racing diff questions

[dan farmer]

Buona sera-

Before the questions, let me give the context. I've got a '67 Giulia Super that's used for vintage racing, with a recently rebuilt 4.56:1 LSD out of a later Alfa. I'm still setting it up, and I find i get inside-rear wheel spin if I upset the car in a corner. If I make a semi-sudden steering correction, there's more body lean and with it comes wheel spin. As well, I'm finding 4.56:1 is too tall a ratio for the tracks I'm on, as I never use 5th gear.

Now for the questions:
1) If we open up the rear end and add some shims to the clutch stack, what break-free torque value should we aim for? How is this measured?
2) Is a 5.12:1 ring and pinion available for the LSD Alfa read end? Can it be found for less than a small fortune?

Thanks in advance-
-df

[George Willet]

Well Dan, let's open up the hornets nest on shimming the Alfa LSD.

There are a lot of tuners that have followed the Competition Advisory Service tech sheets put out by Alfa in the early '70s that showed how to shim the clutch pack to make the LSD act like a locked rear end. This seems to work right up to the point that the very expensive clutch discs wear and go right back to working as designed. Which is for the tapered shafts to work on the ramps designed in the LSD and cause the clutch pack to be compressed. This "locks" up the differential to about 45% (as noted in many Alfa tech sheets) and gives us a "Limited Slip Differential". Maybe the tires that were available then didn't have the cross section and the grip that we have today, and locking up the differential was needed but not allowed, so is that is the WHY of shimming the differential???

I for one would rather not wear out the discs, and concentrate on chassis set-up instead to give corner exit traction. Of course the fastest way around a corner is, according to Mark Donahue (read The Unfair Advantage) running a locked rear end. Having tried that in several chassis, and spent the track time to learn how to make it work, I came to the realization that I got too tired trying to push the damn race car around with a locked rear end just for a second (OK maybe a few tenths) of lap time.

So to get back to the easy on the back muscles LSD. On my GTVs I went through a painful learning process of spinning out at the worst of times with just a little too much throttle application on mid to corner exit, and finally got some help on chassis set-up from some competitors who didn't have that problem, and had a sense of humor (they were laughing at me for showing them the "line" at Portland, and spinning out on the first turn).

I was running the "recommended shock and spring" package, and it turned out that in my case the shock manufacturer had changed the valving but not the part number, so I was running rear shocks with too much rebound resistance causing the inside rear wheel to lift, and we all know that two tires corner better than one. Couple that with a BIG rear antiroll bar and I was "killing" the rear end to make the front end work better.

So to wrap this up before I wander farther from the point(s). Because the Supers have a higher center of gravity, I've found to leave the Super rear suspension and shocks as soft as possible, this inculdes no poly bushings, using the later trailing arms with the larger front stock bushings ('69 GTV and up?) and NO rear antiroll bar. I set up the front to be as stiff as I want it to be, depending on what the car is to be used for: street and track/full race car, and then adjust rear spring rates to give me the amount of oversteer/understeer that suits my driving style (or lack there-of. Also doing the corner weighting is important on the Super with its longer wheel base over the GTVs.

One of the Supers I built for an experienced driver, after a number of laps, was able to finally outcorner a well set-up GTA on corner exit with this type of set-up.

And after my running most of the differential ratios, the 4.55:1 isn't a bad set-up. Send me an email asking for me to send you a gear ratio comparison spread sheet, plug in the speeds you are running at corners at the tracks you run, and you might be supprised comparing corner exit speeds how the stock gear box with a 4.55 works out well against a 5.12.

Hope this helps, George. willet@q.com

[genericwood]

I agree with pretty much everything George wrote. But to further answer your original question, yes 5.12 R&P are available. Several months back someone was trying to sell a new set on eBay but couldn't get any bids at something like $700. New ones can be bought from Giken for more like $1500. If you aren't ever hitting 5th, you might as well be racing with a 4-speed transmission.

As far as "shimming" the LSD, I was finally able to get mine to work well when I quit trying to set it up with preload. It works better when it is shimmed to give the stack a little bit of free play so that the ramps can work. You DO want to add a clutch disc to each side of the stack, however. The additional friction surface will definitely help you get the power down. Of course George is right that getting the suspension right is also very important.

Dan, to further help with each of your questions, it would be helpful to know if you have a close-ratio transmission or a panhard/watts/sliding block in the rear.

Erik

[George Willet]

Well guys I did the RPM/speed caculations on my gear ratio caculator spread sheet for a couple of combinations. I am working on the assumption that Dan is running a 1600cc with a reasonable budget and shifting out at 6500 RPM.

Based on that, here is where the shift points would be With different gear combinations:

Stock gear box/4.55 diff, 6500 shift/speed:
2nd: 47mph; 3rd: 70mph; 4th: 95mph; I didn't include 5th because Dan doesnt get out of 4th.

Stock gear box/5.12 diff, 6500 shift/speed:
2nd: 44.5mph; 3rd: 65.5mph; 4th: 88.4mph; with .79 5th; 95mph @ 5500rpm

Close ratio/5.12 diff: 6500 shift/speed:
2nd: 48mph; 3rd: 70mph; 4th: 88mph; with .85 5th 95mph @ 6000rpm.


Gotta go to dinner... Quickly becomes a cost benifit ratio to spend more money for a fun car,.. TaTa, George

[dan farmer]

Sorry to take so long to reply, and thanks to two of the most experienced racers on the BB for sharing their knowledge. The car is a dedicated vintage race car. It's got a 1600 cc race motor with its best power from 5000-7500 rpm (We're working on improving the midrange.) I believe it's got a close-ratio gearbox, as 4000 rpm in 4th equals 5000 in 3rd (1.25:1) and 3400 in 5th(.85:1). I haven't yet figured out the 2nd-to-4th equivalent yet. The PO had started the conversion to a track car and told me it had a close-ratio box. There's no panhard rod or equivalent on the car (yet). I'm just getting into vintage racing with CSRG here in NorCal, and they're quite conservative about originality. I need to re-read my friend's copy of the 1967 SCCA GCR to see if a panhard rod would be legal. As a new member, I'm not in a good position to go bending the rules.

I think that some work to improve the midrange response of the motor will help me cope with 4.56 gears, as the conversion to 5.12 seems pretty expensive and the parts hard to find. I'll keep my eyes open, since i do want to use 5th gear someday. Letting the rear suspension work will help the traction issue, but I need a bigger front sway bar if I'm going to remove the rear one.

Setting up the suspension is the next phase of sorting through the car. Engine mechanical, brakes, and safety gear have been addressed. Carburetion and ignition set-up are underway right now. Soon, there won't be any excuses left for the car being slow.....

Further advice is much appreciated. Thank you both.

-df

-df

[andyb6]

I have a 5.12 rear end with a Giken LSD in it set up with a pin for either a sliding block or a watts link that is gathering dust in my garage.

I race in the midwest and this seems to be the best gearing with a close- ratio for anything smaller than a full race 1750.

IMO, your handleling in the rear needs to start with getting rid of the T bar and moving towards a Panhard, watts, or sliding block. In many cases you will not need a rear swaybar, especially if you are running Bias Ply tires.

Once you have it settled you will find your 4.56 gearing does not work with a 1600. I would look for a 5.12 and consider some of the higher (5.43 I think) ratios depending on how long your tracks are. I don't think you should be able to throttle steer a properly set up suspension with a 1600 on dry pavement with race compound tires and sorted suspension.

[George Willet]

Here are the numbers for a 7500 rpm engine with the more common gear combos:

7500 rpm shift, stock gear box/4.55 diff: 2nd/58mph; 3rd/85mph; 4th/115mph; .79 5th @ 6500rpm 125mph. This is about the top speed for a 1600cc on the tracks I've run, (no downhill straights).

7500rpm shift, CR gears/4.55 diff: 2nd/62.4mph; 3rd/115mph; 4th/115mph; .85 5th/ 7000rpm @ 125mph.

7500rpm shift, CR gears/5.12 diff: 2nd/55.4mph; 3rd/85.5 mph; 4th/112mph; .85 5th/120mph @ 7500rpm.

7500rpm shift, Ultra CR gears/4.56 diff: 1st/57mph; 2nd/78.4mph; 3rd/96mph; 4th/115mph; .85 5th/125mph @ 7000rpm, (and just to dream: 134mph @ 7500rpm).

The ultra close ratio gear box is the way to go (in my estimation) if the prize money warrants it, but they run over $4000.00. I ran one in a GTV with an all out 1640cc engine and it makes a difference over the other combos. But it was a 6 to 7800rpm engine in an 1885 pound car. And there is a lot of shifting to do, the driver needs to be on the top of his game.

As far as the 4.55/5.12/CR/stock gearbox combos: on 5000 to 7000 rpm 1570cc engines the driver makes more of a difference than the gear combo in my experience. I've changed gear boxes and differentials at the track during test days and the lap times were very close to the same.

Engine torque, suspension set up, tire compound and driver seat time makes much more of a difference over gearing.

All food for thought. George

[andyb6]

I completely agree that the most important thing to a faster race car is seat time. Good drivers are fast in anything and any condition.
Seat time isn't as expensive as car modifications, and it's a hell of a lot more fun per dollar spent!

[genericwood]

Quote:

Originally Posted by dan farmer Letting the rear suspension work will help the traction issue, but I need a bigger front sway bar if I'm going to remove the rear one.-df

[Andrew]

I have a spare 105 5.12 axle from a Giulia TI (not LSD) that will be surplus to my needs once I drive the TI and determine the gears in its diff are good. I'm just across the water Dan if this R&P can help you.

Rick Clemente and others may chime in, but I bet Panhard rods were not de riguer for Alfas in 1967. There were on-the-trailing-arm rear sway bars, and the GTA sliding block setup at the time, but I'd guess no Panhard rod.

Are you talking to Martin Lauber? He's in more or less the same CSRG boat as you.

Andrew

[dan farmer]

Well, it's been a long work week, but I've finally got some time to spend on the forum. Thank you all for your responses.

Andrew- I'd gladly take the 5.12 axle off your hands. I'll talk to Dan Marvin about fitting that r&p onto my diff and get in touch. Please consider it sold. As for legality of a panhard rod, I'm pretty sure it's not legal. However, I've seen at least one CSRG Super with one. Although that'll make a big improvement in rear grip, I think I'll wait till I'm no longer a newbie with the club. Good idea about talking to Martin. I'll have a chat.

Andy- While I agree that seat time is best, there are two factors that I will say in defense of my car modding disease. It's getting to the summer months when the closer tracks (Laguna and Sears) are harder to get into, plus I'm a mechanical engineer and i can't help it. Figuring out how to improve the car, as well as the driver, is a source of diversion and enjoyment.

Erik- I understand your point about losing front grip if I increase the front bar, and my concern is that I'll not have enough total roll resistance if I just drop the rear bar. I'll try it and see.

George- I'll be in touch to get your suspension thoughts. I imagined that I'd have time during the past work week, but that was a joke.

Thank you all again. i'm learning a lot.

-df

[Andrew]

For what it's worth, my mild track2000 GTV became real tail-happy (not in a good way) when I increased the front bar. I had stock front and rear bars, red Konis set 1/2 firm in front and full soft in back, and Rugh track/street springs. Car was very balanced. When I changed only the front bar to a larger Shankle bar, it becamse very tough to keep the back end from coming around. Went back to stock front bar and car is much more manageable and fun.

Dan, note that this 5.12 diff is for sale only/if I determine the diff in the car is good. Otherwise I'll be swapping it in. Will know within a couple months.

Andrew

[George Willet]

Quote:

Originally Posted by Andrew For what it's worth, my mild track2000 GTV became real tail-happy (not in a good way) when I increased the front bar. I had stock front and rear bars, red Konis set 1/2 firm in front and full soft in back, and Rugh track/street springs. Car was very balanced. When I changed only the front bar to a larger Shankle bar, it becamse very tough to keep the back end from coming around. Went back to stock front bar and car is much more manageable and fun. Dan, note that this 5.12 diff is for sale only/if I determine the diff in the car is good. Otherwise I'll be swapping it in. Will know within a couple months. Andrew

[genericwood]

Quote:

Originally Posted by Andrew When I changed only the front bar to a larger Shankle bar, it becamse very tough to keep the back end from coming around. Went back to stock front bar and car is much more manageable and fun. Andrew

[Andrew]

I'm no suspension expert, so I'm happy to be corrected/informed. Could be my car (and my driving) is not developed enough to be on a plateau or edge sufficient for the analysis/comparison to be useful.

Andrew