Will a 9" live axle Cobra w/ spool ever turn into corners?

750hp · #1 (**permalink**) 09-07-2002, 04:30 AM

Having spent the majority of my build budget on my engine, I'm left with less options with regards to finishing the car. Originally, I wanted to have a clutch-type traction lock rear for the street, detroit locker for the track and spool for the drag strip.

Well, the money's fast disappearing, and I already have a 31 spline spool centre ready to bolt in. My question is whether the suspension could be dialled in to enable the car to handle on a roadrace track with the spool rear end. I'm not concerned about using the spool on the street.

My first thought is that I won't be able to turn in, with massive push. Any thoughts otherwise?

Thanks,
Craig

RACER X #99 · #2 (**permalink**) 09-07-2002, 05:02 AM

Craig the locked rear will work just fine in you Cobra on the track provided all your track activities are on a DIRT track.

I have not heard of any Dirt track Cobras running lately so if you mean paved track, I would spring for the Detroit Locker.

Cranky

Richard Hudgins · #3 (**permalink**) 09-07-2002, 12:38 PM

Craig,

Actually, a spool will work much better than you might think. We ran spools in F-5000, Can-Am, and production cars.

The only time that there was a problem was on very slow corners. And then the problem was exit understeer. Never had a problem getting the cars to point in.

It really comes down to which end of the car you like to drive. Some drivers drive the front, (Car feels very planted, no slide) some drivers drive the rear. (Car feels "on top", drive with throttle)

Give it a try, you can always buy a additional center section later.

cobrashoch · #4 (**permalink**) 09-07-2002, 01:29 PM

Craig - I believe RH has it right. But I also think you will most likley bust a axle where the splines meet the axle. If your axles are resplines I'm afraid you may be setting yourself up for a rapid forced turn. Even a hardened racing axle will only take the abuse there for a while. You can help reduce flex in this area on the axle by welding braces on the housing above the spline area to help. Also, the farther you brace out the better. I used two Moroso housing brace kits on my hotrod and the housing is rock solid.(they are cheap) I don't recomend that you road race that setup very long. Buy a cheap junkyard rear if you just have to race it, and live with the breakage. MUCH safer. P.S - A single track rear end takes less power to drive than locking rear ends.
cobrashock

Rick Parker · #5 (**permalink**) 09-07-2002, 02:59 PM

Just bite the bullit and get another center section. You'll be glad you did. No compromises in chassis setup or potential broken axles with dire results......mmmmmmm
You don't want to go there.

Rick

jdean · #6 (**permalink**) 09-07-2002, 04:03 PM

Why don't you try it for a while with just the live axle? There are folks who road race competitively with open rears all the time.

By tweaking the launch bias, some have luck at the drags as well, although not the ideal set up for consistency.

Until you can spring for a posi or locker of some type, I would think this is a safer idea than road racing a spool.

750hp · #7 (**permalink**) 09-08-2002, 04:37 AM

Thanks for the replies guys! I should have made myself a little clearer about what I'm planning on doing with this car. For most of the time I'll be out on the street, but I'll probably take the car to the roadrace track once a month, and I'll get one good drag racing timeslip before they exclude the car from further competition.

Richard - were you aware of any setup changes that helped the spool cars turn in (ackerman angle, camber, toe out?). The chassis is pretty basic and the driver could sure use the help that a driving school provides, but if the car enters corners in a predictable fashion then I'll be happy. As long as it doesn't understeer like a pig, or be totally tail-happy then I can cope with it.

Ron - I have an old 3800lb barge with big block. I've had a 31 spline full spool in that car for nearly 5 years now, and I've given it a pretty hard time over the years. It's currently only a high 11 second 1/4 mile car, but it gets thousands of street miles every year without any failures. Are there greater loads on the axle during roadracing than running 10 sec 1/4's on slicks? I'm using Moser 31s.

jdean - I can see your point about using a "one-legger" for starters, but my motor is making quite a bit of power and I think trying to get one tire to hook up 800hp could be a hopeless task.

Double Venom · #8 (**permalink**) 09-08-2002, 04:56 AM

750.
You know I had to jump in here. When all is said and done, and you've scared the hell our of yourself with that locker, (certainly not taking anything away from JBL, but I think it takes one heck of a driver to use a locker on an autocross/roadcourse!) spring for the Detroit, "Tru-Trak". It is the most amazing center section we have ever used.

On very slow and tight turns, intersection, parking lots, etc., you will get a slight clunk-clunk sound and a small kick in the butt. Pretty cool too! On the strip it is amazing! Instant lock-up, no side torquing, straight and bullitt proof!

We now use these as standard equipment in all our DV's.

One season: Three 9" Ford OEM custom built centers, ripping one 9" case in half, and two Auburns. Last three years, one Tru-Trak, (or is it Tru-Lok, I can't remember!) No problems! Should have bought it in the first place!

See you in June!

DV...Richard...I'm saving a place of honor for that first JBL to show up!

We'll have "TWO" very professional "autocross" tracks set up so you guys can show your stuff!

Richard Hudgins · #9 (**permalink**) 09-08-2002, 10:28 AM

DV,

I will have to agree that a spool will not work for autocross usage. However, for a high speed track such as RA, Willow, or Laguna Seca, they can be used fairly effectively.

A proper differential is superior to a spool in most all cases for road course work.

Unfortunately, all of the differentials that I have seen for a 9 inch rear as well as the 8.8 (Which is what is used in the JBL) have limitations as to setup and operation. (The exception is the McLaren electronic unit for the 8.8, but this is a very expensive unit and not a production item. This means they are build as bespoke units.)

The problem with all of the units (to my limited knowledge) is that they are not adjustable as to percentage slip in the deceleration mode. This is very critical in chassis setup, particularly in the case of a rearward weight bias chassis. This slip percentage is very critical to braking and initial turn in.

Of course the acceleration percentage slip is also very important for mid corner and exit. Current racing differentials will allow for changes in these settings to properly set up the chassis. I certainly wish that someone would make a proper unit for the 8.8 that was affordable and available.

I hope that some of the JBL folks will make it to your "Spring Fling" next year. However, they would all get hammered on an autocross type track with the standard setup. (The rack ratio is way too slow for tight autocross type tracks. As well, the standard dampers would probably have too much rebound control and the chassis would be sluggish in the weight transfer times required by autocross type tracks.)

Craig,

The major differences in setup will surprise you. Note: this is only for slow speed corners, you only change the damper settings on the high shaft speed control. (Not the slow shaft speed, which affects the high speed corners.)

More rearward brake bias and softer bump settings front, lighter rebound rear. This allows the chassis to transfer and rotate into the corner. However, one must pick up the throttle very early (Prior to apex) to catch the rotation. Once you get used to this, it is a quick way around and fun to drive. (Much like a shifter Kart.)

750hp · #10 (**permalink**) 09-09-2002, 03:09 AM

Richard - very interesting! With the setup you mentioned, would you be inclined to set the roll bars very stiff or does a degree of body roll help the turn-in? I have a 125 shifter kart for a bit of weekend fun - if only the Cobra could corner and stop as well as the kart!!!

The track that I'll be using is perhaps the most basic track that's ever been designed from a clean sheet

It runs clockwise. I think I'll make up some ground on the rest of field down the two long straights

DV - I would like a locker style centre eventually. My concerns were threefold....

COST - too much more money spent on this car will eat into my DVSF3 funds!

STRENGTH - If I put some 30x12x15ish sized slicks out back for my one and only 1/4 mile run, how do you think the Detroit will fare?

STABILITY - you've mentioned how the locker kicks in fairly softly at parking speeds, but how do they react if you get on the throttle mid-corner (ie accelerating away from the apex)? Does the engagement of the Locker unload/unsettle the rear suspension?

Thanks guys!

Double Venom · #11 (**permalink**) 09-09-2002, 05:12 AM

Craig,
Remember this picture?

[img] http://pstr-m05.ygpweb.aol.com/data/...q7owRF0180.jpg[/img]

This was the results of a 5,200 RPM launch with slicks! The center section, which you can barely see is the "Tru-Lok" (still can't remember the right name!)

We removed the remeaining pieces of the housing, put it back into a new housing and back on the track! The Rep. from "Detroit" told me if I tore it up with our "PUNY" car he would replace it or refund the money!

On the track, I have never felt it unload. I know Gordan Levy runs a regular Detroit locker and uses the "unloading", as JBL says to kick the car around in the corners.

Geessh, you'd think I got a "cut" for everyone they sold! I'm just so impressed with this unit.

DV

cobrashoch · #12 (**permalink**) 09-09-2002, 07:34 AM

Craig - The answer to your question is yes! yes ! yes !
I too have ran spools on the street in heavy cars. ( Chyrsler Hemi, 4200 lbs ) Never had any problems there either.
But a racing car in twistys is another thing altogether. Remember, you are reversing the stresses and forces in the opposite direction every time you decelerate and brake. Then you get on it hard and everthing stresses in the oppsite direction again. On a drag car and mostly on street cars the forces are in one direction. That working back and forth action is similiar to working a peice of wire back and forth till it gets hot then brakes in two. You do have light weight going for you though, and suspension compliance(slop)built into a cobra that will help. In that respect the tighter the car, the harder it is on something. In this case it's the axles. So I still think you are asking for axle failure. That's were they seem to brake, that and the U joints. The closest race car here in the states that is a aprox. match for a Cobra is a asphalt midgets. Those guys are mad dogs and do stuff that we would never do on a street car but maybe a few of them would chime in here, as that's what they do.
DV's True Trak is the hot shoe so I have been told from "several" local racers for cars like a Cobra. A couple of those guys I really trust. I am using a auburn locker and have never had a failure using them. I did have ring gear wear with a stock set of 456 gears a few years back, but that went away when I switched over to a pro gear. Be cool, till Later,,,,,,,,,,,,,,,,,,,,,,,,
cobrashock

cobrashoch · #13 (**permalink**) 09-09-2002, 07:56 AM

Ed - Was that rear end housing a N case housing? From the ribbing it looks like it might be. I have never seen a N case housing fail that way. I have however seen stock small bearing housings do that though. Just curious.
Cobrashock

Richard Hudgins · #14 (**permalink**) 09-09-2002, 09:16 AM

Craig,

Your question on anti-roll bar rates is a bit more complex than it seems at first glance.

I personally prefer low spring rates and high anti-roll bar rates. I then use the dampers to control the high transient rates due to soft springs. (Note: this only applies to non-aero cars, like the cobra. Aero cars are a whole different beast.)

But, this only works if you have enough torsional stiffness in the chassis to eliminate flex. (Or minimize it to a acceptable range)

If you have a fairly flexible chassis, you will wish very high spring frequencies and damper rates. (Road course work, not drag stuff)

Anti-roll bar rates become a real guessing game in this configuration. The anti-roll bars will introduce considerable confusion as they will be cross loading the chassis and giving you some really goofy dynamics.

Also, the chassis is undamped, therefore this loading will be cyclical to the natural frequency of the chassis. (Just too confusing for me to set up this type of car, the sprint/midget car guys know how to do this and they make them work quite well. Of course they are only turning left.)

So. bottom line is:

If you have at least 2500>3000 lbs torsional rigidity, you can go with high rate bars. (This assumes 1 G cornering loads.) Anything less than the above, talk to your local sprint car guys.

750hp · #15 (**permalink**) 09-10-2002, 04:02 AM

Thanks for your replies!

DV - if your TruLok/TrakLok/TruTrak thingy (

) can come through a centre section failure unscathed, then I'm impressed with it's strength. I like the sound of it's soft engagement / disengagement. Thanks!

CS - Thanks for the heads up about the axles, but I thought the Pro gears were softer (less brittle) to soften the shock loading of the gear engagement when leaving the startline in drag racing applications. I thought it was this softer metal that made the Pro Gears wear at a faster rate than the street gears. Those midgets sure are fun to watch!!!

Richard - yet again I finish reading your posts convinced that I should have bought a JBL and converted it to right hand drive..... In hindsight I'd love to be able to sit back and stop worrying - safe in the knowledge that I had a well designed, proven chassis underneath me. That, sadly is not the case.
While I have not done any beaming/torsional testing on my chassis, let's assume that I have a chassis with no more structural integrity than your basic, average, boring old middle range Cobra replica. Based on that fact, what ( IN THEORY) would you think about having STIFF springs, middle range damper settings, and NO roll bars..... Would that then allow transfer and rotation, or do you still need at least a front roll bar for the sake of adjustment? If my questions are becoming too childish and basic, please tell me to stop wasting your time.... Jeez, I'm an amateur at chassis design

Richard Hudgins · #16 (**permalink**) 09-10-2002, 08:19 PM

Craig,

As I do not know anything about your chassis or the chassis geometry, some basics.

For a relatively flexible chassis you wish your suspension frequencies to be in the 100-120 cpm range. As I do not know your weight distribution or total weight, I will give a guess at wheel rates. (this assumes a distribution of 45/55)

Front: 135 lbs.
Rear: 165 lbs.

A Java wheel rate calculator

Note: these are pretty high numbers, but might work.

Damper rates need to match the frequencies.

I other words, I really do not know how to set up a car of this type from the information you have given. I can only give you generalities.

Please speak to folks like DV, or some of your local Sprint car guys as they will know much more than I about this type of chassis.

Sorry that I could not help more......

Excaliber · #17 (**permalink**) 09-10-2002, 09:02 PM

I can't imagine running a car without an anti-roll bar, sway bar, what ever you want to call it. It's just seems so indispensable no matter what kind of set up you got. I'm using somewhat soft springs on the front and rely big time on the sway bar to transfer weight.

Richard, thanks for the great feed back on "handling" issues in general. I had to dig out my chassis book to hang with you!

Surprise! I'm changing my rear shocks, there to SLOW on the re-bound, as I had suspected.

DV,
I'm going for the Tru-Lok as soon as I figure out WHAT the heck it's REAL name is. I broke what ever "limited slip" unit I DID have and am now driving a "one legger" which is really killing my track times. I was totally embarrased last weekend, spinning one wheel all around the track! I NEED something stronger, so WHAT is it? Tru-Lok? Tru-Trak? or Tru-X? I think Ford made TWO units that had SIMILIAR names but were built WAY different!

Ernie

Double Venom · #18 (**permalink**) 09-11-2002, 03:50 AM

It's "Tru-Trac! I've been all over the web this morning trying to find the "home page" without any luck! Lots of information on the unit. I just can't find the stinkin' home page.

It does use helical gears and no clutches. When we got ours they only had them in the Ford 9". It now appears that they have made one for the 8.8 Oh-ya!

DV

750hp · #19 (**permalink**) 09-11-2002, 04:35 AM

Thanks Richard. Out of interest, have you ever made a RHD version of the JBL?

DV - I've looked at a hundred pages about the TruTrac, but it all seems to be "How to fit a TruTrac to your Bronco/Jeep/F250" I couldn't find any hard tech info about them. It's time for me to hone my search skills! I'll get back into it now...

Double Venom · #20 (**permalink**) 09-11-2002, 07:18 AM

750...
O'Damn & shame on me. I could go back and read everyting but, just call it lazy! Are you using a Ford 9"? Are you using 31 splined axles?

If so...Why don't I send you mine, you pay the shipping

and you can try it out for free! I'm not using it, see no immediate need for it, and I don't mind a bit. IF you break it, then, "what the heck!?"

I'm serious...

DV