Spring Calculations for Crossle

[BorkRacing]

I am setting up both a Zink z10 and a Crossle 55f. Having read the thread where Steve Lathrop made recommendations of 1000/900 for the Zink based on motion ratios, I measured the ratios on the Crossle. I came out with .50 at the front and .86 at the rear.

Using the formula: wheel rate =spring rate x MR x MR or spring rate = wheel rate / MR x MR and wheel rates of 235lbs for the front and 350lbs for the rear. I came up with spring rates of 940/472. This is far different than the recommended set up of 400/350 that came on the Crossle.

The above numbers were based on WR=Corner weight. If I correct WR=Corner weight-unsprung weight then I get 235-50=185 and 350-50=300 for the F and R. Plugging these into the equations still gives me Front SR=185/.5x.5=740 and Rear SR=300/.86x.86=405. Still 740/405 is not near the 400/350 recommended.

Any insight into the difference would be appreciated.

Robby

[rdracr]

I am setting up both a Zink z10 and a Crossle 55f. Having read the thread where Steve Lathrop made recommendations of 1000/900 for the Zink based on motion ratios, I measured the ratios on the Crossle. I came out with .50 at the front and .86 at the rear.

Using the formula: wheel rate =spring rate x MR x MR or spring rate = wheel rate / MR x MR and wheel rates of 235lbs for the front and 350lbs for the rear. I came up with spring rates of 940/472. This is far different than the recommended set up of 400/350 that came on the Crossle.

The above numbers were based on WR=Corner weight. If I correct WR=Corner weight-unsprung weight then I get 235-50=185 and 350-50=300 for the F and R. Plugging these into the equations still gives me Front SR=185/.5x.5=740 and Rear SR=300/.86x.86=405. Still 740/405 is not near the 400/350 recommended.

Any insight into the difference would be appreciated.

Robby

Start with the Crossle 50 has inboard front suspension using a pull rod which would make the spring rates much higner. The recommended rates you are using are for a 32/35.

[BorkRacing]

Can you explain how the spring rates would be higher with a pull rod suspension. Motion ratio is motion ratio regardless of how it is achieved, isn't it?

[DaveW]

Can you explain how the spring rates would be higher with a pull rod suspension. Motion ratio is motion ratio regardless of how it is achieved, isn't it?

That pull rod setup may not be linear and may have a rapidly increasing MR in compression. Rapidly increasing MR's are usually not good for grip.

Did you measure the MR at various positions?

[FVRacer21]

Just off the top of my head, the Crossle 32 had around 220 F and 180 R springs. (and old Apex Speed Thread I wrote said 180 F and 160 rear - might have been for the school cars.

When they got into Nationals they wend up around 300/240 , I have seen people talk about 400/340.

30 and 40 series cars had decreasing rate suspensions.

Now the 50 and 55 had slightly different rocker arm geometries - I seem to remember the springs in the 500/450 range as stock -

The problem is, back then when everyone was running, a consensus was arrived at. The problem with today's tires is that they are today's tires, not the mush softer sidewall tires of the time.

I think your motion ratios are off or someone modified them. I seem to remember figures of around .5 and .45, not .86

Again, all from memory, but the 50 series had rising rate so the angle of the rocker was below horizontal when at ride height..

Springs, shocks and swaybars all have to be matched for tuning - Call Continental Crossle or Porter and they should be able to give you more info.

Just an aside, when the SBRS school cars (B1) went to inboard front, we had cars go over curbs and the front shock spring "toggled over" easy to correct but looked really funny coming into the pits...

ChrisZ

[BorkRacing]

Just off the top of my head, the Crossle 32 had around 220 F and 180 R springs. (and old Apex Speed Thread I wrote said 180 F and 160 rear - might have been for the school cars.

When they got into Nationals they wend up around 300/240 , I have seen people talk about 400/340.

30 and 40 series cars had decreasing rate suspensions.

Now the 50 and 55 had slightly different rocker arm geometries - I seem to remember the springs in the 500/450 range as stock -

The problem is, back then when everyone was running, a consensus was arrived at. The problem with today's tires is that they are today's tires, not the mush softer sidewall tires of the time.

I think your motion ratios are off or someone modified them. I seem to remember figures of around .5 and .45, not .86

Again, all from memory, but the 50 series had rising rate so the angle of the rocker was below horizontal when at ride height..

Springs, shocks and swaybars all have to be matched for tuning - Call Continental Crossle or Porter and they should be able to give you more info.

Just an aside, when the SBRS school cars (B1) went to inboard front, we had cars go over curbs and the front shock spring "toggled over" easy to correct but looked really funny coming into the pits...

ChrisZ

Chris, the 50/55f I have has an upgraded suspension with newer attachments points for the A-arms so that at ride height the suspension is level.

MR's were take at ride height to 1", 1"-2" and 2"-3" compression as this is where most of the suspension activity "should" be. It does change after that greatly after that.

DanW, From what I learned working with Carroll Smith an increasing MR will give you decreasing WR's. This will make the car feel "easy to drive" and to push to the limits since it is changing slower the more you make it work. A good example of this is the 32/35/40 Crossle. It is a classic decreasing rate suspension.

I would think an increasing rate suspension would have traction issues, especially if it was greater than 20% at the front and 5% at the rear and even more troublesome on a rough track. So I would like to hear your view on this.

Robby

[FVRacer21]

Robby,

With the advent of shear plates, it made it very easy to switch from the profile that they used in the UK with treaded tires to change the profile to the slicks we have now. (change roll centers and camber curves)

Somewhere I have an analysis that was done when the 50F when it came out on the Mitchell software - I will have to look at it over the weekend.

My concern is that the pull rods are either too long in the back or too short in the front resulting in strange angles of the rockers. If you can post some pictures it might jog some brain cells - it was almost 35 years ago....

If the car was autocrossed, they might have set the front up very stiff for turn in and the rear softer so you did not get wheel-spin in tight turns. If you are going road racing (where the car belongs) then that might be a handful in fast turns.

Totally agree on decreasing wr being easier to drive - the car was designed to be a FF and FF2000 so with the advent of adding wings and inboard for obvious aerodynamic reasons, the rising rate was a compromise. Not currently being in Ford - maybe someone could chime in on what the current thinking is?

ChrisZ

[DaveW]

...DanW, From what I learned working with Carroll Smith an increasing MR will give you decreasing WR's. This will make the car feel "easy to drive" and to push to the limits since it is changing slower the more you make it work. A good example of this is the 32/35/40 Crossle. It is a classic decreasing rate suspension....

That depends on how you define MR. Some folks in the past defined it as (wheel motion)/(spring motion).

In my understanding, MR is now usually defined as (spring motion)/(wheel motion) which is the inverse of the above. If the spring moves less than the wheel, that is a MR less than 1.00. So with my definition, increasing MR during compression will give you increasing WR proportional to MR^2, which is not great.

It was my understanding that most pull-rod suspensions had, with my definition, increasing MR's.

[BorkRacing]

That depends on how you define MR. Some folks in the past defined it as (wheel motion)/(spring motion).

In my understanding, MR is now usually defined as (spring motion)/(wheel motion) which is the inverse of the above. If the spring moves less than the wheel, that is a MR less than 1.00. So with my definition, increasing MR during compression will give you increasing WR proportional to MR^2, which is not great.

It was my understanding that most pull-rod suspensions had, with my definition, increasing MR's.

The question still remains: If both the Z10 and 55f have similar motion ratios why do we run 1000/800 in the Zink and 400/350 in the Crossle?

[DaveW]

The question still remains: If both the Z10 and 55f have similar motion ratios why do we run 1000/800 in the Zink and 400/350 in the Crossle?

I don't really know why you would want so radically different rates, other than
1. IF you have a rapidly increasing wheel rate, the lower spring rates might make the car more drivable.
2. The chassis on the Crossle may be much less stiff than the Z10's and high WR's might make it subject to snap spins as the chassis unwinds when you hit a bump, etc.
3. Both of the above.

[BorkRacing]

I don't really know why you would want so radically different rates, other than
1. IF you have a rapidly increasing wheel rate, the lower spring rates might make the car more drivable.
2. The chassis on the Crossle may be much less stiff than the Z10's and high WR's might make it subject to snap spins as the chassis unwinds when you hit a bump, etc.
3. Both of the above.

1. good insight!
2. since both are side by side in the shop seems a torsion test is in the future. But I suspect that the frames are comparable.

In regards to why the cars have such radically varying rates may just be as simple as the Zink has Steve Lathrop continually improving and advising and the Crossle is just run with springs from "back in the day". It wasn't long ago that Zinks ran much softer springs, in fact many early FF's ran soft springs with frequencies near 100 compared to todays 180-200.

Bottom line is I was looking at the 50/55f and wondering if it would be better with higher rate springs. Also wondering if there was a SPECIFIC reason for the softer setup. Testing will be the only way to find out.

Robby

[Jayeff]

1. good insight!
2. since both are side by side in the shop seems a torsion test is in the future. But I suspect that the frames are comparable.

In regards to why the cars have such radically varying rates may just be as simple as the Zink has Steve Lathrop continually improving and advising and the Crossle is just run with springs from "back in the day". It wasn't long ago that Zinks ran much softer springs, in fact many early FF's ran soft springs with frequencies near 100 compared to todays 180-200.

Bottom line is I was looking at the 50/55f and wondering if it would be better with higher rate springs. Also wondering if there was a SPECIFIC reason for the softer setup. Testing will be the only way to find out.

Robby

Robby,
Did you ever do any testing on this question and if so what were the results? Curious minds want to know

John

[BorkRacing]

Robby,
Did you ever do any testing on this question and if so what were the results? Curious minds want to know

John

May 16th at Summit Point.

[Charles Warner]

Chris, the 50/55f I have has an upgraded suspension with newer attachments points for the A-arms so that at ride height the suspension is level.

This may be an issue as the suspension might have been designed with the rockers at a predetermined angle at ride height. From my experience with the Reynard atlantics the factory was adamant that the rockers be set at a certain angle as the suspension was variable rate throughout the curve. By altering the attachment points you might have buggered the curve.

Just a thought.

[BorkRacing]

The suspension upgrade was from Neil Porter/Crossle. I have run it through a suspension program and it puts the roll center just above ground in the front and 1" above ground at the rear. Rememeber these cars were designed for the British FF Series and tires.

[Charles Warner]

The variable rate of the suspension, and the starting point of that action, may be more influential than the location of the roll centers.

[tstrong]

Robby, not wanting to contradict Steve Lathrop as he has 1000 times more experience and knowledge on suspension design and set up than I ever will, but my 30 years of racing vintage and club fords on hard slicks (Am Rcr, etc) and vintage treads (Avon, Hoosier, Dunlop) has shown the WR=CW guidance to be much too stiff. I have no experience with Crossle 50 series cars but a fair amount of experience with Titan Mk 6, Mk9, and Crossle 30-40 series cars and myself as well as almost all the top West Coast drivers are running springs that are in the WR= 5.-.75 CW. For the 30 series Crossle that puts you much closer to the range of spring rates that you are hearing people quote. I can also attest to the fact that having driven cars sprung to use soft slicks (Hoosier,Avon) when switched over to Am Rcrs or vintage tires need much softer springs to work. That said if dealing with a Crossle 55 as mentioned in an earlier post (especially if modified by Porter) my recommendation would be to call him as he has seen and done it all with these cars and could easily give you recommendations of what they have found to work. Todd