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Old 11-11-2006, 10:19 AM   #22
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1997 Eagle Talon
(plus a Legacy beater)


Originally Posted by chimchimm5 View Post
So, from reading the links from the DSM forum, it sounds like the helical LSD will wear out faster than the clutch ones (generally speaking, as there are obviously many variables: clutch pressure, clutch material, helical teeth angle, etc)?
Helicals don't wear out that quickly. At least, modified clutch-packs wear out faster. The grinding against the end of the pocket might wear down the pinions in a helical, but there's plenty more. But a scuffed-up clutch is toast.

Originally Posted by chimchimm5 View Post
Are the clutch LSD's then the the only way to have an electroncally controlled LSD?
Yes. But keep in mind that electronic clutch-packs can use any of a variety of signals to increase the clamping force, while a Salisbury only uses input torque. Note, also, that Subaru and Mitsu (both using electronic center clutches) have developed very different approaches. Most off all, the Mitsu center is all about how quickly the center should return to open when no longer needed; the Subaru center is all about being locked in advance.

Originally Posted by chimchimm5 View Post
Is the VD chosen for regular cars (like the WRX) because it is the most economical/long lived LSD for an AWD system and not because it's a "best choice" for the general use application?
As you know, most of the top-end AWDs (and high-power FWDs) have switched from open fronts to helical fronts. In the rear, you don't often find helicals; rather, you find VCs and clutch-packs. My guess is that this comes down to cost. Assuming you aren't some slimey Asian company that rips off the designs of others ... OBX, cough, cough ... then adding a helical adds more than a $1000 to the cost of the car, while a clutch-pack or VC is a lot less.

In the center, you see that Subaru and Mitsu have taken different routes in recent years. Subaru (which has been producing planetaries for years, as in the VTD automatics), went with a planetary for the STi, while Mitsu stayed with a 50/50 spider. At the same time, Subaru stayed with relatively standard rears, while Mitsu created the AYC rear. Well, I think that the jury is in on the initial decisions, what with Subaru backing off the torque split, mostly because the people who bought the 35/65 version didn't seem able to use the gas pedal in any way other than an on/off switch. Whether the lower-split version will work out is yet to be seen. People who known how to drive are often putting 35/65s into their new STis. Several people have also put 35/65s into the older AWD-turbos, such as the 35/65 in Charles Moss' 2G DSM.

Originally Posted by chimchimm5 View Post
On a side note, why did Lotus feel that the Elise did not need an LSD and autox'ers who run the Elise think it does?
Autocrossing is weird. Rarely are cars asked to produce lots of lateral and longitudinal acceleration while moving slowly (and having tons of available wheel torque via gearing). If you aren't trying to turn sharply when on the gas, both rear wheels (esp. in a rear-engine car) will have enough grip to make an LSD unneeded. But asking the same car to power out of a tight turn might shift enough weight off the inside rear to make some sort of limited-slip device necessary. Also, keep in mind the rather low power of the Elise. If you modify the engine for more low torque, you'll need a limited-slip device of some sort. Until then, however, it's better to leave the diff open, as any and all forms of limited-slip cause some understeer and the key to the Elise is its nimbleness. Leave the wallowing power approach to the 'Vettes.

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Last edited by jtmcinder; 11-11-2006 at 10:26 AM.
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