Ride height and High Speed Stability

[kyngfish]

Last winter I redid my suspension - bushings - shocks - torsion bars all around. The setup is probably somewhere near street aggressive. Stiffer than stock. Anyway I also added a front spoiler and my biggest complaint was that the car didn’t feel any more stable at speed. This past weekend I lowered my ride height in the front about a half inch. I can’t remember whether it is front or rear where the wheel arch is slightly higher but either way my front is riding at around 24 and 3/4 which should be giving me a slight rake. Before my rake was 0. Now the car feels super planted at speed. Is it the ride height or the rake that’s contributing more to this? I love the change. Just trying to understand why.

[Flojo]

absolutely your lowering and slight rake now made the difference. simple aerodynamic logic.

FYI: Porsche actually prohibits to use the "turbo" lip without a tale (either duck or whale) for aerodinamic reasons (and vis versa).
the downforce must be balanced front to rear equally.

what is your "high speed"?

I do pusch my SC on the Autobahn around 200 km/h with just the front lip and no issue.
setup: lowered just below Euro ride hight, camber -1.4 front, -1 at rear, running 7/9x16 (205/245), with Bilstein B6 hard.

at 220km/h and above the car gets uncomfortable at rear as the front downforce begins to build a kind of leverage so the rear gets lifted.
but you can handle it

[Targa4now]

I’m running an ‘81SC euro height suspension with a Carrera front spoiler and no tail. I haven’t exploded into a ball of fire yet so I’m guessing you’ll be okay

[Targa4now]

Cool car Flojo

[Driven97]

Did you check / reset your toe after lowering? Could be an additional contributor. IIRC toe changes a decent amount with bump, and by extension, ride height.

Lowering the front may have given you more toe-in which makes the front end of the car feel more stable at speed.

[panzerfaust]

more rake, more weight up front = more stability

areo benefits helps too with or without the added spoiler...

i believe the factory setting had a slight rake

[76FJ55]

Originally Posted by panzerfaust

more rake, more weight up front = more stability

areo benefits helps too with or without the added spoiler...

i believe the factory setting had a slight rake

you're not going to change the weight bias front to rear with a ride height change. Using some very coarse numbers: A 1" change in ride height on a 90" wheelbase rotates the chassis about .25 degrees. assuming a 24" CG above the axle line (which true CG above axle is probably much lower) says you'll move the CG about .1 inches forward at best. Assuming you have a car with 40/60 initial bias the ride height change will now give you 40.1111111111/59.88888888889. Probably not enough to drastically effect handling. You'd get the same forward weight bias change by adding less than 3/4 of a gallon of gas.

The toe change however will significantly change the handling and the aero changes will have a positive affect too.

[Jonny042]

Originally Posted by Driven97

Did you check / reset your toe after lowering? Could be an additional contributor. IIRC toe changes a decent amount with bump, and by extension, ride height.

Lowering the front may have given you more toe-in which makes the front end of the car feel more stable at speed.

I just went through the exercise of getting toe change as close to zero as possible. I did this after making a ride height change of less than an inch, and observing a 3/16" toe change...... not cool!!!

[kach22i]

I once tried raising my front a little, it felt like a boat on the water.

I lowered it to where it originally was and all was better.

Slight rake downward rules.

[kyngfish]

Originally Posted by Driven97

Did you check / reset your toe after lowering? Could be an additional contributor. IIRC toe changes a decent amount with bump, and by extension, ride height.

Lowering the front may have given you more toe-in which makes the front end of the car feel more stable at speed.

Fair, I had a slight toe out before and I haven’t redone the toe yet. I’m resetting toe and camber this weekend as soon as I can rig up a better system than the last time I used jack stands and string (nothing wrong with that method just difficult to set and reset through multiple adjustments).

I think really what I was getting at is that, the rake makes sense - not because of weight distribution but because it essentially tilts the car slightly downward into the wind, which should, help the front end at speed. And I also get that for racecars lower is better, but at some point lowering becomes impractical on the street. So what I was wondering is if say, I kept the rake and just raised the car ever so slightly overall, would I keep the stability?

[panzerfaust]

Originally Posted by 76FJ55

you're not going to change the weight bias front to rear with a ride height change. Using some very coarse numbers: A 1" change in ride height on a 90" wheelbase rotates the chassis about .25 degrees. assuming a 24" CG above the axle line (which true CG above axle is probably much lower) says you'll move the CG about .1 inches forward at best. Assuming you have a car with 40/60 initial bias the ride height change will now give you 40.1111111111/59.88888888889. Probably not enough to drastically effect handling. You'd get the same forward weight bias change by adding less than 3/4 of a gallon of gas.

The toe change however will significantly change the handling and the aero changes will have a positive affect too.

you cant feel the difference with a nearly empty tank vs 3/4 full tank? or a properly weight balanced car on scales?

id say let him drop it down for the proper rake and let him get back with us... it only takes few turns of the TB adjuster bold

[Bill Douglas]

I'm front spoiler and no real wing/spoiler. I've had it up to 235 km/h and it felt great. the car wanted to go faster but I backed it off to a nice sensible 200km/h

Mine looks sort of low but I've never measured it. But it came out of the factory as a 911 SC Sport.

[NZVW]

I'm in a 73 and also have a 72 and both are lowered lower than Euro Spec, corner balanced and set. The one difference I notice is tyre changes. If I swap the Dunlop Semi Slicks for touring tyres the 72 feels a little floaty in the front (it does not have an S front spoiler)
The 73 sits at the near identical height set up and is good way past 200kph and sits flat most of the time.

[76FJ55]

Originally Posted by panzerfaust

you cant feel the difference with a nearly empty tank vs 3/4 full tank? or a properly weight balanced car on scales?

id say let him drop it down for the proper rake and let him get back with us... it only takes few turns of the TB adjuster bold

that's 3/4 of a GALLON, not 3/4 of a tank. I highly doubt you'd feel the difference of 4 lbs in the front.

[panzerfaust]

Originally Posted by 76FJ55

that's 3/4 of a GALLON, not 3/4 of a tank. I highly doubt you'd feel the difference of 4 lbs in the front.

noted... thank you

[kyngfish]

Originally Posted by 76FJ55

that's 3/4 of a GALLON, not 3/4 of a tank. I highly doubt you'd feel the difference of 4 lbs in the front.

Your original take on the difference in the weight bias is right, and not being an engineer, I can’t really make a mathematical argument, but it seems to me that front rake is going to have a compounded impact at speed.

If you stick our hand out the window flat while driving at 80 mph, and then tilt it even slightly downward, you immediately feel force pushing down on the front. Whereas if you keep it flat or even slightly upward, any buffeting wind lifts the front substantially.

That said, my rake was flat before, and I thought adding the front lip spoiler would make a more drastic difference. However adding that rake did, though possibly my toe also came inward. I’ll know in a week when I re-align.

I’m less interested in the front lip w/ or w/out argument. I think it’s been beat to death. If anyone gets on a track seriously and doesn’t use a tail and a front splitter, I don’t think they’ll be very successful. But I think for daily driving or spirited driving and the occasional track day, front lip and no tail is probably fine.

[manbridge 74]

Lowering the front of a 911 makes wheels toe out. As it’s raised up the wheels toe in, as it does on most every vehicle on the road.

[Bill Verburg]

The height of the CoM Δ is ~wheel base * sin Δ (rake angle in degrees)

The forward movement Δ of the CoM is ~ wheel base * cos (rake angle in degrees)

if the rake is 2° sin 2 = .031899, cos 2 = .99939

2° rake imparts only a very small increase in the height of the CoM or more importantly the rear CoM and a very small forward movement of the CoM

inc CoM in back means a longer lever arm for more body roll, again by a very small amount

as Driven says any effects you feel from raising or lowering front only are far most likely to be from toe, camber, caster and aero Δ's where small Δ can impart a significant change in handling

[kyngfish]

Originally Posted by manbridge 74

Lowering the front of a 911 makes wheels toe out. As it’s raised up the wheels toe in, as it does on most every vehicle on the road.

Then the toe variable doesn’t make sense as I already had a slight toe out in the front - so if lowering a half inch increased the toe out and toe out decreases stability at speed - then the increase in high speed stability is either due to the lowering or the rake.

[Bill Douglas]

Originally Posted by Bill Verburg

The height of the CoM Δ is ~wheel base * sin Δ (rake angle in degrees)

The forward movement Δ of the CoM is ~ wheel base * cos (rake angle in degrees)

if the rake is 2° sin 2 = .031899, cos 2 = .99939

2° rake imparts only a very small increase in the height of the CoM or more importantly the rear CoM and a very small forward movement of the CoM

inc CoM in back means a longer lever arm for more body roll, again by a very small amount

as Driven says any effects you feel from raising or lowering front only are far most likely to be from toe, camber, caster and aero Δ's where small Δ can impart a significant change in handling

I thought as much.

But after a few coffees and the sun is shining, I wasn't too concerned about the toe out.