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F1 tyres, a what if?


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#51 RDV

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Posted 12 September 2013 - 19:23

Interesting slow motion at Le Mans, look at the tyres in action.



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#52 Ben

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Posted 12 September 2013 - 20:38

Good debate. 

 

For my sins I designed and developed race car (LMP, GT) and motorcycle (MotoGP) for 8 years. I was proud to share the Le Mans grid over that time with our own RDV - great post BTW. I now try and understand Italy's finest round and black exports in a modest single seater championship ;-)

 

The why wider tyres argument is always a good one. No one ever seems to have an engineering based answer. To a first order contact patch area is a function of load and pressure so a wide tyre with a short contact patch should have the same grip and a narrow but long patch of the same area. Friction is definitely a function of area but most of the theories are poor at explaining why wider tyres offer more grip

 

The only theory that seems to cogently explain why a short wide patch offers more grip (which it does - all things being equal (and as RDV pointed out they rarely are all equal :-) )) is Persson's rubber friction theory. More specifically the complete version of his theory, which includes flash temperature generation. Basically a long contact patch allows flash temperatures to generate and this gives a drop in friction. When you increase the vertical load on a tyre the contact patch tends to get longer rather than wider, hence larger flash temperature effects and less grip - so it's also the origin of load sensitivity.

 

http://www.multiscal...rubber-friction

 

http://www.multiscal...temperature.pdf

 

http://www.multiscal...re_dynamics.pdf

 

Ben



#53 Canuck

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Posted 12 September 2013 - 21:50

Thank you gentlemen.  I was wondering if Ben was going to find this thread or if it was to remain Greg and RDV.  I have learned much and enjoyed myself in the process.



#54 Greg Locock

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Posted 12 September 2013 - 22:50

" I am merely trying to make the point that you are talking about a tyre MODEL."

 

No I'm talking about raw measured tire data from Flattrac machines.

 

Actually there is a clue from Flattrac testing - although both lateral and longitudinal sweeps gather data about maximum mu, longitudinal sweeps destroy the tire much more rapidly.



#55 gruntguru

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Posted 12 September 2013 - 23:06

Interesting slow motion at Le Mans, look at the tyres in action.

Nice video RDV. One aspect that really jumps out of the screen is the compromised mechanical grip resulting from excessively high wheel rates - all to maintain underbody aero geometry. I can't help wondering how fast these cars could be with unsprung aero.



#56 RogerGraham

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Posted 13 September 2013 - 02:25

 

Changing the car to run these tyres involved a huge amount of work, new front bodywork, re-working aero for a different balance, more drag due to limited flow between the front wings, changing disk and pad sizes on the rear suspension due to brake balance issues, given that a bigger diameter and width of front tyres diminished disc tangential speed, thus changing rubbing speed and pad bite, which involved making new rear uprights and coping with an increased rolling resistance from the wider wheels. Front spring rate, damping and anti-roll bars had to be changed, as vertical stiffness rate of the bigger tyre was softer, which then brought in problems controlling ride height and increased issues with splitter touching ground under braking, not to mention CP change with pitch, as tyre spring rate ratio was changed.(Incidentally, tests with the windtunnel model using the new bodywork but with the original wheel size showed that aero drag was mainly due to the tyre size despite being in the wheel well and not exposed to the air, as a single seater.) Ackerman, roll camber geometry and offset had to be changed, plus caster and kingpin angle, plus brake master cylinders just to cope with brake balance bias issue , consequence of the different disc radius and size. 

 

Thanks RDV, that's a brilliant intro for non-tyre experts such as myself.

Apologies if I have misunderstood something, but regarding the bolded text; why are new rear uprights required as a result of reduced front-wheel disc tangential speeds?  Or were the new rear uprights required because you had to change the rear disc/pad sizes?



#57 desmo

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Posted 13 September 2013 - 04:00

Good stuff here. Special mention to RDV and the papers linked to by Ben.



#58 Greg Locock

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Posted 13 September 2013 - 04:28

Very interesting papers, i really like being able to see the effect of cp shape on SAT

 

http://www.multiscal...re_dynamics.pdf

 

Figures 9 and 10, as I have said before, max mu is not dependent on whether it is a short fat cp (longitudinal) or long and thin (lateral), it is still 1.2 ish



#59 RDV

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Posted 13 September 2013 - 08:56

RogerGraham-Or were the new rear uprights required because you had to change the rear disc/pad sizes?

 

...sorry hadn't elaborated further, was actually because the mounting bosses of calipers had to be shortened to bring caliper in as disc O.D. decreased so pad in right place. As upright designed for best mechanical structure, change entailed redesigning upright.

And below an example of contact patches under normal load, will change when lateral load and consequent carcass deflection. In fact it is moving all the time as weight is transfered continuously, with changing Fz, Fy and Fx, (aero, braking and traction loads). Notice different shapes of contact patch despite very similar tyre sizes, due to different carcass, one only has to cope with braking and steering, the other has braking and traction inputs.

....er-somehow having trouble putting images in, says not allowed in this community...mm, so new format screws up another thing...thank you Autosport... :rolleyes: 



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#60 desmo

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Posted 13 September 2013 - 14:07

RDV had the image you tried to post already been uploaded somewhere and if so can you post the URL?  I'm still trying to get this new BB sorted.



#61 RDV

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Posted 13 September 2013 - 14:59

Uploaded from my computer to imageshack, but seem to be refused when try to post here as thumbnail...Here it goes again=

Front contact patch= pqbt.jpg

Rear contact= kuv5.jpg
Measuring sidewall deflection= elpc.jpg
 
Edit-..ah, seems if one posts address directly it works... :blush:

Edited by RDV, 13 September 2013 - 15:00.


#62 munks

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Posted 13 September 2013 - 16:20

When you increase the vertical load on a tyre the contact patch tends to get longer rather than wider, hence larger flash temperature effects and less grip - so it's also the origin of load sensitivity.

 

Interesting point that hadn't occurred to me before! However, I'm pretty sure that's not a cause of 100% of the load sensitivity. See The Pneumatic Tire, chapter 2 section 3.1 for tests with a simple block of rubber (i.e. not a round tyre) on smooth vs. rough surfaces. Simply put, it depends on how much that additional load is increasing the actual contact area by filling in the asperities.

 

In fact if racetracks were made of glass, downforce would be of very little help (perhaps I've just solved all the problems with overtaking ...)


Edited by munks, 13 September 2013 - 16:20.


#63 Ben

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Posted 13 September 2013 - 18:51

Interesting point that hadn't occurred to me before! However, I'm pretty sure that's not a cause of 100% of the load sensitivity. See The Pneumatic Tire, chapter 2 section 3.1 for tests with a simple block of rubber (i.e. not a round tyre) on smooth vs. rough surfaces. Simply put, it depends on how much that additional load is increasing the actual contact area by filling in the asperities.

 

In fact if racetracks were made of glass, downforce would be of very little help (perhaps I've just solved all the problems with overtaking ...)

 

 

 

The issue with those experiments mentioned in the Pnuematic Tire chapter is that they involve elastic rubber asperities being pressed onto a smooth glass surface. It is fundamentally not representative of elastic rubber draping over a rough rigid road surface.

 

Persson's theory covers contact mechanics in detail independently of his friction model, and the results are far more comprehensive than those experiments with rubber asperities and glass surfaces.

 

Ben



#64 Ben

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Posted 13 September 2013 - 19:16

Sorry, Just re-read the section you mentioned, and it's not the rubber blocks on a glass surface section. It does however make some comments that aren't relevant. Persson's model of contact on self-affine surfaces argues that true contact area is a tiny percentage (think 1%...) of what we'd consider the macroscopic contact area, so talking about an effect that's "probably because complete contact is achieved between soft rubber and a smooth countersurface at quite small loads" isn't convincing compared to some more recent work.

 

The theories discussed in the Pneumatic Tyre are based on the (excellent and pioneering work) by Grosch and Schallamach, and many others, but the amount that Kluppel and Heinrich, Persson, etc have added to the subject in the last 10 years has moved things on a little bit.

 

Persson deserves a lot of credit for putting all the work on his website. If you fill in the contact request form he generally replies directly within a day or so. Read the papers and ask him a question!

 

BTW, this is another good summary of the field: http://www.multiscal...nd_adhesion.pdf

 

Ben


Edited by Ben, 13 September 2013 - 19:28.


#65 munks

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Posted 13 September 2013 - 20:10

I appreciate your rebuttal. I have read a little but not a lot of Persson's work and am now inspired to brush (no pun intended) up on it.

 

EDIT: and I concur with your praise of him publishing his work on his site!


Edited by munks, 13 September 2013 - 20:13.


#66 RDV

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Posted 15 September 2013 - 18:19

Link to download Pacejka's Tire and Vehicle Dynamics, Third Edition, as mentioned above.