Measured: Aero Lift, Roll Stiffness & Distribution, Steering Effort, and more

[JasperNoone]

Originally Posted by strat61caster

For how much we obsess about balance, the Mustang has a better F/R weight distribution according to these numbers. And that's the V8...

BRZ: 55.3/44.7
Mustang GT: 54.7/45.3

BRZ with LS swap will be closer to 50/50 than it is from the factory.

[strat61caster]

Originally Posted by JasperNoone

BRZ with LS swap will be closer to 50/50 than it is from the factory.

It's actually around 56/44

http://www.ft86club.com/forums/showthread.php?p=1025230

[JasperNoone]

Originally Posted by strat61caster

It's actually around 56/44

http://www.ft86club.com/forums/showthread.php?p=1025230

Not the particular swap I was thinking of, hm..

Even so, 1% further forward weight distribution < ~230whp you pick up swapping to a real engine. I will concede that my original statement has no support at this time and could very well be wrong until I find the gd picture I'm looking for.

[Rampage]

Originally Posted by totopo

Less weight in back means you can accelerate earlier at corner exit.

Really? That does not jibe with comments of drivers/broadcasters that I watch in the IMSA and Pirelli World Challenge Series races where they claim that the Porsche 911's massive advantage in the corners is its ability to accelerate better out of the corners due to the weight over the rear wheels. I am not arguing. I just seek enlightenment.

[chas3wba0]

Originally Posted by DarkSunrise

Here's one for the Mustang GT:
http://media.caranddriver.com/files/...lete-specs.pdf

The difference in COG, roll-center heights, anti-dive, etc. is enough to remind me of why I bought this car...

[totopo]

Originally Posted by Rampage

Really? That does not jibe with comments of drivers/broadcasters that I watch in the IMSA and Pirelli World Challenge Series races where they claim that the Porsche 911's massive advantage in the corners is its ability to accelerate better out of the corners due to the weight over the rear wheels. I am not arguing. I just seek enlightenment.

I hear people say this all the time too... I wish i had real data or something.

So the reason why RR and MR cars neutral oversteer is because they run out of traction in the rear first. And FF cars understeer because they run out of traction in the front first. The reason for that is those tires have to move more weight. Because the extra traction from the normal force of the weight is less than the extra force needed to accelerate that mass sideways.

So when you are accelerating at corner exit after apex, you are still asking your rear wheels to turn. Because the rear wheels are helping w lateral acceleration, a rear tire on a FR car will have more available traction for accelerating than a rear tire on an MR car, which has more mass to accelerate sideways.

My suspicion why porsches accelerate better out of corners is because of it's completely 100% terrible, no-benefit RR design, it probably gets tons of bonus points in the "Balance of Performance" to make it competitive, so they probably have more hp/weight than the other cars.

[DarkSunrise]

Originally Posted by strat61caster

I've been wondering the same thing and as I fantasize about getting another sports car the MR2 is a strong contender, both the MK1 and MK2 are roughly 45/55, it seems like for less than the cost of a Toyobaru you can get similar straight line performance and the advantage of the MR through the twisties.

A few track guys I met have MR2s from that era. They seem to love them, although apparently those cars aren't very forgiving when pushed. From what I've heard, those cars can be a handful with no ABS and the tendency to transition from moderate understeer to snap oversteer under power. I bet they'd be a lot of fun to master though.

Originally Posted by totopo

So the reason why RR and MR cars neutral oversteer is because they run out of traction in the rear first. And FF cars understeer because they run out of traction in the front first. The reason for that is those tires have to move more weight. Because the extra traction from the normal force of the weight is less than the extra force needed to accelerate that mass sideways.

I feel like this is very dependent on suspension and tire setup. For example, a lot of the guys I talk to at the track who drive MR cars (MR2, NSX, Elise, Cayman/Boxster, etc.) complain about neutral understeer. But that's probably more a function of OEM-engineered understeer (staggered tires and softer rear roll stiffness) than engine layout.

And on the other end, you see a lot of serious autocross guys tune FF layout cars to rotate/neutral oversteer, through thick rear swaybars and stiff rear suspension.

With all those variables at play, it's hard to isolate what the engine layout is actually doing for handling balance IMO. Not saying you're right or wrong, just difficult to see in the real world.

[Reaper]

Had a mk1 mr2 I built for local rally. Traded that car 10 years ago and still miss it. Car was scary fast in the dirt and had to be driven with your brain turned up to 11.

[strat61caster]

Originally Posted by totopo

My suspicion why porsches accelerate better out of corners is because of it's completely 100% terrible, no-benefit RR design, it probably gets tons of bonus points in the "Balance of Performance" to make it competitive, so they probably have more hp/weight than the other cars.

No professional GT class I can find has any 'bonus points' and European Le Mans only has a minimum weight and maximum displacement, Porsche won championships in 2012 and 2010 against Ferrari, Chevy, Audi, Ford, Aston Martin, and many others I don't know because I don't watch, it was the first series I could find rules for.

http://www.europeanlemansseries.com/..._reglement.php

A rear bias car is unstable, in simple terms that means it responds very quickly but at the risk of going out of control. It rotates very easily from a simple rigid body dynamics perspective taking into account wheelbase, mass properties, basic slip equations and steering angle. Your supposition of available grip based on normal grip and lateral force from cornering is pretty good but by understanding the basics of how the vehicle wants to or doesn't want to move you can quantify the advantages and disadvantages of mass placement.

Rear weight bias got it's wicked reputation in the 50's and 60's (James Dean certainly didn't help) but in the 60+ years since has continued to be successful from a performance standpoint. Look at just about any modern supercar and they all have a noticeable rear weight bias (>45/55).

From wiki:
http://en.wikipedia.org/wiki/Car_han...Center_of_mass

If that doesn't win you over to at least respecting a rear weight bias car all I can do is suggest a couple years calculus and a vehicle dynamics course, worked for me.

[totopo]

Originally Posted by strat61caster

No professional GT class I can find has any 'bonus points' and European Le Mans only has a minimum weight and maximum displacement, Porsche won championships in 2012 and 2010 against Ferrari, Chevy, Audi, Ford, Aston Martin, and many others I don't know because I don't watch, it was the first series I could find rules for.

http://www.europeanlemansseries.com/..._reglement.php

A rear bias car is unstable, in simple terms that means it responds very quickly but at the risk of going out of control. It rotates very easily from a simple rigid body dynamics perspective taking into account wheelbase, mass properties, basic slip equations and steering angle. Your supposition of available grip based on normal grip and lateral force from cornering is pretty good but by understanding the basics of how the vehicle wants to or doesn't want to move you can quantify the advantages and disadvantages of mass placement.

Rear weight bias got it's wicked reputation in the 50's and 60's (James Dean certainly didn't help) but in the 60+ years since has continued to be successful from a performance standpoint. Look at just about any modern supercar and they all have a noticeable rear weight bias (>45/55).

From wiki:
http://en.wikipedia.org/wiki/Car_han...Center_of_mass

If that doesn't win you over to at least respecting a rear weight bias car all I can do is suggest a couple years calculus and a vehicle dynamics course, worked for me.

I may respond more later. I didn't say bad things about rear bias, but rear engine. No reason to put the engine behind the rear axle. Re-read that page you posted, the air restrictor size is based on engine placement. I bet the rear engine Porsches get larger restrictors.

They apparently also now use fia gt3 homologation. I can't seem to find specific specs, but the fia gt3 are not all equal, they have some weird "balance of performance" crap to try to make the cars performance more equal.

I think the big benefit with mr over fmr is weight, why most super supercars are mr. But the supercar busting gtr is fmawd and the zr1 is fr or fmr. I really am curious if with equal weight and equal hp at the tires (Mr has less drivetrain loss) if fmr or mr is faster

[totopo]

Originally Posted by DarkSunrise

A few track guys I met have MR2s from that era. They seem to love them, although apparently those cars aren't very forgiving when pushed. From what I've heard, those cars can be a handful with no ABS and the tendency to transition from moderate understeer to snap oversteer under power. I bet they'd be a lot of fun to master though.



I feel like this is very dependent on suspension and tire setup. For example, a lot of the guys I talk to at the track who drive MR cars (MR2, NSX, Elise, Cayman/Boxster, etc.) complain about neutral understeer. But that's probably more a function of OEM-engineered understeer (staggered tires and softer rear roll stiffness) than engine layout.

And on the other end, you see a lot of serious autocross guys tune FF layout cars to rotate/neutral oversteer, through thick rear swaybars and stiff rear suspension.

With all those variables at play, it's hard to isolate what the engine layout is actually doing for handling balance IMO. Not saying you're right or wrong, just difficult to see in the real world.

Yes , you can force a car to be more or less over/understeer, but you loose overall traction. You have a set amount of weight shift to distribute, and the more equal it is between front and rear the more lateral traction you can develop. You can seriously skew it one way, like ff cars that pick up the inside rear, but that decreases overall lateral traction. Sometimes it is worth it though, like in those ff cars where the better slip angles and more traction in the front for accelerating out of corners is worth the loss of pure lateral traction.

[frsvr4]

Those lift numbers are really interesting. I wonder how much down force the stock spoilers generate @ 70 mph. Given the weight of the car, those lift forces seem negligible.

It looks like you could manipulate the lift equation to find an approximate lift coefficient and planform area... then you could approximate the f/r lift at higher speeds.

Thanks for sharing!!

[strat61caster]

Originally Posted by totopo

I may respond more later. I didn't say bad things about rear bias, but rear engine. No reason to put the engine behind the rear axle. Re-read that page you posted, the air restrictor size is based on engine placement. I bet the rear engine Porsches get larger restrictors.

Sorry, I misread "100% terrible" as being against engine behind driver as opposed to engine in front of driver, don't care much for 'FMR' technicalities. I am missing the 'air restrictor' as it relates to engine placement, care to point that out? I see a weight and configuration mention which generally corresponds to piston number and arrangement (6-8-10-12, I-V-H) which very well may give the flat 6 a level shot against a V12 of the same displacement.

Page 7 of this allows for a -5% hit to a cars horsepower, +30kg ballast or -10L fuel capacity at the regulators discretion:
http://www.europeanlemansseries.com/...V11_140514.pdf

Racing aside the discussion is on weight distribution, as I said, if my post didn't convince you all I can do is recommend some calculus and dynamics courses.

[Skurge]

Originally Posted by strat61caster

For going around a track and trying to nail down a great time? Definitely, there's power in being able to swing the back end around a corner and have the traction to seriously power out of a corner, for all the shit Porsche gets it's been effective for over 50 years at many levels of racing. Most of the major supercars in the last 10 or so years have been around there or even more rear weight biased.

I've been wondering the same thing and as I fantasize about getting another sports car the MR2 is a strong contender, both the MK1 and MK2 are roughly 45/55, it seems like for less than the cost of a Toyobaru you can get similar straight line performance and the advantage of the MR through the twisties.

traction, traction, traction...i agree