or otherwise bad I can barely write my name.

Look forward to reading more of you stuff anytime you want to post.