[Fot] in search of

Thu Mar 20 17:33:33 MDT 2025

<< This pre-loaded spring setup does not result in "infinite rate" when reaching the travel limit or droop, but in extension or downward wheel travel, does result in 0 lbf change per 0 inches of deflection.   >>

Yes, that might be a better way of putting it.  This is the other side of the coin from hitting the bump stop.  As the axles droop, there is SOME amount of spring pressure pushing the tires into the track (or the shoulders of the tires, possibly, depending on how long your cable is).  But when you hit the end of the "leash", there is suddenly NO spring pressure.  I was wondering if that sudden change from some to none would upset the car at all.  Evidently not.

Rocky must be a very smooth driver  :-)

Scott (B.)

From: Fot <fot-bounces at autox.team.net> On Behalf Of Anthony Parker via Fot
Sent: Thursday, March 20, 2025 5:54 PM
To: FOT Triumph <fot at autox.team.net>
Subject: Re: [Fot] in search of

This is a very interesting setup and discussion.

Formula VEEs, especially the later "zero roll" designs have elastomeric pucks in the droop limiters to ease the shock on rebound. Might not be worth the complexity in the Spitfire.

I would love to see a back to back experiment between this setup and one with very short dampers which limited the droop to the same degree as the cable system we're discussing. I think a large question now is, with the static camber of this setup, whether jacking and tuck-under still prevail, or is cornering squat possible? If cornering squat is possible, then the cable and inside spring are exacerbating it, but this may be ok if the body rolls enough.

This pre-loaded spring setup does not result in "infinite rate" when reaching the travel limit or droop, but in extension or downward wheel travel, does result in 0 lbf change per 0 inches of deflection.  This, however, does not mean that the rear tire contact patch sees zero force from the ground.  Here, steady state breaking potential for the rear axle will be limited by contact patch shape/size and forward weight transfer. Certainly, droop limiting to zero degrees camber will result in the best contact patch and may reduce forward weight transfer because the C.G. may remain lower in this setup than one with the rear spring helping to lift the rear of the car.

________________________________
From: Fot <fot-bounces at autox.team.net<mailto:fot-bounces at autox.team.net>> on behalf of Barr, Scott via Fot <fot at autox.team.net<mailto:fot at autox.team.net>>
Sent: Thursday, March 20, 2025 5:12 PM
To: rocky spitfire4.com <rocky at spitfire4.com<mailto:rocky at spitfire4.com>>; JOE CURRY <spitlist at cox.net<mailto:spitlist at cox.net>>; FOT Triumph <fot at autox.team.net<mailto:fot at autox.team.net>>; ralph hansen <quikrx at yahoo.com<mailto:quikrx at yahoo.com>>
Subject: Re: [Fot] in search of

What I meant by "infinite spring rate" is, based on my understanding of the system you're describing, the condition when both rear axles droop and hit the end of the cable "leash".  As I understand it, you essentially had both rear axles hanging from a single cable that slid from side to side under cornering loads but, under braking, with both axles traveling downward at the same time, would be arrested by hitting the end of that cable "leash".  Am I understanding the setup correctly?  I would think that would be similar to having a leaf spring hit the bump stop and going from whatever the spring rate is to *no* spring, or "infinite spring rate".

I asked because I considered creating a similar system once for my Spit racer, as well as for my dad's swing axle GT6 racer, but wanted either to add a spring in the middle of the cable or to run the cable over a spring-loaded central pully of some sort - just to ease the transition from normal spring operation to full leash arrest.   Maybe that wouldn't be necessary?

Scott (B.)

From: rocky spitfire4.com <rocky at spitfire4.com<mailto:rocky at spitfire4.com>>
Sent: Thursday, March 20, 2025 3:05 PM
To: Barr, Scott <sbarr at McCarty-Law.com<mailto:sbarr at McCarty-Law.com>>; JOE CURRY <spitlist at cox.net<mailto:spitlist at cox.net>>; FOT Triumph <fot at autox.team.net<mailto:fot at autox.team.net>>; ralph hansen <quikrx at yahoo.com<mailto:quikrx at yahoo.com>>
Subject: RE: [Fot] in search of

Yes, it caused no trouble for me. Neither road racing nor autocrossing. Essentially no droop. And FWIW, on numerous occasions when other people have driven my car, they often commented on how well it handled. Very neutral. I never did the turnbuckle thing on mine

Originally it had a limited slip diff, but I finally wore that thing out. Replaced with a welded-up rear. It was even better, tighter. (But harder to push around in the paddock.) I usually had a 4:55, best for autocrossing and most MiDiv tracks. Originally it had a 4:11 but I haven't used that for decades. Borrowed a 3:89 to do the 2011 Runoffs at Road America.

Not sure what you mean by "infinite spring rate." The only spring is the transverse springs over the top of the diff (and I still had the stock spring!). When I went to do a straight-line stop, it stopped. In a straight line. It would trail brake nicely and predictably, with both tires on the ground. As long as I didn't run out of talent and screw up the braking, upsetting the balance.

--Rocky

From: Barr, Scott <sbarr at McCarty-Law.com<mailto:sbarr at McCarty-Law.com>>
Sent: Thursday, March 20, 2025 2:30 PM
To: rocky spitfire4.com <rocky at spitfire4.com<mailto:rocky at spitfire4.com>>; JOE CURRY <spitlist at cox.net<mailto:spitlist at cox.net>>; FOT Triumph <fot at autox.team.net<mailto:fot at autox.team.net>>; ralph hansen <quikrx at yahoo.com<mailto:quikrx at yahoo.com>>
Subject: RE: [Fot] in search of

It must work great or you wouldn't have used it so successfully for 40 years, but...when under straight line braking, when the droop get arrested by the cable, doesn't it go from whatever the spring's rate is instantly to infinite spring rate?  Am I understanding that correctly?  And that change caused no trouble for you?

Scott (B.)

From: Fot <fot-bounces at autox.team.net<mailto:fot-bounces at autox.team.net>> On Behalf Of rocky spitfire4.com via Fot
Sent: Thursday, March 20, 2025 12:51 PM
To: JOE CURRY <spitlist at cox.net<mailto:spitlist at cox.net>>; FOT Triumph <fot at autox.team.net<mailto:fot at autox.team.net>>; ralph hansen <quikrx at yahoo.com<mailto:quikrx at yahoo.com>>
Subject: Re: [Fot] in search of

Are we talking the camber compensator for the Spitfire that has a middle mounting bolted to the bottom three bolts of the differential?

The scariest moment I ever had in my 1964 Spitfire, back when it was my daily driver (ca. 1967), was when my diff locked up as I was passing a semi on I-35 in Kansas City. I barely got the car over to the shoulder without getting run over, then spent six hours on the roadside waiting for AAA to send me a tow truck! (That's when I decided not to renew my AAA membership, but I digress).

Turned out the middle mounting on that camber compensator had broken off the sections on the two end bolts because of the constant flexing on it, so only the middle bolt was holding it in place. The constant action on that one bolt eventually made the round bolt hole oval shaped, and all the diff fluid drained out. A dry diff really doesn't work all that well.

I drove the car without a CC for a few years, then gave it to my brother (who's a mechanic) to turn into a race car, He came up with a solution which I understand originated with Volkswagen owners to address the Beetle's swing-axle issues.

He tied a cable to the bottom shock mount, ran it up to a pulley attached to the top shock mount, then across to the same setup on the other side.

[A diagram of a shock bolt  AI-generated content may be incorrect.]

The biggest modification was to take that big bolt at the top of the shock, drill and tap it through the head to take a shouldered 1/4-inch bolt which becomes the hub for the pulley. Not shown, a simple retainer over the top of the pulley to prevent the cable from jumping off.

He did that in 1971. He raced the car for three years, then I got it back and raced and autocrossed it through 2011. In all that time, 40 years, I replaced the cable just once.

Since it was going on a race car, we jacked the rear suspension as high as we could (filled the trunk with heavy stuff while the car was on jackstands, then sat someone in the trunk while we attached the cable) which both lowered the car and stiffened the ride. I later bought another '64 Spit as a street car and did the same trick, but let the suspension droop more so it was softer for street use (I also didn't autocross that car). On that one I found a pulley that came with a shroud, didn't need the retainer.

It works as a camber compensator without that evil spring. The swing axle doesn't swing any more. Both rear wheels remain constant with each other.

I've given this design to several other drivers. One came up with a clever trick - he cut an opening in the diff tunnel under the gas tank (which also provides nice access to the bolts holding the spring to the top of the diff). Then he installed a turnbuckle in the cable at that location. Adjustable suspension! Cheap and easy. And IMHO safer.

-Rocky Entriken

From: Fot <fot-bounces at autox.team.net<mailto:fot-bounces at autox.team.net>> On Behalf Of JOE CURRY via Fot
Sent: Thursday, March 20, 2025 10:02 AM
To: FOT Triumph <fot at autox.team.net<mailto:fot at autox.team.net>>; ralph hansen <quikrx at yahoo.com<mailto:quikrx at yahoo.com>>
Subject: Re: [Fot] in search of

It has been that way for years.  It became impossible for me to market them at the price that I knew was affordable for most Spitfire owners.  Therein lies the rub!

Good luck.

Joe

On Thursday, March 20, 2025 at 07:48:44 AM MST, ralph hansen via Fot <fot at autox.team.net<mailto:fot at autox.team.net>> wrote:

hello everyone,

I'm looking for a leaf spring manufacturer to make up about 25 single leaf springs. Preferably close to Milwaukee but just about anywhere might work. Must do good work and be reliable. These springs would be for the TR camber compensators. Doing some shopping around since one I've used has doubled the cost in 5 years. Thanks in advance.

Cheers,

Ralph

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