Rear End for Drag Racing
 

I want to beef up my rear end for drag racing.. I'm pushing about pushing about 460hp out of my SBC and I plan on spraying it with no more than a 100hp.. My Trans is a TCI th400 with 3000 stall.. I just ordered some 3" halfshafts, which come with solid dana spicer u-joints, tube yokes and flanges .. The entire rear end is pretty much stock with the exception of a fiberglass mono leaf, which came in the car when I bought it.. I have no idea what rate the spring is but I do know the rearend sinks pretty good when I launch the car, even though it still has gas shocks in the rear.. So I'm hoping the addition of some QA1 shocks and front drag springs will give me maximum weight transfer and traction, especially since allot of my racing will be done on places such as airport runways..... My plans are to go with a rear gear in the 3.90 to 4.11 range with 28" tall tires and I need to budget this as much as possible so if someone may have the additional performance parts I would need, please let me know (especially the rear end).. What else I should consider/replace?

As always, thanks!!

I have been looking into this also....You can definetly spend as much money as you want beefing up the ass end of these things. I would like to find out how much of this stuff is REALLY needed for weekend warrior duty with an automatic. I figure at a minimum they need 3" thick walled half shafts, solid u-joints through out and probably a 3" steel drive shaft. I am sure the 30 spline inner/outer axles are a good idea also...I am wondering if I can get by with the stock vette rear end guts though...(ie. ring/posi/caps etc)..

This should be a good thread...

:lurk:

http://www.tomsdifferentials.com/cat23.jpg

$1988 for the bad a$$ axle kit... I am more worried about slinging half shafts then I am about tearing up a ring gear or a posi unit....:ack:

Me too.. I have the stock 3.08 posi and have been hooking hard and still havent broke anything yet so like you, I am just looking for what I need to replace minimum.. If I dump another 2 grand into the rear end my better 1/2 will come unglued..

See, my problem is, I don't want to R&R the 3rd member yet until I decide what gears I need to run. I have 3.08s and I am hoping to keep them. If it turns out to be a dog with em, then I will go to 3.55s but....I would like to keep the 3.08s for decent hwy driving as I now WON'T be going to an OD tranny....

I am thinking I am gonna just replace the outer axles with the 30 spline units with the flanges, get some 3 1/2" half shafts with the flanged yokes on both sides and the 1350 solid u-joints. I will probably just install some 17 spline inner axles with the flanges so I can run the 1350 u-joint/flanged setup, yet keep my stock posi unit.(for the 17 spline spider gears.) I am hoping my 3rd member will hold up as its in pretty good shape internally. I know most go to the 12 bolt guts but damn, thats $1800+$1988 for the 1350 axle kit+$1000 for the trailing arm R&R...thats over $5k not counting the spring/struts/loops or anything....It would be easy to dump $6-7k in the ass end of these things.:crazy:

I wonder how many people actually break the 17 spline inner axles? I can see the outer axles giving up, especially if there is any wheel hop but.... This will definetly be down the road a bit for me.:ack:

From our experience (the Houston gang) the first weak point is the outer axles. And just keep slowly going inward from there. The last thing to worry about would be the driveshaft.

One thing that really hurts u-joints is the excessive squatting. Stiffen the rear spring and put in some longer snubbers and that's the cheapest thing you can do before replacing any axles or rear end parts. I've got a 410lb spring with QA1's and some long snubbers I found at AutoZone. Fully compressed, my half shafts are pretty much horizontal to the ground.

As others said, stiffening up the spring rate will help so prevent breakage.. As you are running automatic, I would not worry too much about breakage. I used to break posi-cases (where the pin goes through the case) with 465 HP (when I had my 383 Stroker about 7 years ago). However, that was with slicks and stick shift.. With automatic, there's a lot less shock on the rear and if the rear spring rate is stiff enough. Less rear end travel will definetely help to prevent breakage. If you have fresh rear end components, a stiff rear spring solid spicer u-joints, you should be good to about 550 HP without going through parts.. Of course, nothing lasts forever and eventually, you will break parts but I'm sure that if everything is properly set up, you can get hundreds of passes out of the original components as long as you run automatic without transbrake.. It's not so much power that breaks things.. it's shock! For example, I'm launching at 5000 RPM with the transbrake with nitrous coming in immediately.. If I'd put a stock rear end in my car, it would grenade the first time that i'll let go of the transbrake button..
I'd say that you should go with about 400 - 420 lbs/inch rear spring rate (preferably composite spring to safe weight) and QA1 shocks, set to a very tight setting when racing.. As you will lose weight transfer in the rear, you will have to add weight transfer in front. This can be done through soft front springs (either '65 - '68 no A/C front springs (the originals and NOT the reproductions). The original front springs had about a 280 lbs/inch spring rate while all the reproductions have about 500 lbs/inch spring rate. You can easily tell if it's a original or a repro as the original springs are longer with thinner coils. If you can't find those springs, go with MOROSO Trick front springs. The 213 lbs/inch springs work on Vettes but about 1.5 coils will have to be cut off for the identical ride height of the original springs. In my case, 2 coils were cut off which lowered the front somewhat. Cutting coils also adds to spring rate.. With 2 coils cut off, my front springs went to approx. 250 lbs/inch. You also want to go with adjustable shocks and set them up to have minimum resistence for extention and maximum resistence for compression. That way you allow the front to come up easy to get weight transfer and to come down slowly so that the front won't bounce...

Excellent info guys, thank you!!
Is there anyway to tell what spring rate my composite rear mono leaf is because I cant find a tag on it..

Just got my new front runners installed and my rear tires are tucked under the wells :thumbs:

http://img.photobucket.com/albums/v3.../DSCF00025.jpg

QUESTION??

I UNDERSTAND THAT THE PURPOSE OF USING THE STIFF REAR SPRING IS TO ELIMINATE REAR SQUAT. WHAT PURPOSE DOES "ELIMINATING REAR SQUAT" SERVE? IS IT JUST TO GET ALL OF THE UNIVERSALS/HALFSHAFT ANGLE LINED UP, OR ARE THERE OTHER REASONS?

IN OTHER WORDS, IF THE REAR HALFSHAFT ANGLES ARE STRAIGHT UNDER POWER (SQUAT) WILL THIS SERVE THE SAME PURPOSE AS INSTALLING A STIFF REAR SPRING?

JUST TRYING TO GET IT STRAIGHT IN MY HEAD. :thumbs: LOL

THANKS

:cheers:

BILL SR

:lurk:

This kit will help stabilize your half shaft angles.

http://www.dragvette.com/basid%20drag%20kit.html

My diff case started leaking and I discovered all the bolts but one had sheared off. I was one bolt away from splattering my diff all over the track! I had all the bolts holding the diff together replaced, installed the basic drag kit, 1/2 shaft safety loops and upgraded U joint caps and U-joints. I make about 450 hp and run 1.6 sec 60 fts. So far so good. :cheers:

Eliminating (or at least reducing) rear squat, and thereby keeping the halfshafts straighter under power reduces the strain (and consequent breaking) of the halfshaft U-joints because of the severe angle incurred.
Stiffer shocks are also needed to aid the leaf spring in this task.

Bump:d

Stiffer spring help plant tires better too. Our stuff works a little different since trailing arms really can't transmit motion like a 4 link or a regular leaf spring setup. As the rear of the car goes down the spring *pulls* downward by the end bolts which forces traction. The more of that motion you use up by squatting is just that much less force applied to the arms initially. Don't get me wrong...there are cars that 60' very well with a significant amount of squat....but it's wasted motion and hard on parts as mentioned. Some drag folks extend snubbers so the car rides out solid on them...but that is pretty harsh and not as consistent. It's especially hard with a stick car. If spring is weak and you get good traction you will get into a *porpoise* mode where the car loads and unloads pretty violently. I've been there and while wild to watch....it's not much fun to drive! When you really get traction you won't know which lane you're coming down in!

Mine uses a steel 5 leaf Daytona spring with an added thick main leaf for 6 total. Vette brakes made it for me. I use QA-1 adjustable shocks, stock trailing arms and no added traction bars etc. The front end is critical...you want some lift. I use Moroso springs just like Olivier and I have CE 90/10 front shocks. Otherwise it's just stock arms etc.
I have Tom's outer axles and custom made 3.5"x.134 wall halfshafts with 1480 series u-joints. My differential is a custom built Hemi Dana 60 made into an IRS configuration.

Alignment is important here too. Vette's go through some wild toe in issues on launch with lots of bump steer. The front ends gain positive camber on lift vs going to negative as Camaro's and most everything else does. Not bad...just need to set it up to make it drive OK.


JIM

The stock 10-bolt internals are not going to last in this application. The reduced shock loads will make a difference but over time the 10 bolt is going to break. You need a 12 bolt and all the parts or one of those rare Dana's Jim has. You can convert to a solid axle as well some have here, I like the IRS in a vette but everyone is different.

450hp, reduced shock loads, you may get by a while with a tuned 10 bolt, setup with 12 internals. It would cost less but the budget for either is well over $2k.

If I was running this I would have a 12 bolt, 3.5 shafts, 31 spline outers and the spring.

Note to those who think a 12 bolt is a bolt together job, think again. every part needs to be machine/hand fit. The last 12 bolt I built is now 1/2 way around the world.

Is the 460HP at the crank or wheel? Do you run slicks? If you pull the spring mounting plate under the diff, there will be series of numbers written on the spring. Look for K=... and that will be the rate (assuming the rear spring is the same width from one side to the other).

So gears dont typically break first? Would studding the case, main caps instead of bolts make any difference? What differential case the best/strongest to use? Would the Auburn Pro work in our rear?

I wont be pulling the spring just yet because I want to do the rearend work over the winter. Right now, its still streetable so I might as well get the last few rides in before the snow sets in but thank you for letting me know where the spring rate is located... The 460hp is at the crank but I do plan on spraying another 100hp. I plan on running slicks at the track too but also have some radials for the street (pictured above)..

GTR1999 & 427Hotrod
I was hoping I could get away with the 3" halfshafts because I already ordered them :lol:
My previous Camaro ran 10.80's but had a 9" Ford rearend with leafspring/slapper bar set-up and in that case, and like you mentioned, works completely different because the slapper bars push up on the frame and drive force down on the rear end.. If you watch a slapper bar set-up launch, the backend usually rises slightly, instead of sinking.. Pinion angle also played a big role in traction on a standard leafspring set-up like my Camaro, which ran at about 8 degrees but I'm totally new to these IRS suspensions and what they require for drag racing....

This is a great thread!!

Another option is to check with Duntov for their racing rear
http://www.duntovmotors.com/Differentials_63-79.htm

Here's a video of one in my car after I broke 3 stock diffs.

http://s19.photobucket.com/albums/b1...t=01c64a2c.pbr

HIGHHAT,

THANKS FOR RESPONDING.:thumbs: THIS IS A VERY INFORMATIVE THREAD.

SO WHAT SPRING AND SHOCK PACKAGE DO YOU HAVE UNDER YOUR VETTE? IS IT A STREET/STRIP SETUP OR STRIP ONLY, AND WOULD IT BE REASONABLE TO USE ON THE STREET?

THANKS

:cheers:

BILL SR