I start with a confession. I know nothing about either the chemistry or the construction of tyres. However, I do help customers with the suspension set-up of a wide range of race (& road) vehicles, & I have definite views on tyres - views that do not always appear to be shared by tyre manfucturers.
Starting with the blindingly obvious. Tyres are the only points of contact between a vehicle & the road surface. It follows that the first sentence of the opening post is a true statement - with no contact between (any of) its tyres and the road surface, a vehicle is going nowhere. It also follows that the nature of the contact between the tyres and the road surface (specifically, tyre "grip") will have a major effect on the time required to negotiate a given trajectory. Not too much to argue about so far, hopefully.
Tyre "grip" depends upon many variables. These include tyre construction, compound, contact patch area, mean vertical force, vertical, lateral and logitudinal force variation, pressure, temperature, road surface & (no doubt) a few other parameters. Some of those listed are inter-related. For example, tyre pressure will vary directly with temperature, contact patch area will vary directly with pressure and vertical force & the way a compound interacts with the road surface (develops "grip") will depend upon temperature and the type of road surface. On a given outing, tyre temperature will depend on how the tyre is worked, which will depend upon the mean vertical force and the vertical, lateral & longitudinal force variation. Finally, vertical force variation will depend upon the road surface, the suspension set-up, and the forces induced by a driver. Still not too much to argue about, I trust.
Now to some more speculative (&, hopefully, more interesting) observations & deductions.
Slick tyres that are too cold, & "wet" tyres that are too hot lack "grip" (do not "work" properly) and lap times suffer significantly (sometimes by more than a second a lap for a typical road circuit). I would conclude that compounds are designed to "work" over a relatively narrow band of temperatures, that the specific temperature band is controlled by the tyre designer/chemist, and it is the aim of a driver and his race engineer to ensure that the tyres operate at the correct temperature (most of the time).
It is conventional wisdom that tyres can be brought up to their operating temperature more quickly by increasing damper digression and increasing roll bar stiffnesses. The first can be demonstrated on a rig (more aggressive dampers increase the work done by the tyres that react damper loads).
It is also conventional wisdom that increasing tyre pressures bring tyres up to operating temperature more quickly. Logic (& rig tests) suggest otherwise. I suspect that increasing tyre stiffness (both vertical & lateral) and increasing bar stiffness changes the way a driver will "lean" on the tyres, and it is probably this that increases the work input to the tyres. It follows that a driver can make a significant contribution to the rate of heat input to the tyres.
I would conclude that a driver (in the way he is prepared to drive a vehicle) plays an important role in making the tyres work. Make him comfortable and confident, and the tyres will work better for him. It is observably true that a vehicle with a cold tyre lateral imbalance will take longer to reach competitive lap times than one with a good cold tyre balance.
Tyres are part of the suspension of a race vehicle. Suspension set-ups cannot be taken too far away from a region where they work sympathetically with the tyres. The particular tyre property that is important in this context is tyre vertical stiffness & this depends mainly upon tyre construction, although minor adjustments can be made with pressures and cambers. It follows, arguably, that a good natural tyre vertical stiffness "split" is one that matches the longitudinal centre of gravity position of the sprung mass. It could also be argued that tyre vertical and lateral stiffness are closely correlated, and a similar lateral tyre stiffness split will minimise sprung mass sideslip under lateral acceleration (in a turn). Sideslip is (again arguably) a primary driver cue, and minimising it will help a driver operate closer to the ultimate performance of the vehicle & its tyres.
It happens that current F1 tyres, uniquely in my experience, do not conform to the above mantra. This, in my view, provides an explanation for F1 developments since the start of the 2007 season. Observable developments have been more and more complex front wings during 2007/2008, the unaesthetic 2009 front & rear wings, and the almost universal rejection of KERS. All to allow the centre of gravity to be pushed forward (along with the centre of pressure) to match better the tyre stiffness splits. Overall, it would appear, that is more important than the occasional power boost offered by KERS.
Grooving of certain oval tracks in the USA (starting with Indianapolis) provided an interesting insight into how tyres work (or not). The track at Indianapolis was grooved (I believe) as a solution to a dramatic reduction in grip experienced after it was re-surfaced before the start of the 2005 season. I believe that Firestone experienced problems making the tyres durable on the grooved surface, and Michelin (who knew nothing about the surface before the F1 circus arrived at Indianapolis), had to withdraw their tyres before the F1 race after several spectacular failures during practice and qualifying. I uderstand that Goodyear experienced similar durability problems with their NASCAR tyres. Three of three is more than coincidental, & one can only conclude that matching tyres (construction and compound) to track surface is massively important.
So back to the original statement. Both the construction and chemistry of tyres are very important to the performance of all race vehicles. They are also hugely complex and little understood. Hopefully, the ramblings above will provide some background into, and stimulate discussion about, the difficulties of making them work successfully in a racing context.
DaveW wrote:It follows, arguably, that a good natural tyre vertical stiffness "split" is one that matches the longitudinal centre of gravity position of the sprung mass. It could also be argued that tyre vertical and lateral stiffness are closely correlated, and a similar lateral tyre stiffness split will minimise sprung mass sideslip under lateral acceleration (in a turn). Sideslip is (again arguably) a primary driver cue, and minimising it will help a driver operate closer to the ultimate performance of the vehicle & its tyres.
It happens that current F1 tyres, uniquely in my experience, do not conform to the above mantra. This, in my view, provides an explanation for F1 developments since the start of the 2007 season. Observable developments have been more and more complex front wings during 2007/2008, the unaesthetic 2009 front & rear wings, and the almost universal rejection of KERS. All to allow the centre of gravity to be pushed forward (along with the centre of pressure) to match better the tyre stiffness splits.
Dave, I'm really interested in why you think F1 tyres don't conform to the norm. I think I follow your explanation of the normal situation - you want a tyre with good vertical and lateral stiffness so the sprung mass (the mass of the rest of the car) doesn't move around too much in a turn. This gives the driver a good feel for grip and encourages the driver to lean on the tyres more and thus bring them upto working temperature quicker. So what's different with an F1 tyre, specifically since 2007? Wouldn't the same reasoning apply?
DaveW wrote:The particular tyre property that is important in this context is tyre vertical stiffness & this depends mainly upon tyre construction, although minor adjustments can be made with pressures and cambers.
I thought that tyre vertical stiffness depends mainly in pressure, more than in construction. Then I realised that tyre ride rate (and not stiffness) was what I was thinking about when thinking about pressure adjustments given a tire construction stiffness. Im a bit confused here.
I would like to measure vertical and lateral stiffnesses of a tire given different pressures.
What are your experiences in this case guys?
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I estimate tyre stiffness as a matter of course when rig testing a vehicle. Many caveats - wheels are not rotating, tyres are usually (but not always) cold, they are loaded vertically but not laterally, etc. Customers choose baseline pressures.
Typically, tyre stiffness varies by around 8 N/mm/psi &, perhaps, 10 N/mm/degree of camber (apologies for the mixed units).
Across a range of open wheeled, mid engined aero vehicles (IRL through to F3) the ratio of rear/front tyre stiffness is usually between 1.1 and 1.3. The ratio for F1 tyres between 1994 & 2004 averaged 1.17. None prior to 2005 was less then unity.
F1 grooved tyres during 2007/2008 averaged 0.92, & the 2009 slicks appear to be (one sample only) slightly worse at 0.91.
Aero centre of pressure should be close to the centre of gravity for consistent balance, & both should be at around 47.6 % wheelbase behind the front wheels for zero sideslip/ny with the 2009 F1 slicks. That would have an unacceptable impact on traction for a rear drive vehicle, so the (not very comfortable) compromise is, perhaps, a centre of gravity close to the geometric centre. Not easy to achieve with a mid-engined layout (hence heavy front wing assemblies & no KERS), & the result must still be an evil drive (sideslip AND probably a lateral balance shift with airspeed). A possible explanation, perhaps, for some of the inconsistencies & tribulations observed over the last three years.
Dave, many many thanks for such an informative post =D>. Good to get some actual figures.
Just to put it in laymans terms. Are you saying that for most open wheel racing vehicles, the ratio front to rear tyre stiffness is in the region of 1.1 to 1.3, that is to say that the fronts are stiffer than the rears. But that post 2007, F1 has the reverse situation where rears are stiffer than fronts requiring the moving of the centre of gravity towards the front of the car to compensate - try and get heat in to the front tyres which is more difficult due to their relative lack of stiffness. But this compromises rear traction... (god I hope I'm getting this right)
Is this problem anything to do with the relative width of front and back tyres, or is it purely a design choice in the constuction?
Apologies, s-t, my ratios are rear/front. Rear tyres normally have the higher rate, which is (more) consistent with the "natural" c.g. position of a mid-engined vehicle.
p.s. Tyre (or contact patch) width changes will affect balance, of course. However, vertical stiffness is also affected by tyre construction.
A few years ago I worked with an LMP2 vehicle. A rig test suggested that the initial rear tyres were too soft. The manufacturer produced an alternative set that were the same size, same compound, but different construction. The construction change yielded an increase in vertical stiffness that was equivalent to the original tyres with (I recall) inflation pressures increased by 8 psi. The new tyres reduced lap time at a track test from 63 to 60 seconds. Some of the improvement was (I like to think) because the vehicle was better controlled mechanically, but I'm sure the major part was down to driver confidence. The change certainly brought a smile to his face, & his comments were all about lateral performance ("now I know what it is going to do").
Current F1 rear tyres do not have a higher rate than the fronts, but that is not caused by a large pressure split (although low rear tyre pressures can improve traction, courtesy of an increased contact patch area - but, perhaps, at the expense of other desirable properties).
A small F1 paper chase for you. Throughout the following I will use the ratio of rear/front vertical tyre stiffness (a "magic ratio", perhaps), as observed during rig tests. The ratio will, of course, be greater than 1 if the rear tyres have a higher vertical rate that the fronts.
My starting point is 2005 - an odd year because (uniquely in modern times and, perhaps, significantly) tyres were not allowed to be changed during a race. I saw both Bridgestone & Michelin tyres towards the end that year. They had ratios, respectively, of 0.91 and 1.32. Question: how many poles/races did Bridgestone runners win in 2005 compared with, say, 2004 & 2006?
My records for 2006 are sparse, but I did see one set of Bridgestone tyres with a ratio of 1.09. Not brilliant (in my view), but a distinct improvement over 2005.
2007 was a turbulent year, which saw previously successful ex-Michelin runners at a huge disadvantage (mostly on-track). I saw my first Bridgstone control (grooved) tyres in January & was surprised to find that the magic ratio was back down to 0.92. You might want to check and speculate upon vehicle wheelbases chosen for that year. It might be of interest to look up driver comments during pre-season testing. You might also like to compare, for example, Monaco pole time for 2007 with that for 2006 (hint - a forward c.g. hurts traction). Finally, you might like to review front wing developments during the 2007 season, particularly those of ex-Michelin runners (a more forward centre of pressure would suggest a more forward c.g.).
A major change to the aerodynamic rules occurred before the 2009 season which hit the reset button, particularly so for some teams. The revised front & rear wing dimensions, devised by the OWG, are particularly interesting, as is the fact that the magic ratio for the Bridgestone slicks was the same, if not slightly lower, than that of the grooved tyres they replaced.
Now, each of the above could have its own individual, perfectly rational, explanation. On the other hand, with the hypothesis that good engineers don't forget how to design a good race vehicle (especially with relatively minor rule changes over the period up to & including 2008) you might like to review my earlier posts & speculate on whether a common theme might exist.
Some very interesting thoughts. Particularly regarding the impact on KERS.
What would have happenend if all teams would have agreed to provisionally fit some grooves where needed until the situation can be addressed properly next season.
Formula One's fundamental ethos is about success coming to those with the most ingenious engineering and best ..............................organization, not to those with the biggest budget. (Dave Richards)
carefully?
