Slip and Axle Stiffness

[GEspo]

Hello Eng-tips experts. Is there a short answer to why increasing axle stiffness at one axle will create more slip at that axle relative to the other axle? This question is related to how to get more oversteer/understeer ie suspension tuning/roll bar tuning.. Thanks!

 

[GEspo]

citreon is HIS.. 2008 911 GT3 RS is 2nd, supposedly

newer 911 strays from the original.. its said 997 is the last true 911.. just cant afford it, 2006/2007 gt3 engine alone is $45k, add another $20k for the gearbox, they pretty much giving the body away..

 

[GregLocock]

HIS being an acronym of great significance in this case but not an especially obvious meaning?


Cheers

Greg Locock


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[GEspo]

interconnected suspension

 

[GEspo]

On the 911 architecture there have been tests comparing 911 to cayman.. on accel weight at rear wheels 911 74% vs 67% cayman.. stopping 58/42 for 911 vs 63/37 cayman..

so if stop and go goto the 911.. and correcting a skid to rear engine, 911 wins?

im a fan.. who ironically has the exact opposite car

 

[GregLocock]

Qashqai is second, beating all the Porsches, of which they have tested far too many.

Of course there is the old engineering observation that when you start measuring something the measurement becomes the target. Nurburgring lap times comes to mind.


Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[SwinnyGG]

If the car is turning, it is rotating in the yaw axis and has rotational momentum. Higher MOI (ie rear engine) means that rotation is more difficult to stop and control or stop and reverse, not less difficult.

If you've ever driven an air cooled 911 at speed, you know this. More 911s have slid into armco backwards than any other platform in existence.

The 'moose test list' 911 fanboys talk about being at the top of is an ad hoc list published by a Swedish magazine. It is not a complete list of every car in existence. Porsches perform well, but so do numerous other wide/low sports cars and GTs.

 

[BrianPetersen]

Just because the weight distribution from having the engine way out over the back is favorable for acceleration and braking doesn't mean it's favorable for going around corners without ending up backwards through the hedges!

 

[GEspo]

Theres no question the rear will loose traction first on the rear engine, question is what car will be able to “swing back” the other direction to right itself.. Im wondering what motivates people to say “mid is better in turn but when you loose it you cant get the car back”.. ie what math describes this

the concept of “more predictable” is probably floating around here

thanks all for entertaining this btw

 

[GEspo]

im going to take a stab at this: the reason theres NOT alot of mid engine drift cars, when the rear brakes loose on the rear engine theres still some traction on the fronts so the car can be controlled a bit.. when traction breaks on the mid engine the more evenly distributed grip is lost both front and rear..

 

[BrianPetersen]

I suspect the statement "mid is better in turn-in" is probably relative to a front-engine car, not a rear-engine car.

The concept of the car "swinging back in the other direction to right itself" implies that the driver has lost control. Maintaining control in the first place means having a margin of stability and that means having understeer margin. Having engine weight hanging way out over the back is not conducive to having understeer margin.

If the car does go unstable, the other part of it is whether what needs to be done in order to recover is possible within humanly realistic reaction time (or stability-control-system reaction time) and whether the driver is given sufficient information to be able to judge what the right recovery action is. Full steer-with-a-pinkie-finger power steering is not conducive to doing that. 4 turns lock-to-lock steering isn't, either.

Vehicles with a really long wheelbase, wheels pushed all the way out to the corners, seem to slow down these reactions.

 

[BrianPetersen]

There's not a lot of mid-engine drift cars because drifting requires losing traction at the rear and keeping it at the front. 90/10 weight distribution with a monster engine up front and rear axle drive with a limited slip diff is the best recipe for that. Besides, there's not a whole lot of mid-engine cars out there that are suitable and cheap enough to be hacked up.

 

[GEspo]

enter “more predictable”: when the rear starts to slide on the rear engine youll notice it and still be able to control the car

I suppose id like to test all this since so many variables, tires etc.. ie when youre making huge slip angles there lots of factors to whether a mid is less controllable in slide

 

[cibachrome]

Just a few comments: Short wheelbase cars don't do well at speed for many reasons, Ackermann gradient for 1.

For 2, MOI is not the principle 'driver', Izz/Wt is. Barbell or Sputnik.

3), Drifting cars need mostly front grip, especially in the tire Mz world. Watch the videos. You turn the car via steering by applying steering system moments. Then you vector the power with drive wheels.

Put your car's rear wheels on grease plates and do the spinnerama play.

 

[GregLocock]

There seems to be some idea that the moose test is just some sort of journalistic prank. It's actually ISO 3888-2 and is used to evaluate ESC systems and limit handling in Germany, the USA, Australia, and no doubt elsewhere. It's used by the Feds to check whether an ESC system is more than a light on the dashboard, you have to be 2 mph faster through the gates with ESC on rather than off.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[SwinnyGG]

I wasn't implying that the 'moose test' itself is not highly valuable or standardized.

What I was saying is that when Porsche fanboys talk about Porsches dominating the 'moose test record' they're talking about one specific list published by a magazine, compiled from tests with different drivers under different conditions over many, many years. The magazine list itself doesn't mean all that much; that isn't an impeachment of the test itself.

As far as why drift cars are almost always front engine/rear drive... I think that has way, way more to do with the fact that front engine rear drive 'sporty' platforms are abundant and cheap, while mid engine rear drive platforms are rare and expensive, than it does with the last 1% of handling optimization.

 

[cibachrome]

Front engine, rear drive cars have "emergency brakes" /parking brakes on the rear wheels. A necessary element of drifting or the "Reverse 180" spinnerama maneuver.

"Honey, do you love me, or is that the hand brake handle ?"