roll centers, rear uprights

[mickmate]

Hi Dom, thanks for the e-mail. I'm posting my reply on here until someone can tell me why I shouldn't, or better yet explain this cloak and dagger stuff about dimensions of an original. The owners handbook says the track on racing versions is 55 1/8". Every single other piece of literature I've seen on these says track is 56" my conclusion, one is the track and the other is flange to flange. My rear upper arms are 11.5" long and my rear lowers are 12.5" long. The uprights (that are partially done) are 12.4375" height with 5.5625" above CL. My rear shocks (Probably QA1 DR5855B) will travel from 12 to 16'' in length. You can play with simple CAD stuff to try your geometry changes on paper first figuring stuff like camber changes and RC's. I have it layed out on a bench top full size with wire arms to length and can see it work without getting into any high tech CAD stuff for modelling it. It sounds like your front RC is still too low but what the heck do I know! If a car with foot wide tires can't get around a corner real well there's definetely something wrong. From what I've heard from guys that toss them around it's all about the nose heavy front end geometry with minimal adjustments. You got me inspired so I'm going to do some figuring on it right now. We'll cipher more.......
Cheers

[Dominik]

Okay,
let’s "roll”:

I went into overdrive now, borrowing some computer time at the local Universtiy:

the rear 427 upright is 6.73” from center up and 5.62” down (total 12.35”)
the rear lower a-arm is 12.4” long, the upper 11.4”.
Roll center for the rear: a "lousy" 2.2”!
Track rear: 55 1/2”

the front lower is 10.9”
the front upper is about 8,62” – that is an estimate.
I have no value for the front upright, I estimate 11.18”.
It appears to be adjustable, looking at the manual and pictures.

With the estimated values Roll center front:
4.5” at ride height.
3.6” mm at 1” bump
2.8” at 2” bump.
5.43” at 1” droop
6.5” at 2" mm droop.
Ground clearance 4.72”.
Track changes by 1/2” in both directions in bump or droop.
Roll center moves about 10” sideways.
Track front: 54 1/4”

So our 427 roll-centers are not nearly where we want them...

My brain is boiling, I will introduce new feartures in suspension behaviour as "runnning changes” and stick to what I have for now.


Cheers,

Dom, Cape Town

[mickmate]

Hey Dom you crazy number cruncher. I like the look of your numbers except I believe you have the rear upright upside down from the axle CL. We are within a few thou of each other which is encouraging. That rear RC looks like a good height according to what I'm reading. The rear shock is a crazy angle, around 28 deg which explains the drag race and acceleration out of corners performance you mention. The front upright is a funky thing, you can scale drawings all day long and not know where that top ball joint finishes up. Your front upper arm length looks a little short but I'm avoiding the steering end till I've built my rear and my confidence to tackle the front. I would suggest lowering that front RC which is why I suggested the adjustable height balljoint so you can play with its position. I can smell the smoke coming out of your ears at the moment. Get back to it!

Cheers

[Dominik]

hi mick,

the drag race traction out of corners should also be related to the anti-squat under acceleration.
Paul V. Valkenb.. pg 21:
Anti-squat in the rear is doubly beneficial, in that the desired tire patch movement is rearward, which absorbs bumps. An excess, however, can cause wheel chatter under hard acceleration.

Yes, I listed the values upside down for the rear to test you.. ;-)
Sorry, I had the drawing inverted.

The roll center though is based on the correct values.

rock 'n roll,

dom

[speed220mph]

dom

Don't forget the most important aspect to rear wheel travel--bump steer--little or no toe-in in bump and absolutely no toe out. Also, keep camber gain to less than 1/2 degree negative per inch of bump travel in the rear.

Anti squat: Keep it to a minimum and you'll be ok. You can't get the tire patch to move back enough unless you have a 12 inches of bump travel. As you said, this can cause wheel chatter, the thing that breaks drivelines.

[Dominik]

tom,

good points!

the next chassis I will redesign the rear to get a similar set-up as on the 289, but with coil-overs.


the real challange building a 427 (or any car for that matter) is making the trunk and passenger compartment... - especially if you do not have any measurements.

whoever out there is planning to build a cobra with homemade chassis, I can only recommend using somebody elses existing body - with inner tubs!

mick,

with that upper a-arm length the 427 would have 11.7 degrees kingpin inlination
if it was 10.12" long instead of 8.62" the angle would be a reasonable 8 deg.

my own is longer to give me 7 degrees which is built into the upright I use..

dom

[mickmate]

Hi Dom, you've certainly provoked some interesting dicussion on the subject and I see it's had plenty of views. I'll certainly bear some of these good points in mind putting my front end together. I guess 10 or 11 deg is a common number on the uprights. Hey another thing I remembered was seeing spindles that unbolt from the bottom of a strut and a bracket bolted in its place to locate the upper balljoint wherever you want to.

[muz]

Hi guys,
Here is some specs on coil sprung chassis.
Front .camber=0
caster =3*
kingpin inclination=11*
toe in =1/16
no of coils =10
coil dia =4.1/2''
wire dia =0.5625
spring length free =11''
spring length fitted=10''

Rear camber =2*neg
no coils =8
coil dia =4.1/2
wire dia =0.5625''
spring free length =11''
spring fitted length=10''

Ground clearance =6.1/2''
Track front =55''
rear =54''
Tyre size =185x15


185x15 uck. 289 only got 6'' rims, hence different track
S.C. 7.1/2'' all round. Some were lucky to get 9.1/2''
Competition early 6.1/2'' and 8.1/2''
late 7.1/2'' and 9.1/2''
Cobra wheels have 3.1/2'' backspace I am told.?
Rear camber neg 2 degrees makes it a little harder to calculate hub to hub spacing.Allow for hats and spinner adaptors if required.
Front camber=0, K.P.I.=11 degrees. If you have bottom arm sorted draw a line up at 11*.Top arm angle to obtain desiered R.C. ,lntersection is top ball joint.
This K.P.I. line should meet the ground level line at track width, wheel centre for 0 scrub radius.
Cheers
muz.

[Dominik]

mick,

I never checked the views... J

The rest of the week I use to clear my brain by working on the engine.
The spinning assembly is finished, on comes the oil-pan – not.
It’s a deeper sump, I had to extend the pick-up first. On goes the pan, off it comes again. It needs a drain plug...
These are minor headaches compared to the whole project.

The lowered sump now positions the oil pan 3.54” (90 mm) above ground with the chassis featuring 5” (127 mm) ground clearance.
BTW, the crank centerline (FE) I positioned 12.6” (320 mm) above ground – measured at the parting line main cap to block.

That’s quite low, especially for the bellhousing. It’s almost touching the chassis rails with a 2” high transmmission mount.
I use solid mounts to keep things from moving and to add some torsional stiffness to the chassis.

I recommend to move the motor 1” up for more clearance, which of course is detrimental for the center of gravity – whereever that is located!
I got values from 12.6” to 14.1” (360 mm) for crank centerline to ground on variuous Cobras with FE engines, measured at the pulley bolt.

That is why I recommended elsewhere in the forum to buy the Daytona body/chassis kit which was offered for USD 5000,00 intead of building your own. One may save USD 1000,00 by do-it-yourself – IF nothing goes wrong and you have all the equipment!

I wish I had better equipment and more space...

hey muz,

that is a good one. I rush out now to compare to what I got!
womder what is new in cape town as the Shelby SASA plant...

Dominik

If we are here to help others, they are here to get our rusted brains

[Ant]

This is an interesting subject as my roll centers are below the ground on the front and not high enough on the back, then there is all the other stuff to check out!

I would like to know how long the original "top and bottom arms" were on the front and rear of a 427 coil sprung car?

At present I have a front end similar to a 427 but the rear is T/Bird with a large bottom arm with rose joints for toe in/out and camber adjustments, but it has no anti-dive or anti-squat on either end of car!
My arms are:
Lower front 11.81"
Upper front 7.87""

Lower rear 12.20"
Uppear rear 7.67"

[What'saCobra?]

Dominik
Your friend's M8F is seriously out of wack or something.

I have many seat hours on the VDS Teddy Pillette M8F6-72 and you can be sure it takes full throttle in 3rd, 4th and 5th anywhere on the track. It has an 850hp 8.2 liter all alloy fuel-injected CBB, with a highly upgraded LG600, all modern CV joints and takes full power once the AVON tires are hot.

Where is he located, if you would? I can help him get it correct, if he wishes. That should not be happening unless something is quite wrong and in that car, anything wrong is not safe.

For instance, we had to remove the rev limiter for the last race at Daytona, even though we had the largest pinion drive ratio that would fit in the Hewland case. We were bumping the limiter half way down the front and back straights.

In the end, the last 5 or 6 laps saw 8250 rpm on that big block before braking and we recorded 215 MPH! That would never have happened unless we could use full power all the way from the bus stop exit on the back section to the front "straight" shut down point.

Of course, with the downforce that body generates, given the side air dams and all, we were firmly riding on the front bump stops. Pretty strange ride over the twin bumps formed by the twin tunnel infield entrances in the last turn just before the front straight. I kept thinking about how thin the tire carcases were, although new and how the bumps could throw the nose in the air an inch or two and lose underbody low pressure.

Of course, when the engine was disassembled for OH, we found considerable damage, but at least it didn't break on the last race of the season.

What a car and what an engine.

[mickmate]

Allright Dom, now you're getting some interest. Hey Ant take it from an Aucklander get your RC about 2-2.5" above the deck, as previously mentioned if it's too low it'll try and roll it over. The rear numbers we have posted look pretty close to coinciding with each other (fools seldom differ?) and Dom has thrown out some theoretical front ones. Muz some interesting numbers though the coils sound too large in diameter, where are you getting your numbers buddy? Track measurements are changed by offset not rim width as they are to the centre of the rim and tire. Dom when you plot the RC the back is easy as the pivot point is the rear (straight) leg of the a-arm. BUT What do you guys do at the front for RC calcs with an antidive angle between inner mounts. Do you take the height from the middle of them? Some good stuff lads keep thinking about it!
Cheers

[Dominik]

What’s a cobra:
Thanks for your input and offer to help!
Serious, Peter used some very old tires frequently for racing, because – well I do not really know. I cannot understand why one would race with old tires J But that was in the 90s. Things have improved.

Anyway, he bought the car (#02) in 1975 and raced it ever since in Europe.
It does handle and all the rest of it, I think he was joking more than beeing serious about the wheel spinning. BTW, he also put a Group-C body on one M8F to compete in Group-C in the 80s.

He ran the M8F before the Hockenheim-Ring was closed down for reconstruction for F1. More a show race, when DTM cars and "what have you" came to run.
Fastest speed that day was Peter’s 203 mph coming out of the forest, second fastest 20 mph less from a DTM. Other drivers asked him if he was crazy... J
We started a Can-Am thread a while ago, but I was batling soo much to access the forum and post pictures that it died away.
I lived in Miami in 1996.

Back to the topic:

Ant,
Plain a-arm length will not result in the same roll centers.
It also depends on where they are mounted in respect to the chassis centerline and wheel centerline. Check my values earlier in this thread.

To calculate roll center always make sure that you find the instaneous center by plotting the lines through the center of the balljoint and chassis mounts. Often the center of a ball joint is 1" or so off the centerline from the a-arm. Get a book or have someone fax you page 36 of Fred Puhn’s book. It’s easier to understand by looking at a picture.

Mick,
yes: we used the center of the anti-dive front upper a-arm as reference.
Same in the rear, where the upper arm is tilted for anti-squat.

For comparison I shall calculate the roll centers of that "handling wonder" which I raced 4 weeks ago and shipped to Europe now.
Front 5.1",
rear – 1" (below ground!) no anti-roll bar in the rear...
No anti-dive or squat.

Dom

[What'saCobra?]

Thanks for your comment, Dom.

Do you mean Hoffman?

[Dominik]

yes,
peter hoffman. I helped him sometimes before I moved to south-africa.

today I got the upright "box" from the laser cutting.
welding may start next week. as soon as it is rolling I take a pic outside the messy garage, without the body.

cheers,

dom

[What'saCobra?]

You are correct, he is certifyable, but very quick and subject to the dreaded red mist when he feels his position is threatened.

He has been banned a few times by ISS.

But, he is also a completely dedicated McLaren driver and does take his car to the limit and now, rarely over.

Nice to mee'cha.

[Ant]

Dom,

Thanks for that, we are going through the proper process of altering the chassis to suit my requirements. I have just purchases some different uprights with 2" raised stub axle this will give a nice roll centre on the front, the back is to low, but thats easy to change the lower innner pickup points.

Then onto bump steer, anti-dive and the rest of it!
While I was on the job I thought get rid of the pathetic T/Bird rear brakes and fit some nice 11.75" circuit use rotors and four pot billet Dynalites to the rear.
Problem no handbrake, but we can get an authority card here and have a hydraulic system. I am using 12.19 rotors and billet Superlite on the front, which will be just adequate due to 15" Trigos!

The book "Advanced Race Car Dynamics" by the Milliken brothers, in my opinion it is just to heavy for the average guy, and there must be far better books that can be digested by more simple folk!

[Dominik]

Steering

Ant,

good luck!
I develop a "street" version with locally available brakes for either 5-pin wheels or knock-off and will adapt the parts from a locally built race car for the "race" version, which will result in about three times the cost per "corner" (brakes, upright...)

If I knew then what I know now, I would not have duplicated the orig. 427 rear suspension setup. There is nothing wrong with it, but it is a pain to weld.
The idea from FORD was to get anti-squat, that the driver benefits under heavy acceleration. I wonder if they thought the same for racing, because here you don't have nearly as much wheel travel as on the street.

My steering arms (front) are 130 mm (5.1") long from the center of the lwr balljoint, but here is room for little variation. Just leave the steering rack brakets undrilled or move accordingly.

One should be able to correct bump-steer in that area. 427s had the 5/8" spacers under the rack to position it in the correct height. Must be a band-aid, the brackets could be higher to start with. Those spacer heigths can be changed to correct bump-steer when lowering the car.

I made sure bump steer is a minimum that the spacing between the tie rod end's knuckles is the same as the spacing between the a-arm chassis mounting points (24,5") 622,3 mm.

Not verified yet! Could be my weekend exercise...

I was afraid so that Milliken’s book is over the top for a regular. When I studied I never liked those engineering books. I learned more fom HP Books.

Handbrake:
My street brake if from a VW V6, a cable operating the piston. For serious braking I will use an upright with two calipers. Hydraulic handbrakes are not legal in Europe, in case I sell one to that side.

"What’s a":
a little biography from Peter in your mailbox.

Rock ‘n roll,
dom

[Ant]

Dom,

Regarding Ford building in anti-squat, as my car has none, and as it will have limited suspension travel - "is it that important to have?

The Milliken book is probably very good, I have started to browse it and in the suspension section there is heaps of stuff, and working out spring rates using the calculations, but nowadays I think there are websites with the enter your measurements and it calculates it for you, or a good eg Koni supplier, thats involved in motorsport can work it all out, when they do the shock valving etc!

[Dominik]

Ant,

I do notthinkanti-squatis a necessity. It appears that about "3177" or two more ;-) Cobra replicas have none and perform very well.

I wish there was a web-site with pictures of suspension used in sports cars.

Race cars rarely have anti-squat, because that only becomes effective with a lot of wheel travel.

I check a Superformance soon, but by looking at the picture of the rear suspension I think that they do not have anti-squat.

A typical XJ suspension does not have it, unless the differential is mounted in an angle anf the track rod allows for that movement.

Don't ask me how, I can only tell more onece mine is finished and I can check the camber and toe curve by moving the wheel (or better the chassis) through their travel.

Milliken's book is like I thought by what you tell. More for the Chassis Engineer at GM or FORD who is building a completely new car.
Good if one understands it, but easier to consult a field engineer for questions. Like KONI's.

Cheers,

Dom