CR Rear suspension mods

[400TT]

Do your best Mike. I was certainly looking forward to seeing your car in the flesh for the first time at this Shelby Fest.

[Aussie Mike]

I haven't given up yet. I just spoke to the guy building my wheels and he reckons they will be finished and ready to go today.

Cheers

[Tenrocca]

Mike - these results look fantastic, very exciting stuff for us CR guys. Cant wait to hear your driving impressions with the new setup.

Genius!

Cheers

[Philm]

Mike,
I read trhis post about 10 times this morning.

Absolutely fantastic.

When you finally produce the Murphy Kit Cobra, you have my order.

Phil

[boxhead]

Great to see Mike, I am another of the CR guys sitting and watching with Tenrocca.

[Morris]

Mike

Do you think there's a reason that control arms are Straight from the upper chassis pick up point to the upright????

And bump steer needs to be .015"thousands of an inch per inch of travel...... from ride height in a bump or droop situation.

And not on a piece of wood...... not flat and accurate......

Don't mean to bust your bubble.... but if you are going to do what you are doing .....get it accurate or you may hurt yourself.

The bump steer can be controlled by rotating the upright...... and your upper control arm would have me concerned if you have any power applied to the rear wheels and they have any grip.......

Just some thoughts....

Morris

[Aussie Mike]

Hi Morris, Thanks for the input. I apreciate your thoughts as you seem to have done a lot of research into car setup yourself.

I don't quite inderstand what you mean about the upper control arms? Unfortunately they have to have a curve in them to clear the upper chassis rails. The original cast steel Ford arms are curved also. I had thought of notching the chassis rails to run straight arms but I wanted a bolt in solution.

The majority of the suspension loads are taken through the lower arm and I think the upper arm will be more than strong enough. It's made from 1.5" x 0.120" wall 4130 tube. The only mod I've been thinking on is a bar between the 2 legs to complete the "A" but I'm not sure it's nescesary since the legs on the upper arm are quite short. As far as putting HP through it I think the brackets on the chassis would colapse before the upper arm bent.

I have a friend in the engineering department of one of the local universities and he's offered to do some finite element analysis calcs on the arms. I'll probably take him up on it.

The bump measurements I've done so far are just to get an idea of how it's going. I've got a sheet of 6mm aluminium plate that I've got to finish machining to replace the craft wood. I'm also putting together some revised uprights for the dial indicators that will let me raise and lower the whole stand. This will let me set the suspension at ride height and work back and forth from there.

The rear end problems I've been feeling on the track are now fairly evident from the numbers. When the local tyre shop does the rear wheel alignment he sets the rear toe to 2mm at ride height. If we take it that ride height is 1/2 of the suspension travel then at full droop the car will have about 18mm of toe out on the rear. When I lift on the main straight at Winton and shift back to 3rd for the left hand kink the rear end feels quite squirmy (that's a technical term ). I believe this is because as I decelerate and shift down the weight transfers to the front and subsequent lift of the rear means I have toe out. I stiffened up the damping on all the shocks and this settled down a bit but it's still there. The stiffer shocks reduce the weight transfer and keep the rear from lifting but its a band-aid solution. The real fix is to tune the bump steer out which is what I'm trying to do.

So from your reccomendation of 0.015" of bump per inch of travel I'm aiming for about 2.5mm of toe change over the whole suspension travel. Is that the maximum reccomended or minimum? should I try to tune it to less than that if possible?

I can see some argument for having a bit of bump in there. e.g: As the car pitches into the corner the outside rear wheel turns in slightly and helps turn the rear of the car. What do you think?

Cheers

[Morris]

Mike

When you start trying to push 700-800 Hp thru to the rear tires the upper control arm will come into play moving the car in a forward and aft drive.

So the side force along with the forward bite will put a strain on the upper control arm........ and the issue is flexing of the upper control arm..... which would change the geo of the rear suspension.... even thought bent control arms are used to get around frames and other componets ...... the line is still theoretically straight ...but the arm is going to bend more or less .....as opposed to a straight arm.....

And yes the .015"thousands per inch is max allow and if you can get it better it's always desired.

You are right on the mark with the rear end getting loose on you when hitting the brakes at the end of a straight .....as the geometry moves from a Toe-in condition to a Toe-out condition which is a wandering condition......

Toe-in is what you want on the rear end ..... you want the car to track like it should and not go from a Toe-in to toe-out and back condition as the back end will move around from side to side......and this has been a issue with these cars since they were made...... Many folks will set so much toe-in so they don't go into toe-out and then they have just scrubbed alot of speed.

Unstable suspension geometery will cause many ill-effects to the handling of a chassis.

Morris

[acecob]

are we there yet , are we going , how long, are we going to Winton,when is it gonna be ready, will it take long, should we start packing,

Just muck'n around.

So will u be there, lol
how close are ya.

[400TT]

Only 5 more sleeps to Shelby Fest. That's if Mike will actually be getting any sleep before then.

[stephen low]

Mike

Are your rims all Ali, or do they have bolt in steel centres?

What's normal for the good rims?

Cheers

[Aussie Mike]

Hi Slowy,

The rims are alloy. They are a 3 piece design. The 2 rim halves are spun from aluminium sheet. It's a neat machine to watch when they are being made.

The ring of bolts run through the 2 rim halves and the wheel center and clamp the whole thing together. Theres a bead of silicon sealer run round between the rim halves to seal everything. I also got my rim halves welded together for a bit of extra strength.

Cheers

[Rebel1]

Mike, do you have a phone contact for Ozwheels, Oz-wheels, whoever?

I can't seem to find the number.

Cheers

[Aussie Mike]

Hi Les,

The guy that owns Ozwheel now is a fellow named Peter. He seems to be producing a good product without the long delays I experienced with Tom.

Here's a link to his web page: Ozwheel Australia: citysearch

Cheers

[Aussie Mike]

Well I got back into the shed today. I decided I'd work on dialing in the rear bump steer and try a few experiments and see what resulted.

I made a new spread sheet that graphs the bump steer in relation to ride height. With the wheel bolted on I was able to work out what the aproximate ride height would be and where that would fit in to the total suspension travel.

I made a few educated guesses when I welded my new shock mounts in as I didn't have a wheel I could use to check the ride height. I wanted ride height to be about in the middle of the shock travel. I have 170mm of wheel travel and ride height came in at 90mm of travel. That's close enough for me. That gives me 80mm of compression before I hit the bump stops.

I figured that when the wheel alignment is done they set your toe adjustment at ride height. So all measurements need be referenced to ride height. In the graph below I've set toe to zero at ride height (90mm) and worked the bump figures back from there.



With the tweeks I've made to the setup there is virtually no bump steer from about 20mm below ride height and up. With the wheel at full droop there is only 5mm of toe out but with a couple of mm of rear toe added at the wheel alignement so I don't think this will be a problem. I don't reckon I'll be able to tune it any better without changing the points where the arms attach to the chassis.

For comparison I've included the graph for the stock Ford arms. I'm very happy with the improvement.

Cheers

[LoBelly]

that's a pretty startling improvement

have you done both sides? - there's an open practice at winton on good friday...

LoBelly

[Aussie Mike]

Nearly finished the passengers side but I won't be doing the track day. I'm just going to concentrate on getting everything sorted prior to the nationals. I've got a heap of other things I want to do. Front brakes, rear brakes, new fuel tank, roll bar etc.

Cheers

[Aussie Mike]

There was a bit of a discussion on one of the other threads about fitting aftermarket calipers. Part of my mods is upgrading the rear brakes so I thought I'd post a few pics of how to build an adaptor bracket for a radial mount caliper.

Radial mount calipers are much easier to adapt to a custom setup than a lug mount caliper. Start with a block of alloy about the right size (I used a 32mm square section). Mark out a center line on the block as all your measurments will be taken from here.

With the caliper sitting in place on the disc resting on its pads measure the distance from the mounting face on the caliper the center of the bolt holes in the the hub. Mark a line on the block at this distance. Now you can drill the holes in the block for the bolts that attach it to the hub. The spacing needs to be equal either side of the center line. You can measure the distance between the lugs off the old caliper as it's easier to measure on a hole that's not threaded.

Once you have the block bolted to the hub you can double check the pads are centered on the disc face. If they are out to far you can machine the face of the block down to get the pads in the right position.

Next you can get the caliper centered either side of the disc. Measure the width of the slot int the caliper. Mine are 31mm wide and my discs are 26mm thick. This gives me 2.5mm clearance either side of the disc. work out the distance from the inside edge of the caliper to the center of the mounting hole. Add your 2.5mm clearance and to this number and mark that hight on the block relative to the face of the disc. Now you can drill and tap the holes in the block where the caliper will bolt to. Work out the distance between the mounting holes (mine were 154mm) and mark that on your block in relation to the centerline. If the caliper mounting holes and holes to bolt to the backing plate are both referenced to the center line then the caliper should be concentric to the disc.

Here's a pic of my roughed out bracket. The caliper is mounted in the right position and everything is square. I've just got to machine off the excess material and make the bracket look pretty.





Cheers

[LoBelly]

Had to look up what a radial mount was..

..mmmmm.....brake porn

LasciviousBelly

[Aussie Mike]

Here's the finished bracket looking a bit sexier. Some time on the mill has cleaned it up and removed a lot of excess material that didn't really add any extra strength.







Here's the disc and caliper in place. I'll probably slot these discs to match the front. DBA didn't bring out the slotted versions till after I'd bought these







One of the things that can be a problem with fitting aftermarket discs to these rear ends is there isn't much clearance between the disk and the wheel hub. The calipers I ordered are the low profile version so I gained about 1/4" over the regular version. I also spent a fair bit of time working out how deep the boss needed to be on the back of the wheel and where clearance could be gained. Peter at Ozwheel did a great job finding clearance with the numbers I gave him. He couldn't make the wheel boss any deeper because of the pin drive hubs. He machined the backs of the spokes to gain the extra I needed. There's about 3 or 4mm clearance between the caliper and the spoke which is plenty.







Cheers