Why do default-understeer cars need huge rear tyres?

[centurion]

I was discussing with another forum member over MSN the other day on tyres.

Besides aesthetics, wouldn't putting a 265 tyre on the rear make a BMW, for example, a 335i, even more understeery and eliminate the possibility of doing a 4-wheel drift?

In other words, wouldn't a 265 rear be way worse than the default 225/225 configuration?

 

[Puny]

Re: Why do default-understeer cars need huge rear tyres?

I think the increase in rear-wheel grip far outweighs the ability to drift esp since it's a RWD configuration to begin with.

WOT, the stock 335i already loses traction in the rear wheels from a dig. Having smaller tyres will just result in a more twitchy ride I reckon..

2nd, and this is just my observation, drifting is not solely about losing traction due to high lateral Gs.. A certain level of power needs to be there to propel & rotate the wheels to lose tyre-contact grip. If there's sufficient power, the car can certainly still drift. It boils down to the driver control in the end..

In that sense, unless the car is a competition prepped drift car, it makes more sense to have rear tyres wide enough to match the power regardless of street or track.. "Worse" is dependant on the application of the car..

My 2 cents

 

[Shaun]

Re: Why do default-understeer cars need huge rear tyres?

Wow nice explanation Alvin! Didn't know you were into tech..

===

Caysman, steady state characteristics have little to do with sport driving. With more tire under the rear, high speed understeer gives driver the confidence to find limits, and coming off the slow and/or tight corners you can get on the gas a whole lot harder without having the car step out and have to spend time correcting. Hard decel entry to corners is also more stable with the rear anchored better. If you get corner entry right and aren't correcting mid corner trying to gather the car, then mid to exit is going to be a lot more consistent and quicker.

Back to steady state, skidpad numbers can be misleading. Two similar chassis, similar engined (powered), similarly tired cars can pull different numbers, yet the one that does worse on a skidpad can be very much quicker actually lapping because in the steady state it is unbalanced, but under accel / decel, it is better balanced.

 

[axl]

Re: Why do default-understeer cars need huge rear tyres?

"Two similar chassis, similar engined (powered), similarly tired cars can pull different numbers, yet the one that does worse on a skidpad can be very much quicker actually lapping because in the steady state it is unbalanced, but under accel / decel, it is better balanced."

So the different skidpad numbers can be attributed to what factors then? Can't be suspension setup right? Since that already comes under chassis.

Similarly tired cars ar? They did Le Mans 24hrs, hence so fatigued? Sorry, can't help that one. Hehe...

 

[Shaun]

Re: Why do default-understeer cars need huge rear tyres?

It is attributed to suspention set up. In earlier thread I mentioned that I tend to separate chassis from suspension . This is in line with the industry, though maybe not with popular press. From recent memory I know that at least Aston Martin Racing technical director has the same definition, along with everyone at work I deal with.

When I said similarly tired I should have further specificed same total contact patch areas and shapes, but not necessarily similarly distributed front to rear of car, just to make the point clearer. It works either way though...

"Similarly tyred", you anglophilic banana...

 

[MRacer77]

Re: Why do default-understeer cars need huge rear tyres?

wah.. cheem ah... I just learnt something new today... thanks.

 

[centurion]

Re: Why do default-understeer cars need huge rear tyres?

I guess competition driving should also be separated from FUN driving .... welcome back Shaun after a pretty long hiatus.

 

[TripleM]

Re: Why do default-understeer cars need huge rear tyres?

If the rear goes up from 255 to 265
Then up the front from 225 to 235 as well
Typical BMW is 30mm difference in staggering the set up.

In fact those who wanna track the car v often, can up the wheel sizes
say from stock 8J to 9J front and 8.5J rear to 10J rear and slap on 255 tires in the front and 275 in the rear...swee2x

 

[MRacer77]

Re: Why do default-understeer cars need huge rear tyres?

agreed with jack.. thats the standard for BMWs... but I thought that the standard M3 came with 235 in front and 255 in the back?

 

[Azrielsc]

Re: Why do default-understeer cars need huge rear tyres?

Jack sir, any comments on 235 fronts and 275 rears?

 

[MRacer77]

Re: Why do default-understeer cars need huge rear tyres?

wouldn't that make the car more prone to understeer?

 

[wilman2772]

Re: Why do default-understeer cars need huge rear tyres?

wouldn't that make the car more prone to understeer?

Yup!! It'll make more understeer.

I'm running 245/35/19 for the front & 275/30/19 for the rear when tracking in Sepang.....car understeers quite bad, rear grip too good liao.....

So I set my PSS9 front to setting 2 & rear to setting 1(rock hard). Front swaybar to medium & rear to full hard settings. Front camber to -2.3deg & rear to -1.5 deg.

Now car dun understeer no more & it's so dam nice thru turn 1,2,4,9,14 & 15.....

Can't wait for 8th June.....majullah Sepang.....kekekekeke

'The Great White'

 

[Shaun]

Re: Why do default-understeer cars need huge rear tyres?

Ignoring aero, the trends are...

Greater levels of rear wheel torque require more rear biased stagger
Greater rearward bias in CG requires more rear biased stagger

If you're breaking traction easy coming off the sharper turns, then more rear tire is going to help you off them - esp so when you bump torque with the 335 since that engine type makes it easy. You may not like the feeling of understeer (push) in the momentary steady state portion of T5, but almost everywhere else it will help. A car that has tendency to push because of tire requirements or limits can be made better use of by changing driving line and style to one of quicker and deeper turn entry and quicker throttle application from middle to exit.

Of course if you can stuff tire under the front as well, why not. The requirement for slight push as a limit indicator really only applies to very high speed, zero runoff, and/or endurance type stuff which few to none of us run.

 

[TripleM]

Re: Why do default-understeer cars need huge rear tyres?

agreed with jack.. thats the standard for BMWs... but I thought that the standard M3 came with 235 in front and 255 in the back?

Nope
All E46 M3s came in 225/255 both 18 and 19

 

[TripleM]

Re: Why do default-understeer cars need huge rear tyres?

Jack sir, any comments on 235 fronts and 275 rears?

It's fine but I'd prefer wider in the front by 10mm to 245..
My prefered min set up is at least 30mm diff, no more but can be lesser too.

U will find, however that using wider tires in the front will make your steering bit heavy but at high speed driving and bit of aggression ard the corner is wonderful.

Generally speaking : narrower front induces more understeering.