LSD, TBD, or nothing? Amateur Autox

jcsjcs · jcsjcs's Garage

Hey all -

I am an amateur auto-crosser - one season under my belt with pretty uninspiring times - but still having a blast.

I plan to auto-x for a year or two more - and maybe start on the track.

I am about to have my transmission rebuilt due to synchro ring issues and was considering LSD or TBD since the transmission will be open.

BUT does this really make sense for someone with my limited skill level considering it about doubles the price of the rebuild. Would I be better off spending the $ on other upgrades like better tires, suspension, etc.

Money is tight at this point - so any money for LSD / TBD is money not spent on other work.

Thanks - jcsjcs

fbarrett · fbarrett's Garage

jcsjcs:

Knowing nothing about your car (hp, weight, tires, etc.), I would still say that for autocrossing, tires and skill are the biggest factors, so I would suggest foregoing the limited-slip differential. Unless you have a surplus of power and have thus noticed the inside rear tire smoking in turns, you can spend your money more effectively. But then, what do I know--I've been autocrossing for 45 years without a limited-slip.

Frank

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by jcsjcs

Hey all -

I am an amateur auto-crosser - one season under my belt with pretty uninspiring times - but still having a blast.

I plan to auto-x for a year or two more - and maybe start on the track.

I am about to have my transmission rebuilt due to synchro ring issues and was considering LSD or TBD since the transmission will be open.

BUT does this really make sense for someone with my limited skill level considering it about doubles the price of the rebuild. Would I be better off spending the $ on other upgrades like better tires, suspension, etc.

Money is tight at this point - so any money for LSD / TBD is money not spent on other work.

Thanks - jcsjcs

Most A/X'ers prefer tbd, as that option doesn't cause under steer like a lsd will

HarryD · 04-01-2010, 09:19 AM

If you got money burning a hole in your pocket, spend it here: Home

crikett · crikett's Garage

Ideally, add the LSD! It does make a nice difference. For AX it may leave your car a little understeery, but I've never had an issue getting the car back to neutral through alignment and sway bar adjustments. And if you progress on to visiting the track, you'll be really happy you went this way.

That said, they are not cheap (back to that in a moment), and for AX alone and driving on the street, a TBD can be pretty attractive as you get to save some money and still realize most of the benefit for those situations.

And then there is just leaving it as is, and saving the money (or investing it elsewhere).

Being somewhat on the novice side, chances are there are many places you will gain more than upgrading the differential (tires, more aggressive alignment and corner balancing, anti-roll bars - all will gain more than you'll likely see from the diff at an AX). But, like all of those other areas, it does add something and allows you to expand what you can do as a driver a little further.

That said, for your current situation, you will not be saving that much in labor to have this done now rather than in the future (at least with the older 911s - not sure what you have), so it's strictly a matter of if you want to spend the money now and have the upgrade (again, other upgrades will get you more right now).

The LSD will become much more important if you start tracking the car, and also if you get it more stiffly sprung (TBDs seem to be much less effective as the spring rates go up). For the oloder cars with a stock or mostly stock engine, the factory ZF unit is a great option with the Guard units being the premium best of the best (with the Guard and Quaife TBDs coming in next... avoid the so-called 1.5 Way units that often pop up on eBay or through "import performance" outlets).

Shoepop · 04-01-2010, 12:43 PM

Quote:

Originally Posted by Bill Verburg

Most A/X'ers prefer tbd, as that option doesn't cause under steer like a lsd will

Bill is right on for auto X.

Matt Monson · 04-01-2010, 01:05 PM

Quote:

Originally Posted by crikett

That said, for your current situation, you will not be saving that much in labor to have this done now rather than in the future (at least with the older 911s - not sure what you have), so it's strictly a matter of if you want to spend the money now and have the upgrade (again, other upgrades will get you more right now).

The LSD will become much more important if you start tracking the car, and also if you get it more stiffly sprung (TBDs seem to be much less effective as the spring rates go up). For the oloder cars with a stock or mostly stock engine, the factory ZF unit is a great option with the Guard units being the premium best of the best (with the Guard and Quaife TBDs coming in next... avoid the so-called 1.5 Way units that often pop up on eBay or through "import performance" outlets).

See the bold. I agree with this. While it seems like a "while you're in there" sort of mod, it's in another part of the gearbox all together and is independent of the rest of the rebuilding work they are doing. You save very little by doing it now because the LSD can be installed by a good technician with the gearbox still in place in the car.

The comment about the TBD being less effective as spring rates go up is also often tied to running really stiff sway bars that lift a wheel. TBD's have zero lock up and behave just like an open differential if you get a wheel off the ground.

Aside from that, thank you for the compliments. We take great pride in our products and appreciate when you guys like what we do and share it with others.

jcsjcs,
If you are new to it all, spend some time learning the car and put some money into becoming a better driver. Then you'll figure out what you love about this car and track versus auto-x and be better suited to making an educated decision. Plus, you'll have spent some time at the track seeing other people's set ups and talking to them about their cars. There are so many racers who do not use the internet but have a lot of firsthand experience to share when you get involved with your local PCA regional events. Some of this stuff makes sense in print, but when you get a ridealong and feel what's going on cuopled with an explanation it becomes one of those "a-ha" moments where you get it.

Regards,

Matt Monson
Guard Transmission LLC

Flieger · Flieger's Garage

Perhaps you can help me,

I was interested to improve my understanding of the dynamics of gear-type torque-biasing differentials when I saw this thread on the Early 911S Registry. LSD for a '73 911? - Page 2 - Early 911S Registry Bulletin Board

I think I understand how a gear type TBD works but then I had this thought: Does not a traditional spider gear differential send more torque to the outside wheel in a turn, assuming the inside wheel is not spinning?

How can a differential sense traction? The inside wheel has less potential traction but uses less than the outside wheel. Therefore, it accelerates at a slower rate than the ring gear. The ring gear thusly drives the helical planets around the sun gear on that wheel. The spinning planets then drive the second set of planets which drive the outside sun faster.

With spider gears, the ring gear drives the spider gear around the "fixed" sun, causing the opposite wheel to be "levered" faster if one looks at a top view and sees the spider gear as a circular lever.

Where does torque biasing ratio come in that differentiates a TBD from an open diff?

As I visualize, the spiders still bias torque to the outsidde wheel, as the TBD does, if potential inside wheel traction is not exceeded.

Does a gear type TB-LSD actually turn the car like an active diff? I like the idea of rotating the car around curves with power.

Flieger · Flieger's Garage

emsdoc · 05-28-2010, 08:17 PM

I put in a TBD for my autocross car and it changed the handling dramatically..had to put a much bigger rear sway bar on and then would get a lot more front tire lift. The most noticeable change was that in the past with an open diff. I could slid through corners and then power out...prob not the fastest autox technique, but was a lot of fun! Can't do that with TBD...I guess it did it's job of more bite with the tires...still had more fun with the open diff though....

ix0ifan · 09:27 AM

Quote:

Does not a traditional spider gear differential send more torque to the outside wheel in a turn, assuming the inside wheel is not spinning?

Open differentials connect the drive axles using a bevel gear set which divides the torque equally between axles but cannot support any significant torque difference between the the axles and provide virtually no resistance to differential action.

As soon as you try to deliver more torque to one axle the result will be a rotation of the gear set

This means that if one wheel loses traction the attempt to deliver more torque to the wheel having the best traction will cause the wheel with poor traction to 'spin' up'.

In practice the best you can normally achieve with an open diff is a collective torque of approximately twice the amount of torque that can be supported by the drive wheel with the least traction.

The Quaife ATB uses a 'high' helix angle side and planet gears and it is the floating planet gears that are provide the differentiating capability and this s similar to an open diff and it will split torque equally to both wheels if the torque applied doesn't exceed the grip of the 'weaker' wheel.

In practice the torque flows from the ring gear to the diff housing and then to the planet gears which are located in their own cylindrical housings being driven by the flattened tips of the helical gears.

Torque is then transmitted to the side gears and hence the axles.

When torque is split equally the end thrust of the helical planet gears are balanced exactly by the opposing end thrust of the output side gears.

This allows the planet gears to float freely in their cylindrical chambers.

When one wheel has more traction than the other and that wheel starts to slip, the end thrusts of the helical gears will become unbalancedand the steep helical angle will force these gears towards the ends of the cylindrical chamber.

This will cause a wedging action which biases torque towards the wheel with the greatest amount of grip.

The torque bias is linear and proportional to the difference in traction and can only be varied by changing the helix angle.

A Quaife typically varies between 50/50 and 20/80.

The inside wheel does, however, need to remain on the ground and have at least 20% of the weight applied if there is to be maximum torque biasing and hence maximum thrust.

Flieger · Flieger's Garage

So the advantage of helical gear diff is that it reduces the required traction of the inside wheel from (for example) ~90% total to ~20% total so you can transfer weight faster with a stiffer rear anti-sway bar, thus allowing a quicker initial turn-in rotation. Is this correct?

ix0ifan · 06-01-2010, 12:23 AM

I believe that the idea of any LSD type device is to allow torque to be transmitted to the wheel with most traction and the ATB is just one type of device that achieves this.

The Quaife will allow a bias of around 4:1, which is better than a typical plate type diff but it doesn't work at all on coast.

78SCRSMAN · 78SCRSMAN's Garage

Anyone have experience using a spool in regaurds to AX? Track? Street?

gestalt1 · 06-01-2010, 04:47 AM

Quote:

Originally Posted by 78SCRSMAN

Anyone have experience using a spool in regaurds to AX? Track? Street?

i had a toyota tacoma with an electronic locking diff. i tried it on dry roads a couple times to see if a welded-up or spool is streetable. it is not. in a strait line it is obviously fine and the acceleration and braking were noticebly more stable. normal cornering on the street was very jumpy and that was with suv type tires, with sticky performance tires it would be worse.

Rob Channell · 06-02-2010, 05:19 PM

I have a Quiafe TBD and I like it for autocrossing as it helps the car rotate very easily. On faster tracks and through faster sweeping corners the entry into the corner is not as stable as I would like. When I still had the open diff I could trail brake (not a lot, but a little) into a fast corner. With the Quaife that is out of the question. It is just too difficult to modulate the throttle when the limited slip function only works under acceleration. The car feels too unsettled in the fast corner entry. I plan to swap over to a plate type eventually as I think I would like it better.

For comparison, I have a 911SC with 23/30mm torsion bars, 22mm F/R sway bars, custom valved Bilsteins and tires in either 235/255 or 245/275 widths. Car weight is a little under 2600 lbs without driver. I am a fairly average driver with a good bit of experience.

My opinion, FWIW.

Steve@Rennsport · 06-02-2010, 07:20 PM

Quote:

Originally Posted by 78SCRSMAN

Anyone have experience using a spool in regaurds to AX? Track? Street?

I have some experience with a spool in an early RSR and 935,....

These are VERY predictable for competition/racing, but they do generate ferocious understeer at low to medium speeds so they are totally inappropriate for anything besides racing.

For driving a high-powered car with the throttle, they are amazing.

IMHO, anything else should get an LSD for their sheer ability to stabilize the car under braking and corner entry, if for no other reason. It makes a very big difference.

Matt Monson · 06-03-2010, 07:32 AM

Quote:

Originally Posted by ix0ifan

The Quaife will allow a bias of around 4:1, which is better than a typical plate type diff but it doesn't work at all on coast.

Can you better explain this statement? How do you mean that it is better than a plate LSD in it's bias?

911st · 06-03-2010, 01:58 PM

I might caution that you be aware of the rules under which you might run. I thinke in SCCA and a stock class I suspect anything other that a stock LSD could bump you up a class or two. If your goal is to latter run PCA DE track events, an LSD adds a bunch of points to your car and can bump you into a group of faster cars. Just somthing to be aware of.

If you find you are spinning the inside rear wheel a bunch at AutoX, try stiffening up the front or front shocks to transfer traction to the inside rear tire. If that dose not do it and the guys you run against have LSD's, then concider a LSD of some type.

My vote is get the car done and use the money on tires, track fees, and get to know the guys you run against. Use this mod to your advantage latter to maybe move to another class where there is better or more competition.

Matt Monson · 06-04-2010, 05:44 AM

I cannot speak to SCCA, but in PCA a number of years back, the rules were deliberately rewritten to allow our LSD in stock classes. This applies to all classes A-L and does not include spec classes which have their own special requirements on a case by case basis. The wording is very careful:
Any limited slip differentials (LSDs) derived from a mechanical type that was delivered in a street-legal Porsche is allowed. No locked differentials will be permitted.

The key word being derived. They treat our LSD as an "evolution" of the stock ZF LSD.