Questions about rake and ride height

[tirwin]

I'm having a local shop align and corner balance my '83 after a bunch of new suspension work. I can go into detail on that if necessary to answer my question, but for now let me ask a couple of questions in hopes that there is an easy answer.

My recollection is that the 911 normally has a positive rake angle of something like 1º.

When I search ride height threads I see a lot of suggested heights as 24in rear and 24.5 in front, which is confusing me. I would've expected the opposite. Do those heights not imply a negative rake angle?

If those heights do in fact indicate negative rake, then I'm assuming that makes the back end squat more and was perhaps done for some reason.

Basically, I'm trying to figure out if I should care about rake at all and just worry about getting the balance right. Or does rake make a difference?

I'm hoping there is any easy answer, but like I said, happy to go into more details on my particular setup if necessary.

[DRACO A5OG]

Actually, the factory rake is ZERO ergo .5" higher at the fender lip.

If you want a forward 1 degree rake make the fronts and rears even. Height of your choice, but remember lower than Euro height, you should consider a bump steer kit.

Are you using it for AX/DE/Track days or aggressive street?

[DRACO A5OG]

Set the height and/or rake you want then corner balance her. She may end up lop sided but she will handle like she is on rails.

Mine sits on the street with the passenger side front a bit lower but when I am in the car she evens right out. Oh yeah, make certain they put your weight in the driver side to properly corner balance her.

[Bill Verburg]

Originally Posted by tirwin

I'm having a local shop align and corner balance my '83 after a bunch of new suspension work. I can go into detail on that if necessary to answer my question, but for now let me ask a couple of questions.

My recollection is that the 911 normally has a positive rake angle of something like 1º.

When I search ride height threads I see a lot of suggested heights as 24in rear and 24.5 in front, which is confusing me. I would've expected the opposite. Do those heights not imply a negative rake angle?

If those heights indicate negative rake, then I'm assuming that makes the back end squat more.

Basically, I'm trying to figure out if I should care about rake at all and just worry about getting the balance right. Or does rake make a difference?

I'm hoping there is any easy answer, but like I said, happy to go into more details on my particular setup if necessary.

When I search ride height threads I see a lot of suggested heights as 24in rear and 24.5 in front, which is confusing me. I would've expected the opposite. Do those heights not imply a negative rake angle?

No, those are fender measurements and the fenders are different heights

go by the factory height measures which are off the t-bar/axle ctr Δ
stock for a 911 has been consistent for all RoW cars @108+/- 5mm front the rear has varied a lot from 12+/-5mm in the 70s to 16+/-5 for the '80s

the lowest oe was for the SC/RS 143+/-5mm front and 14mm+/-5mm rear

note that the '70s rear 12mm was actually the lowest rear spec. but over the years aero became more of a consideration which is why some rake is desirable. The later cars w/ more aero tend to leave the rumps up a bit more than the very early cars for this reason. There is no actual rake spec, some is good and some like more than others.

By the time you get to the SC/RS type specs you should be using at least a steering rack spacer to help minimize bump.

[Ronnie's.930]

Tim, the confusing front-v-back numbers is because those are being measured at the fender lips and the rear fender archs are lower, relative to the ground, than the fronts.

Regarding rake - I think a lot of driving manners, personal preference goes into this . . . I like the quick steering feel of a lot of rake (I like a lot of rack, too, but for other reasons and that is a different subject altogether!!!!), but too much makes the car overly twitchy in a straight line at high speed.

[tirwin]

Ok, got it. Makes sense now.

My intent is to lower it from stock. The front spindles have been raised and the tie rods have the adjustable bump steer ends on them.

I am referencing the 24/24.5 numbers because that's how people report their numbers when they show pictures of their car. I'm trying to approximate that visually (if that makes sense).

The goal is the right handling, not looks.

Thanks, everyone!

[javadog]

If the goal is good handling, you might want to be higher than you think. People running 24" or 24.5" fender heights have their cars lowered into the weeds. Unless they have made significant changes to the cars, they don't have enough suspension travel for the real world and while you can change the spindle heights for toe correction in the front, you also need to consider where you are in the camber and toe curves.

Stock fender heights are around 27"/26-5/8, not that the factory ever measured suspension height that way. Look up the threads where people raise their cars back up from running them really low and see what they had to say about the improvement in handling.

99% of people run low ride heights because they like the way it looks. The other 1% think a low center of gravity is the most important thing to think about.

There is no official "rake angle" as Bill noted above. If you have a coupe and run spoilers, you'll notice a difference if you run lower in the front.

JR

[tirwin]

JR,

I think you're right. I would like lower than stock but not slammed either. I was thinking that the purpose of rake was intended to be more of a function of aerodynamics but it is also linked to suspension. I have a duck tail rear spoiler and I will be adding a front chin spoiler in the future.

The shop I'm using does a lot of race prep and I think they'll help me get it dialed in. I was just confused by the lower rear fender height that people report.

[javadog]

Rake is something that is almost irrelevant on these cars. It probably started as a result of people lowering their cars slightly more in the front, probably because there's often a little more space between the front tire and fender than that of the rear, especially when the fuel tank is low on fuel. Most people care more for how the car looks than how it drives and they don't like this extra gap. When the gaps are more or less the same, you can often see a little downward tilt towards the front on the rocker panel. This became rake, which then became the "official 1 degree of rake" which is just internet horse****.

I doubt you'd ever notice any difference in aero lift from playing with the rake. It does seem that running the front lower to the ground does help a little; I've noticed that after driving 35 years in 911s with tails.

A shop that does a lot of track prep work can have a lot of knowledge, but what you want for the track and what you want for the street are not the same thing. Be specific about your goals when you talk to them.

JR

[tirwin]

One would think that lowest center of gravity is best. The problem seems to be 'lowest' is relative to other factors that impact handling, and as you say, also depends on how you drive.

I think the front ride height on my car had been lowered as much as possible. There were no bump spacers and it seemed to me that the geometry was suboptimal. I felt a good amount of kickback at times. A goal in the suspension refresh was to increase the adjustability. I'm hoping that the changes I've made will allow me to 'normalize' the geometry of the control arms and dial out the bump steer.

There will probably be some trial and error involved.

[javadog]

A low center of gravity is nice. It's one of a half dozen things that interact with one another, so it's not like you can have a low center of gravity in isolation.

Porsche designed the suspension to work through a specific range of motion. If you look at the camber and toe curves with respect to bump and droop, you can see this. When people lower the front of a 911, they think about bump steer. They don't think as much about the reduced travel, the fact that the toe and camber changes aren't optimum, they probably don't think at all about the roll center, etc..

It's useful to get a book about suspension design and see how everything interacts, then see what Porsche did and what the curves look like at the ride height the suspension was designed to operate at. It's also useful to identify what things you are trying to fix, then see what the best way to fix them might be, rather than just change something for the sake of change.

JR

[tirwin]

My suspension had to be addressed. The struts were shot, especially the rear. I talked to several friends, read a lot and spent some time talking to Chuck Moreland in person before coming up with this plan. I feel pretty good about it now. Excited to get the car back to see how it feels.

I am (hopefully) at the end of a long list of projects to get this car back up to par. Until recently I have not felt comfortable with the idea of doing AX, DE or even taking a road trip. There were just too many issues - CIS, electrical gremlins, no A/C - and I didn't trust it.

I hope I've done honest work. At least that has been my intent. The only way you really learn is by doing. Going to the Hilton Head Concours in a couple of weeks and then Amelia Island next year. (Not showing the car, of course, just driving it.) I'm hoping next year I'll get a full season of DE and/or AX.

Only issue left is to deal with the loose nut behind the wheel and my wife says it can't be fixed!

[mazing3]

Hey T, Who is raking your ride here in town? Got my paint woes sorted and have been feverishly re-assembling. Hell, I'm at OLOBA right now.

Java is so so dead on too about the travel factor and the roll center aspects... I learned that the hard way on my first go at mine. Just thought I had gone overboard on the front TBs and that was why the nose was so bouncy...Elephant Chuck set me straight and I got to enjoy one of those "now I get it" moments by coming up an inch. There is a deadly "show me your stance" thread out there that led me into inaccurate functional territory.

[tirwin]

Goldcrest in Kennesaw. They are the only place around that can do a corner balance that I know of.

Just for clarity, I wasn't shooting for 24/24.5 myself. I was just using that popular example from several of the ride height threads to understand if that meant those setups had negative rake. I get it now.

Glad you got your paint sorted out! Looking forward to the next get-together down there. I think I'm done with the C&O scene.

[tirwin]

While we're talking suspensions, I was watching the season finale of Fast 'N Loud last night. They put an air ride suspension on a 2005 Ford GT they rebuilt after being wrecked. I have no idea why and it seemed like the only reason offered was "because".

[Bill Verburg]

while the CoM(AKA CG) is important the distance between the roll center and CoM is far more important. This is determined by the geometry of the suspension.

compare A1 to A2 in this diagram, the lowered car will roll more(bad because it affects camber pattern and bump) than the stock height(all other factors the same) because the lever arm A2 is longer

[tirwin]

Thanks, Bill. Good stuff. Referencing your lower diagram -- in such a situation, wouldn't tire sidewall stiffness and the sway bar be examples of the things that could be used to help decrease roll?

I have chosen to not change from the factory sways until I have some driving time with the rest of the new setup first.

Will keep you posted. I should be picking it up tomorrow.

[Bill Verburg]

Originally Posted by tirwin

Thanks, Bill. Good stuff. Referencing your lower diagram -- in such a situation, wouldn't tire sidewall stiffness and the sway bar be examples of the things that could be used to help decrease roll?

I have chosen to not change from the factory sways until I have some driving time with the rest of the new setup first.

Will keep you posted. I should be picking it up tomorrow.

wouldn't tire sidewall stiffness and the sway bar be examples of the things that could be used to help decrease roll?

The force that causes roll is a product of the mass and length of A. In the diagrams A2 is longer than A1 because the lowered car has had it's instant center lowered. Because the control arm angle was changed to lower the car. This is where 'geometry' enters the fray.

Yes, roll bars ans spring rates and tire sidewall all deal w/ the amount of roll foce but having proper geometry limits the magnitude of the roll force that has to be dealt w/. Theoretically if A was held to zero there would be no roll force to deal w/ no matter how fast the car was going.

In 993 up this is dealt w/ by using wheel carriers that adjust geometry to be more benign at lowered ride heights, but these are only used on the RS/GT3 & RSR versions of the cars. On a 911 the only way to deal w/ it is to raise spindles, lower the steering rack, use o/s tie rod ends or some combination of the 3.

[aoncurly]

When I had my 81SC set up with stock 16-inch Fuchs, 25 Front/24.5 Rear caused me to scrape the hook in front and my tailpipe at the rear going in and out of my driveway (and my driveway isn't very steep). My car is now set up at 25.5 Front/25 Rear and I don't scrape anymore. I believe this setting is considered "Euro" setting, and at least to my eye looks better.

[javadog]

Originally Posted by aoncurly

My car is now set up at 25.5 Front/25 Rear and I don't scrape anymore. I believe this setting is considered "Euro" setting, and at least to my eye looks better.

That's what everybody thinks is "Euro" height but is close to 1-1/2 inches lower than "Euro" height. A common misconception...

It does look better, I'll give it that.

This might be worth a read:

Ride height - factory measurement method

JR