911 rake front/rear

[Jim Garfield]

Randy, you're just trying to get this thread to go another 7 pages...

[stlrj]

I was browsing the wind factory wind tunnel pages in Paul Frere's PORSCHE 911 STORY and noticed all their tests were done on a level chassis with no rake at all.

Why does the factory insist on no rake for their test data?

If they lowered the front end, I wonder how much that would change the front/rear lift numbers?

Cheers,

Joe

[Randy Webb]

Yes, I wanted to see if it can qualify as the most off the original topic, most opinionated w/no bacground for the opinions, and least practically rewarding.

It's close to winning all 3 awards.

[Tyson Schmidt]

Originally posted by stlrj
I was browsing the wind factory wind tunnel pages in Paul Frere's PORSCHE 911 STORY and noticed all their tests were done on a level chassis with no rake at all.

Why does the factory insist on no rake for their test data?

If they lowered the front end, I wonder how much that would change the front/rear lift numbers?

Cheers,

Joe

Probably because that would increase oversteer. Porsche used every trick in the engineering handbook to build in understeer so these cars wouldn't swap ends at the hands of the inexperienced.

This is what I take away from all of this.

On a 964 or newer Porsche which have a truly flat bottom, it's better to have no rake, and, if anything a front splitter that doesn't drop lower than the underbelly so that the pressure under the car is as consistent as possible from front to rear. The splitter would send more air over the top and less under the bottom without inducing as much downward shift in the front at the expense of lift at the rear that a front air dam or body rake would cause.

For the '89 and earlier cars which have very turbulent and uneven underbellies, the less air under the car, the better. (This could be achieved by overall lowering too.) Front air dams, spoilers, and rake would be good, since the underside is a mess compared to the later cars. A rear wing should be used in conjunction with the aforementioned for optimal results.

I have a question about the upward slope of the rear of the underbelly that no one seems to have answered for me yet.

Why do they do this? I see this on almost all the race cars, yet according to Bogieman, this causes rear lift.

Is it beneficial in another way? I was trying to sleep last night, but couldn't with all this crap flying around in my head. The only thing I could come up with is that it must give the high pressure air under the rear of the car a smooth escape route to the low pressure area directly behind the rear bumper that causes drag, thereby reducing overall drag, and maybe a little of the rear lift?

Is this right?

[masraum]

I think for the upturn to do much/anything you generally want tunnels or these things



with either the tunnel or these fins you do get the suction effect, but with a normal car at a normal height and nothing to keep air out from under the car I don't think it's that beneficial.

It may be in some situations where they don't have the vertical walls to keep air from sliding in the side they are just going for smoother transitions

... or it could just be that I am totally off and talking out of my butt like Jim Carey, but that's my opinion.

I'd love to builld a small wind tunnel and test this stuff, but from what I understand when you start reducing the size to get realisticly scaled results the windspeed necessary climbs exponentially, so since I can't build a 3000 mph wind tunnel or a full sized wind tunnel or even afford to rent on for an hour...

[masraum]

Tyson, to, I think, add to your reasoning about the pre'89 cars, don't forget that the original Ruf CTR used a rubber airdam similar to the front lip spoiler under the car in front of the motor/tranny area.

[Tyson Schmidt]

Originally posted by masraum
Tyson, to, I think, add to your reasoning about the pre'89 cars, don't forget that the original Ruf CTR used a rubber airdam similar to the front lip spoiler under the car in front of the motor/tranny area.

I wonder if that was to help the engine cooling fan work more efficiently by using the venturi effect in the rear to help evacuate air from the bottom of the motor, or to use the venturi effect to reduce rear lift?

I'm done speculating, since much of this stuff is entirely counterintuitive.

Earlier in this thread I was fighting the hurdle of thinking in terms of compression/decompression, when apparently this is almost a non-issue below mach-1.

[350HP930]

Originally posted by Tyson Schmidt
I have a question about the upward slope of the rear of the underbelly that no one seems to have answered for me yet.

Why do they do this? I see this on almost all the race cars, yet according to Bogieman, this causes rear lift.

Is it beneficial in another way? I was trying to sleep last night, but couldn't with all this crap flying around in my head. The only thing I could come up with is that it must give the high pressure air under the rear of the car a smooth escape route to the low pressure area directly behind the rear bumper that causes drag, thereby reducing overall drag, and maybe a little of the rear lift?

Is this right?

Thats not a bad theory you have their Tyson. You could be right on that one.

[stlrj]

For the '89 and earlier cars which have very turbulent and uneven underbellies, the less air under the car, the better.

Tyson,

Theoretically, it makes perfect sense, but I'm still having difficulty understanding why all the factory test data was done on nose high pre 89 911's with the very dirty underbellies. And to complicate matters, I have yet to see where the addition of rake to any degree has been beneficial in mitigating understeer.

Cheers,

Joe

[Eric Mckenna]

This is a great thread but mother,, man I am Like Tyson.. struggling to take all this info in and make sence of it all..
So over all what is the best set up for a 88 ish and under earlier car?

I have a 78 SC with a RS fibreglass tail and a stocker OE rubber lip and the car sits about a 1/4" higher in the front what do I need to do to set it up .. raise the rear and lower the front, and leave the nose and tail alone.



Eric Mckenna

[Tyson Schmidt]

Originally posted by stlrj
Tyson,

Theoretically, it makes perfect sense, but I'm still having difficulty understanding why all the factory test data was done on nose high pre 89 911's with the very dirty underbellies. And to complicate matters, I have yet to see where the addition of rake to any degree has been beneficial in mitigating understeer.

Cheers,

Joe

Well, I think it makes sense that the Factory would do testing on the cars the way they would be delivered to the public.

Also, the front of the 911 isn't actually nose-high as delivered from the Factory. It just looks that way. If the front fender lip is .5" higher than the rear, this actually translates to a 1 degree rake. The front fender opening is larger than the rear.

The factory also has to take into account that with a full tank of fuel, and full cargo capaicty in the trunk, that the car can't be scraping the ground with the nose over every bump and driveway, and be prone to oversteer at high speeds due to the excessive rake from having a full trunk and fuel tank. They have to set things up as a compromise under all real-world conditions.

I've experimented with varying amounts of rake on the track. A nose high attitude definitely makes the car push more, and is far more susceptible to side-winds. Willow Springs Raceway will teach you that in a hurry, with some scary side gusts as you are already in a 4-wheel drift at about 120-130mph in turn 8. I've been blown over a few car widths in this situation, and it's a pretty scary experience, especially if you are already running out of track.

My boss's 914-6 was set so low that it was hitting the bump-stops. (By the previous owner) I raised the front so the suspension would have some travel, and the car wouldn't be such a handful over the bumpy stuff. (It was raked quite a bit.) We ended up changing it back because he just couldn't get the car to turn-in at speed with this attitude. At that track, you learn very quickly to mind your aero, and a simple wing or rake adjustment can net you significantly better laptimes.

[stlrj]

Tyson,

On the 914, did you forgot something when you raised the the front, like maybe setting the toe to compensate for all the toe-in you ended up with after cranking it up? I know what a handfull they can be with too much toe and what a huge difference a bit of toe out makes for turn in.

Seems a lot of people blame poor handling after raising the front when, in fact, the toe never seems to get addressed since it's too easy to raise the front of a 911 or 914 and a PITA to set the toe, so guess what, it never gets done right and everyone blames the lack of rake for the poor handling.

Cheers,

Joe

[speeder]

Originally posted by stlrj
Tyson,

On the 914, did you forgot something when you raised the the front, like maybe setting the toe to compensate for all the toe-in you ended up with after cranking it up? I know what a handfull they can be with too much toe and what a huge difference a bit of toe out makes for turn in.

Seems a lot of people blame poor handling after raising the front when, in fact, the toe never seems to get addressed since it's too easy to raise the front of a 911 or 914 and a PITA to set the toe, so guess what, it never gets done right and everyone blames the lack of rake for the poor handling.

Cheers,

Joe

Joe, How on earth is it "a PITA to set the toe"??

You're making no sense there, Kimosabe.

[stlrj]

Denis,

Compared to turning an 11mm screw, you bet your a$% it's a pain. Doubt that half the people on this board even know how it's done, have done it themselves or even understand the correlation between ride height and toe change.


I would even go as far to say that breaking loose the tie rod lock nut can be major deal for most of us.

Cheers,

Joe

[Tyson Schmidt]

True Joe. But I'm not one of those people. I've been doing suspension and alignment for over a decade, and have a thorough understanding of all the geometries involved in car set-up.

I agree, if I hadn't reset toe, it definitely would have increased understeer. But that wasn't the case.

Dave also runs a front swaybar, but no rear swaybar on his 914-6, just like most who track and autocross 914's. His inside front wheel is pretty much off the ground the entire time through turn 8 and 9 at Willow. That would pretty much eliminate toe settings as a contributing factor, wouldn't it!

There are though, many issues that hurt handling when you lower a 911. You throw off the bump-steer characteristics, the camber compensation characteristics, and you increase the amount of bump-understeer.

Bump-understeer is when the outside tire toes out as that corner compresses upon entering a corner. The steeper the upward angle of the tie-rod at ride height, the more bump-understeer you'll have. This applies only to a car which has the steering rack behind the front axle.

A little bit of bump-understeer is good, since it prevents the car from turning-in more than the driver wants. But you want to keep it to a minimum, so that the car won't lose it's willingness to turn-in to a corner.

The angle of the control arm is also compromised by lowering, and is in an almost horizontal angle on a lowered car. This compromises the suspensions ability to compensate for camber loss from body roll, and will result in lost grip due to the tires contact patch pulling away from the pavement as the inside of the tire is lifted, and the ouside of the tire and sometimes even the sidewall are all that is left in contact with the pavement.

The proper way to lower a car is by raising the spindles on the strut housing, and lowering the mounting point of the outer tie-rod ends to the steering arms the same amount, so that the tie-rods and the control arms remain parallel to one another, minimizing bump-steer.

Bump steer is simply the toe change as the suspension moves through it's travel, caused by non-parallelism of the tie-rods and control arms. Also the tie-rods should be as close to the same langth as the control arms as possible.

[masraum]

I'd say that's a pretty good guess. Now if you read up a little you'll really have it down!

[Randy Webb]

1. "upward slope of the rear of the underbelly ..."
2. "trying to sleep last night, but couldn't with all this crap flying around in my head. "

1 - my guess is that this helps break up the turbulent eddies that attach to the rear otherwise -- same effect as a Kamm (sharp cut off) tail. Just a guess - but an educated one. The red car photo between this post & yours shows a more advanced design along the same lines. It has directed (big black vertical things) diffusers.

2 - You will be pleased to hear that Georg Cantor, the famed mathematician, went insane from thiking aobut the real line.

[Tyson Schmidt]

Originally posted by masraum
I'd say that's a pretty good guess. Now if you read up a little you'll really have it down!

Was that for me? What does it mean?

[masraum]

Yep, that was for you. It was sarcasm, your post sounded like something from a textbook. I was thinking when the other poster said that you must have forgotten to check the toe and alignment after raising the 914 that he just wasn't familiar with your work. I would say that your post would tend to back that up. It wasn't a dig at you.

And now something else for you from Fred Puhn's book How to Make Your Car Handle
page 49. This quote seems to validate your point that rake can/will create suction between the car and the road.

In addition to a low nose, the racing car often has less ground clearance at the front than at the rear. This adds downforce on the entire body, even on a stock sedan body.

So, You have at least one person that seems to be on your side about rake counteracting lift at speed. Awesome, I feel better about it now. I would say that he should be a pretty good reference.

[tsuter]

Mine is so low the wifey can't get to the oil drain plug!