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Silly me: I bought a 944 for E-street Autocross

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Old 11-28-2017, 09:36 PM
  #106
edfishjr
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I just posted some new info on where I am on lowering the 944 and the effects.

It discusses weight transfer, roll centers, jacking force... you know, all that stuff you think about every day.


https://edfishjr.com/2017/11/29/auto...s-of-lowering/


edit: link fixed per PedalFaster

Last edited by edfishjr; 11-30-2017 at 02:56 PM.
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Old 11-30-2017, 12:10 PM
  #107
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FYI, the link you provided doesn't work. Try this one: https://edfishjr.com/2017/11/29/auto...s-of-lowering/.
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Old 11-30-2017, 02:53 PM
  #108
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Originally Posted by PedalFaster View Post
FYI, the link you provided doesn't work. Try this one: https://edfishjr.com/2017/11/29/auto...s-of-lowering/.
Thanks!

It just occurred to my why messing with the suspension on the 944/924/968 can produce such wonky results: the effects of lowering front vs rear are opposite w/r/t roll stiffness.

The MacPherson front gets softer in roll as the car is lowered. The blog post linked above explains why.

The semi-trailing arm rear gets stiffer in roll as the car is lowered. This is because the semi-trailing arm roll center is very stable with rear suspension articulation, but the CG is being lowered. Therefore, the moment arm is getting shorter so the car rolls less in the rear for the same lateral-g input.

So, just lowering the front and rear equally seriously messes with the roll stiffness distribution and can produce a large tendency toward oversteer without doing anything else! (911's should react similarly.)

This also explains why the jacked up rear ends of U.S. cars, as delivered, supposedly for bumper height reasons, would have produced a tendency to understeer as compared to ROW cars.

Last edited by edfishjr; 11-30-2017 at 03:09 PM.
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Old 12-01-2017, 03:17 PM
  #109
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That is information that is both relevant to my interests, and also helps explain some of the results I've had.
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Old 01-09-2018, 06:27 PM
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I noticed that when a 944 is slightly raked....even the slightest...it introduces some considerable roll.
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Old 01-09-2018, 10:10 PM
  #111
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Originally Posted by mdnt08 View Post
I noticed that when a 944 is slightly raked....even the slightest...it introduces some considerable roll.
Once this thing gets back on the road I'm going to try to measure roll under different conditions. We'll have a test and tune in not too long where I can go around a skidpad with a digital level taped to the dash and see what I get. Not sure how that will work.

If you want to try it, here's one possible way: measure the roll with the rear set all the way down vs. all the way up. If I'm right about how the rear suspension works, the car will be stiffer (roll less) with the rear end low and roll more with it high. (This is the opposite of the front end.)

What I'm saying is that, say we start with both ends of the car hiked up and the car level, i.e. no rake. As we lower both ends, keeping the car level, the front gets softer in roll and the rear gets stiffer in roll. This will move the handling balance from understeer toward oversteer as the lateral load is "attracted" to the stiffer end of the car.

By the same token, if you start with a car that's level and lower just the front, creating positive rake, it will roll more. And vice-versus.
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Old 01-09-2018, 10:48 PM
  #112
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Originally Posted by edfishjr View Post
Once this thing gets back on the road I'm going to try to measure roll under different conditions. We'll have a test and tune in not too long where I can go around a skidpad with a digital level taped to the dash and see what I get. Not sure how that will work.

If you want to try it, here's one possible way: measure the roll with the rear set all the way down vs. all the way up. If I'm right about how the rear suspension works, the car will be stiffer (roll less) with the rear end low and roll more with it high. (This is the opposite of the front end.)

What I'm saying is that, say we start with both ends of the car hiked up and the car level, i.e. no rake. As we lower both ends, keeping the car level, the front gets softer in roll and the rear gets stiffer in roll. This will move the handling balance from understeer toward oversteer as the lateral load is "attracted" to the stiffer end of the car.

By the same token, if you start with a car that's level and lower just the front, creating positive rake, it will roll more. And vice-versus.

Yes I started it level and slightly lowered (1.75") with 250lb weltmeister springs and 26mm CIP1 torsions adjusted down with the most minimal pre-load. As the front settled, the roll increased..so I re-adjusted the rear to level out again....this corrected the added roll.

I tried a lower stance and loved it on the track, I think it was about 3.5" pretty low but handling was actually predicable... given the track is smooth and flat. Street wise it was not comfortable and did not pass our annual inspection in Hawaii so I raised it back up to the 1.75" front and leveled the rear. I may take it down another .5" all around.
I just have all poly bushings and turbo sways front and rear. upper strut and C-pillar bar since I'm already in SCCA SM due to other mods.
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Old 01-09-2018, 11:05 PM
  #113
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Originally Posted by edfishjr View Post
Once this thing gets back on the road I'm going to try to measure roll under different conditions. We'll have a test and tune in not too long where I can go around a skidpad with a digital level taped to the dash and see what I get. Not sure how that will work.

If you want to try it, here's one possible way: measure the roll with the rear set all the way down vs. all the way up. If I'm right about how the rear suspension works, the car will be stiffer (roll less) with the rear end low and roll more with it high. (This is the opposite of the front end.)

What I'm saying is that, say we start with both ends of the car hiked up and the car level, i.e. no rake. As we lower both ends, keeping the car level, the front gets softer in roll and the rear gets stiffer in roll. This will move the handling balance from understeer toward oversteer as the lateral load is "attracted" to the stiffer end of the car.

By the same token, if you start with a car that's level and lower just the front, creating positive rake, it will roll more. And vice-versus.
The digital level won't give you reliable roll numbers due to lateral g-forces. It will only work at rest. You may have better luck setting up a camera to measure roll from the stills.
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Old 01-09-2018, 11:05 PM
  #114
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Originally Posted by mdnt08 View Post
Yes I started it level and slightly lowered (1.75") with 250lb weltmeister springs and 26mm CIP1 torsions adjusted down with the most minimal pre-load. As the front settled, the roll increased..so I re-adjusted the rear to level out again....this corrected the added roll.

I tried a lower stance and loved it on the track, I think it was about 3.5" pretty low but handling was actually predicable... given the track is smooth and flat. Street wise it was not comfortable and did not pass our annual inspection in Hawaii so I raised it back up to the 1.75" front and leveled the rear. I may take it down another .5" all around.
I just have all poly bushings and turbo sways front and rear. upper strut and C-pillar bar since I'm already in SCCA SM due to other mods.
I kept trying to edit my post (it said I was still logged in, but I wasn't) to include a clear statement that I thought you were absolutely right: anything that creates positive rake makes the car softer.

Where do you take the 1.75" and 3.5" measurements from?

Last edited by edfishjr; 01-10-2018 at 01:14 AM.
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Old 01-10-2018, 01:04 AM
  #115
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Originally Posted by sjfehr View Post
The digital level won't give you reliable roll numbers due to lateral g-forces. It will only work at rest. You may have better luck setting up a camera to measure roll from the stills.
Oh. Yeah.

My bad.
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Old 01-10-2018, 01:25 AM
  #116
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Both wheels, back to back


end weld of the added 1" strip

Got my widened phone-dials back from Weldcraft today... now 8" wide.
245/40-15 Rival S tires should arrive tomorrow.
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Old 01-10-2018, 10:59 PM
  #117
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Originally Posted by edfishjr View Post
Thanks!

It just occurred to my why messing with the suspension on the 944/924/968 can produce such wonky results: the effects of lowering front vs rear are opposite w/r/t roll stiffness.

The MacPherson front gets softer in roll as the car is lowered. The blog post linked above explains why.

The semi-trailing arm rear gets stiffer in roll as the car is lowered. This is because the semi-trailing arm roll center is very stable with rear suspension articulation, but the CG is being lowered. Therefore, the moment arm is getting shorter so the car rolls less in the rear for the same lateral-g input.

So, just lowering the front and rear equally seriously messes with the roll stiffness distribution and can produce a large tendency toward oversteer without doing anything else! (911's should react similarly.)

This also explains why the jacked up rear ends of U.S. cars, as delivered, supposedly for bumper height reasons, would have produced a tendency to understeer as compared to ROW cars.
Soooooo what i take from this... is I should maybe run just a little nose high? being a little more neutral with the ability to hit oversteer would be fantastic. I did notice an improvement when leveling my s2 vs is previous stance now that i think about it. It was lowered all around but down in the nose a little bit, i brought it back to level and its less twitchy out back...

Edit: i don't know if this helps your research at all, but this is the raw video from an event i did a few years ago that just tracks the front left wheel. Cars on koni yellow inserts, 968 M030 bars and -1.5 camber. 225/45/17 and 255/45/17 stagger. I just dumped the raw footage, so the action is from :30 to 1:40. Also my sd card was going bad so there are glitches.


Last edited by Arominus; 01-10-2018 at 11:17 PM.
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Old 01-11-2018, 01:35 AM
  #118
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Originally Posted by Arominus View Post
Soooooo what i take from this... is I should maybe run just a little nose high? being a little more neutral with the ability to hit oversteer would be fantastic. I did notice an improvement when leveling my s2 vs is previous stance now that i think about it. It was lowered all around but down in the nose a little bit, i brought it back to level and its less twitchy out back...

Edit: i don't know if this helps your research at all, but this is the raw video from an event i did a few years ago that just tracks the front left wheel. Cars on koni yellow inserts, 968 M030 bars and -1.5 camber. 225/45/17 and 255/45/17 stagger. I just dumped the raw footage, so the action is from :30 to 1:40. Also my sd card was going bad so there are glitches.

https://youtu.be/X9TPjXt8TbM
I don't think one can say anything absolutely about how to rake the car. It will depend on what springs, what sway bars, what tires, what adjustability you have (M030 or not) etc. The rate at which the front loses stiffness in roll as it's lowered doesn't match the rate at which the rear gains stiffness as it's lowered or loses it as it's raised. I think with the 968 M030 bars you are probably best as level since this is a Porsche developed combo. For autocross, you could probably use an even stiffer front bar rather than raising the front.

A completely stock, non-M030 car? I'd say you probably want it level as Porsche seems to have intended. I would add a stiffer front sway bar long before I lowered the rear to produce negative rake. Then, toe-out the front tires to quicken the turn-in (easily restored once the event is over by counting flats or other marks) and adjust over/understeer balance at autocross speeds with rear toe adjustments. Not that you need this, but general caveat: when the rear-end is lively at autocross speeds it may be too unstable at higher, track speeds as the car tends toward more oversteer as speed increases.

As for the video, it doesn't seem to me like you have excessive roll based on how much the tire moved within the wheel-well. Of course, stiffer is generally better for autocross. Nice downshift into the hairpin!

Last edited by edfishjr; 01-11-2018 at 01:56 AM.
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Old 01-11-2018, 03:41 PM
  #119
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245 on 8 (left) and the tread measures (from overhead) exactly 1" wider than 225 on 7.
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