roll centers, rear uprights

[Dominik]

I am developing "quickly" rear uprights for my new Cobra chassis.
The current plan results in a rear roll center 2 3/8" above ground and front 2 3/8" as well.

To lower the rear roll center I could just make the upright shorter, resulting in more parallel wishbones and a lower rear roll center - if I am not mistaken, and if that is what I need.

The more heavy end of the car (Cobra weight distribution 50/50) calls for a higher roll center.
So equally high roll centers seem to be the answer.

Although, to reduce jacking of the rear of the car and get a better camber curve I shall position the rear roll center lower.
That would also be what a suspension engineer in town suggested.

Any comments?

I am sorry if suspension questions have been discussed before, but my 33k dial up connection does not really allow to "search" clubcobra effectively.

Dom, Cape Town

[Clois Harlan]

Dom,

I wish I could help you but this is way over my head.

Clois

[Dominik]

Clois,

thank you for your interest. :-)
that you cannot help me only confirms that this is a secret to more than us.

maybe we get some brains to start "revving past the red line".
with the discussion elsewhere in the forum about handling, I could only be beneficial for all of us.

I always had to smile when "dealers" in the 80s pontificated (I had to look this one up in the dictionary):
"this replica does not handle very well - of course.
the originals did not handle. what do you expect?"

I thought: sure, that's why they never won anything...

dom, cape town

[rdorman]

No expert here but as I recall.....

If you what the roll center lower, wouldn't it be easier to move the inboard mounting points? Seems to me for performance use a roll center of an inch or so above ground or less is preferred (not to far below!) and that the rear roll center should be lower than the front. These are of course generalities.

Desired camber curves would depend a lot on the rubber you will be running etc (radials like more than bias plys).

You may want to buy some suspension software or try scale 'dolls' and play with the different combinations. If I was doing a chassis from scratch I would probably use an already available upright to control cost. A-arm, suspension pickup points etc are relatively cheap compaired to the cost of machining an upright! There are lots of after market uprights available. Just a thought!

So what is the project you are working on? Sounds interesting!

Rick

[Dominik]

rick,

good thoughts. thanks.
the rear lower than the front? no problem, I can change that with the upright.
I could consider two mounting positions for the upper (rear) wishbone to the upright.

a lot of uprights available is correct, but typical engineering shop cost of USD 20 to 25 in south-africa easily allow me to make an upright (with a bolt-on bearing housing/retainer from a GB FORD Sierra/Sierra)

the chassis is very loosely based on the 427 chassis.
I always wanted one and I am willing to trade some handling for originality.

my best bet is to use a 289 style rear suspension instead of 427, but then I loose the anti-squat built into the 427.
tracion out of the corner may suffer, the first chassis is 90% finished and I do not want to modify it - design is "frozen" ;-)
maybe the next one.

I read all of C.Smith's books and paul van valkenb... as well as fred puhn's.

it's not really explained what one really wants.
I do not expect it to be perfect, but I just do not want to mess it up completely.

I can lean on the original 427 upright's dimensions, to simulate their behaviour, but it may be a compromise for street use.
427s were also (mainly?) built for street use.

as I understand, the front roll center of a 427 is about 2" above ground and the rear 1".

rubber:
BFGs... - unfortunately. Importing is very expensive, so we use the cheapest. I would prefer Hoosiers if I had that option.

rear roll center lower than the front seem to be the way to go.

thanks,

dom

[mr bruce]

Chassis dynamics is way more involved than you would believe. What tyre are you going to use, where is the engine located, what is the proposed wheelbase, track width, shock parameters. My theory with these cobra clones?, Most of the handling is in the tyres and shocks.The Backdraft cars cleaned up at Run and Gun this past year , they have BMW based suspension, with McStruts in the front, I believe. Go look at one of them, try to find their secret. one more thing, No car is going to make you or anyone the next Senna,, Shoemacher(spl?) or Fangio.

[Tom Kirkham]

Normally the rear roll center is higher than the front. A good book on the subject is Race Car Vehicle Dynamics by William F. and Douglas L. Milliken ISBN 1-56091-526-9. Some other great books on the subject are the Carroll Smith books.

[rdorman]

See! I had the roll centers backward! I love the Carroll Smith books but he does not attempt to tell you exactly what works. To many variables. The Herb Adams book on chassis and suspension design while basic does attempt to give you 'ranges' that generally work well. Which is generally an unequal, un parrellel upper and lower a-arm setup.

20-25 and hour! A third or less of what they are here!

Good luck with your project and post some pictures will you!

Rick

[coyled]

Dom, as Bruce stated this is a complicated subject. The backdraft car had 600hp and a great driver that had raced for 20 years. Tha name of the parts on the car(bmw, mustang, jaguar) are not as important as the set up. From what I have learned, mostly related to original style suspensions, the biggest problem with a cobra is in the front not the rear. The front roll center is too low, the track is too narrow, there's a bunch of bump steer and not enough ackerman. In roll, the roll center moves down which unloads the outside wheel right at turn in. As I understand it, thats mostly why the cobra does not turn in well, it just sort of rolls over. I know a guy that started with a hi-tech, turned into a can am cobra. Almost all of the changes were at the front. Changes included different spindles which changed the roll center, and steering arms. Good luck, Scott.

[Dominik]

tom,

I contacted Milliken and must say their books are quite expensive, which they admitted.
the rear now higher than the front? that makes more sense looking at the measurements of a 427.

that would explain why the the 427's rear upright is higher (longer) than mine. I can change that easily.
It's only a dummy for now.

rick,

I post pics once I am happy with what I can post - especially the background, which is my messy garage :-)
unequal, unparallel a-arms is what I got. so I am at least not incorrect here.
the car I raced (see below) had almost parallel / equal length suspension in the rear.

scott,

I am shipping an excellently handling car this monday to germany.
I was comfortably cruising into second behind John Atkins' race prepared (still street legal) 289.
just one second slower per lap, with less power, and I am a beginner.

that car also features (FORD) McPherson strut in the front and a 289-like homemade rear with coil-overs.
it was built by a very well known racer/car builder in south-africa.

bump steer is a good point. I defenitely consider your input.
Tony Martin from BDR was there and watched me in the first heat.

bruce,

yes, you are right. but like I said, I just do not want to mess it up completely while I have the chance.
and I saw some Cobras really change behaviour once they went to 17" tires - which is not what I want.
I am too old to become a schuhmacher.
fangio? maybe... ;-)

Okay, back to the drawing board and playing with roll-centers. I keep the rear now where it is (quite original), as high as the front (or move it higher) and carry on.

otherwise I will never finish... - although, I wait for more input.

and yes, I would buy a Kirkham for originality (I had a good look at Fritz' car in Germany), but importing is quite expensive - I aquired a house instead...

thanks again for your thoughts,

dom, cape town

[Dominik]

here is now my best guess:

to get the front roll center a little above ground I found a front upright that is 3/8" (to 3/4") longer between the center of the ball joints than the spacing of the upper and lower mounting points at the chassis.

to get the rear roll center higher above ground than the front, I design an upright which is 1 1/2" longer than the spacing of the mounting points at the chassis.
the wider track in the rear also moves the rear roll center slightly up.

I think it is a good start.

I am thinking about using two different mounting points on the rear upright, one higher than 1 1/2".
the wider track in the rear also moves the roll center slightly up.

a next chassis may feature two holes per wishbone to play a little more with the wishbone positions at the chassis.

keep well,

dom

[speed220mph]

Dominik,
I'm wondering why you are moving the points on your upright and not the inner pivot point on the chassis. Seems like you're doing it the hard way unless you are starting from square one. With that said, the books mentioned are good. But a less expensive one that covers this subject in depth is HPBooks' How to Make Your Car Handle by Fred Puhn. Even Caroll Smith said Fred's book eliminated the need for him to do a suspension book.

"Simply" put, the rear rc should be either the same level or slightly higher than the front rc. Lower in the rear creates an inherent oversteer condition, which is what you must avoid. Too high, on the other hand, creates a mechanical lockup condition, also an oversteer condition. So don't go too high. Also, control arms should be as long as possible, the lower control arm being the longest. This and a near parallel-to-the-ground lower arm and a slightly angled down upper arm will provide an instant center that will create minimal camber change, an rc that's about where it should be and handling that's easy to tune with sway bars front and rear.

Oh, don't forget bump steer. You can get the rc right on, but if the suspension ends up with bump steer, especially toe out in bump, and you'll end up with an ill handling (dangerous) vehicle.

Good luck.

[Dominik]

Speed-Tom,

thanks. Yes I build from scratch using 427 blueprints.
I found out that there is no thing as THE position for the roll centers, because it's always a compromise.

"a slightly angled down upper arm will provide an instant center that will create minimal camber change"

The slightly angled down upper wishbone positions the roll center below ground (F. Puhn, pg 36), which is "not usually used on high performance cars, but it does exist on some sedans".

So here is a compromise already. It is not explained why high performance cars don't use this.
I consider that and defenitely have an eye on bump steer.


cheers,

dominik, cape town

a friend of mine races a McLaren M8F, yes at 150 mph on the straight he can still not go full throttle...

[mickmate]

Hi Dominik, looks like you're in deep! A couple of options on relocating the pivot points for your upright. There are several different brands of ball joint available for race cars that are adjustable in length to shift your pivot points to desired height. If you need more info on them let me know, they're quite inexpensive. Once you have established dimensions that work, whether it be pivot points or a-arm length's, you can use your adjustable set up dimensions to make something more permanent if you wish. The adjustable set up can often be used as a jig to fab the permanent set up. Uprights (spindles) can also be custom made by outfits like www.stockcarproducts.com. Keep reading about it but don't drive yourself crazy! I am VERY interested in your project as I'm about ready to commit to rear outer pivot point locations on original spec rear suspension. Keep posting pics and talking about it. I'll get a few of my rear suspension in my gallery shortly.

[mickmate]

http://www.stockcarproducts.com./susp18a.htm
http://www.stockcarproducts.com./susp7.htm
These may be of some use to you.

[speed220mph]

Donomik:
Great, you have Fred's book. He covers everything . . . just a little hard to find it.

An M8F and it can't go full throttle? Sure isn't the suspension as the limiting factor unless there is a serious deflection. Wonder if it's ever been wrecked and a suspension point didn't end up in the right place?

About location of the rc below ground, notice from the drawings that the rc can't go below ground if the lower arm is parallel to the ground. Of course this changes with ride height. Another thought: It's best to keep the control arms as long as possible to limit this change. Notice the typical F1 suspension. This limits camber change with wheel travel, particularly important with wide tires.

The reason suspensions don't (shouldn't) have an rc below ground is simple, the car would roll too much. The idea is to end up with an rc pretty far away as possible and at about two to four inches above ground with the front one being lower.

[Dominik]

mickmate,

yes, I am building the 427 chassis. I also have drawings for some missing details.
Although I do not know the length of the front upright...
And I try local uprights for little money before I copy original parts. My a-arms are quite "straight-forward". Round tubing with rod-ends.

thanks for the suspension links and your input!
as mentioned before, it's quite a nuisance to follow up with a 33k dial-up connection. So pics posting and downloading takes my patience (and Nicole’s!) to the extreme. You may email me directly.
I like to make parts over here in cape town, because they are so much much more affordable than importing from the US. I will look into importing in spite of the cost later.

". Keep reading about it but don't drive yourself crazy!"
That is excactly the point! I just want to follow some basics, like Tom’s position for the roll centers.


Tom,
I was a little exaggerating with the M8F’s performence, but with about 550cui, 3.1" throttle plates and tires that have been used by the competition before(!), traction suffers J
He used to accelerate way behind the braking points of the english GT40s (including upshifting!) during free pactise at the Nurburgring.
Meaning: GT40 brakes, he flies past on the throttle, shifts into 5th, on the throttle for another second and brakes only then...

You said:
"notice from the drawings that the rc can't go below ground if the lower arm is parallel to the ground."

Please help me here:
Fred P drew a picture on page 36 with the lower a-arms level and the upper ones pointing down, resulting in a RC below ground.

"It would roll too much."
See, that is how little I know so far. I thought it rolls less when the RC is lower...

Thanks,

Dom

[speed220mph]

Dom:

The top drawing on page 36 is the way a suspension ought to be . . . forget the other two. The one illustrated at the bottom would be a disaster. Another good example is the top drawing, page 37. It illustrates how the rc height changes with pivot point changes. Notice that all show the rc is above ground when the inner pivot on the upper arm is lower than the outer. The closer to paralled the arms become, the farther away the ic is, resulting in a less severe camber curve.

Something I didn't mention before is the upper arm should always be a little shorter than the lower arm to provide some camber change.

You can read about this in Fred's book, but as the rc is lowered, the car has more tendency to roll because it increases the roll couple. Keep on reading and you'll see that the front and rear roll centers establish the roll axis, which is what the car rolls around, sort of like a hinge. See page 39--42 for an explanation. Read it over and over and over, then it may become clear.

[mickmate]

Hi Dom, according to Paved Track Technology the RC should be 2-3". I have some good detail and dimensions on the rear if you would like to trade secrets. My approach is very similar to yours although not quite as far along. I am fabricating the rear upright to my required geometry. Problem is same as you, what do you want/need for required geometry. There's alot of reverse engineering going on here with something that may not have worked that well to begin with. Still we'll have the look and feel of a 60's race car when we're done right. I'll be back in touch.
Cheers

[Dominik]

mick,
I spoke to a local guy with a lot of knowledge from the 427 chassis.
He pointed out, that copying the rear suspension layout will give the best traction out of corners and on the drag strip, but not cornering speed.

I should have used a rear suspension layout similar to the SPF or my car that just left. I can always do that for one of the next chassis.

I will defenitely not run from change to change before finishing the first car. Sort of like building a Harley and trying to catch up with the latest trend and actually never finishing one! ;-)

Today a locally made upright from another race car landed on my table. Now I am tempted to make that one fit...

I should aim for design-freeze now and live with it. :-)

Dom