Rick Staples writes;
>Brent,
> I have been a fan of the Torsen differential since it first appeared
some
>25 - 30 years ago as the Gleasman Dual-Drive differential. I agree that
>would be the ultimate differential on almost any vehicle, and would love to
>try one in my GMC. Just a couple of concerns.....
Sure;-)
> Is there physical room in the GMC/Toronado final drive to fit a slightly
>bulky Torsen diff.? It's a little tight in there, and this may be a
problem
>for the ARB unit too.
This remains to be seen- there isn't an awful lot of room for any sort of
different differential carrier, and of course the GMC also has a pretty big
crown gear. Torsens are relatively compact however, and can stand a lot of
torque within a smallish unit as they never get real shock loads.
One recent development is the manufacturers of these differentials seem to
be introducing more carriers lately, I know one was recentlt released for a
~8.25" gear in a modern GM differential like the Chevrolet Caprice police
cars you see use. There may be some way to adapt one for the GMC by mix and
match and perhaps a machined spacer etc, such as many aftermarket units use.
Also, considering the many different axle ratios currently in production for
the THM 425, there is likely a large enough market if perhaps 1/4 of people
who buy gears want limited slip to justify the tooling up for production of
a purpose built GMC/THM425/Toronado Carrier. All that might really be needed
is a unique carrier casting, holding standard Torsen parts within. Air
lockers would certainly have little market outside GMC, as the Automatic
Level Control on the Toro and Eldo are run by those old vacuum pump things
that would die attempting to engage, and theres no other air source.
> The other concern relates to the peculiarities of
>front wheel drive handling. One reason limitted-slip/locking differentils
>have seldom been used on fwd vehicles is that it can cause problems on
>slippery roads. Since the driving wheels are the steering wheels,
>manufacturers felt it was better to let ONE wheel spin and maintain
steering
>control with the other, rather than risk BOTH front wheels spinning,
causing
>the vehicle to slide right off the road.
Its even more than that- FWD car with connected axles are extraordinarily
hard to handle even on dry surfaces. When Toronado was introduced in 1966,
GM used the friction modifier additive Positrac cars use to rid the car of
the last bit of internal differential friction between the spider gears and
internal carrier parts. Essentialy, no FWD car uses any oher style limited
slip except for Torsen, and for good reason. Some Jeeps and trucks use
locking diffs, or clutch type positraction etc, but no OEM manufacter has
dared to. They really don't work on FWD. Bear in mind you are at the
steering wheel at one end, and a 455 with potentially about 3500 ft/lbs of
torque is at the other with between a 1/4" and 4" lever- makes handling
snaky indeed.
Both front wheels spinning isnt too bad in any event, they do that all the
time with open differentials;-) Independent suspension cars really gain
minimal benefit from limited slip devices. They already have equal torque to
two wheels carrying identical weight, and theres no unbalancing force like
driveshaft torque in a RWD car with a live axle to counter. The only
variable is the co-efficient of friction between the tires and road surface,
or occasionally unbalanced loading due to high centering.
The primary reason a Torsen style Limited slip will work in a FWD vehicle
where a clutch type or locker will not, is the Torsen is extremely
transparent in operation. To sum the differences, (badly) an OPEN carrier
sends equal torque to both front wheels 100% of the time. A Clutch type like
cars with Positraction/engaged lockers sends very unbalanced torque that
varies with the speed differences left and right by attempting to drive the
slow wheel with the fast one, and to a large extent, this is what a locker
also does; a Torsen style releives drive from the FAST moving wheel with
the slower moving one. Essentially the difference is an open one reacts at
the tire contact surfaces, whereas a Torsen does this reaction within the
carrier. Although an unlocked Locker would be just like stock to drive, when
engaged, it might be a real handful to say the least.
By varying the pitch of the worm gears in the carrier the Torsen can be made
more and less aggressive in its action, but the action is always VERY
progressive and linear. At rates of slippage encountered in normal driving,
it has no noticeable action, but it will never permit an unloaded wheel to
accelerate very much, and has won favour in FWD racing for this reason. You
may occasionaly see European Touring Car Racing, many cars used in this
class have Torsens, and the action is apparent on hard corners when a inner
front wheel is lifted clear of the Pavement- they dont start spinning wildly
in the air, and the car is just driven thru the corner by the outboard
wheel.
One especially nice feature of the Torsen also is they last a long time with
no deterioration in performance. The mechanical construction of the worm and
sectors means even if there was significant wear, the action would remain
consisitent. No seals, clutches etc to fail. The worm pitch is the point of
action, not the actual metal surfaces per se.
> One advantage of the ARB system is that when it is disengaged, there is
no
>drag on one wheel or the other. I'd like to try either on my GMC.
I am thinking another good possibility would be adaptation of an ABS to
brake a fast moving front wheel on one side to allow the other to drive the
coach. With the current availability of electronics, perhaps an automotive
system exists that could be adapted. Really, the basic issue is there just
isn't a sufficient proportion of the GMC's weight carried on the front
wheels for it EVER to have outstanding traction on slippery roads. However,
something like a Torsen likely would give the coaches a little advantage and
better wet and slippery road handling generally speaking, which is worth
something, and might make the difference in isolated cases. Personally, I
favour no modifications, and just driving on the crown of the road;-)
Brent Covey
Vancouver BC
>Brent,
> I have been a fan of the Torsen differential since it first appeared
some
>25 - 30 years ago as the Gleasman Dual-Drive differential. I agree that
>would be the ultimate differential on almost any vehicle, and would love to
>try one in my GMC. Just a couple of concerns.....
Sure;-)
> Is there physical room in the GMC/Toronado final drive to fit a slightly
>bulky Torsen diff.? It's a little tight in there, and this may be a
problem
>for the ARB unit too.
This remains to be seen- there isn't an awful lot of room for any sort of
different differential carrier, and of course the GMC also has a pretty big
crown gear. Torsens are relatively compact however, and can stand a lot of
torque within a smallish unit as they never get real shock loads.
One recent development is the manufacturers of these differentials seem to
be introducing more carriers lately, I know one was recentlt released for a
~8.25" gear in a modern GM differential like the Chevrolet Caprice police
cars you see use. There may be some way to adapt one for the GMC by mix and
match and perhaps a machined spacer etc, such as many aftermarket units use.
Also, considering the many different axle ratios currently in production for
the THM 425, there is likely a large enough market if perhaps 1/4 of people
who buy gears want limited slip to justify the tooling up for production of
a purpose built GMC/THM425/Toronado Carrier. All that might really be needed
is a unique carrier casting, holding standard Torsen parts within. Air
lockers would certainly have little market outside GMC, as the Automatic
Level Control on the Toro and Eldo are run by those old vacuum pump things
that would die attempting to engage, and theres no other air source.
> The other concern relates to the peculiarities of
>front wheel drive handling. One reason limitted-slip/locking differentils
>have seldom been used on fwd vehicles is that it can cause problems on
>slippery roads. Since the driving wheels are the steering wheels,
>manufacturers felt it was better to let ONE wheel spin and maintain
steering
>control with the other, rather than risk BOTH front wheels spinning,
causing
>the vehicle to slide right off the road.
Its even more than that- FWD car with connected axles are extraordinarily
hard to handle even on dry surfaces. When Toronado was introduced in 1966,
GM used the friction modifier additive Positrac cars use to rid the car of
the last bit of internal differential friction between the spider gears and
internal carrier parts. Essentialy, no FWD car uses any oher style limited
slip except for Torsen, and for good reason. Some Jeeps and trucks use
locking diffs, or clutch type positraction etc, but no OEM manufacter has
dared to. They really don't work on FWD. Bear in mind you are at the
steering wheel at one end, and a 455 with potentially about 3500 ft/lbs of
torque is at the other with between a 1/4" and 4" lever- makes handling
snaky indeed.
Both front wheels spinning isnt too bad in any event, they do that all the
time with open differentials;-) Independent suspension cars really gain
minimal benefit from limited slip devices. They already have equal torque to
two wheels carrying identical weight, and theres no unbalancing force like
driveshaft torque in a RWD car with a live axle to counter. The only
variable is the co-efficient of friction between the tires and road surface,
or occasionally unbalanced loading due to high centering.
The primary reason a Torsen style Limited slip will work in a FWD vehicle
where a clutch type or locker will not, is the Torsen is extremely
transparent in operation. To sum the differences, (badly) an OPEN carrier
sends equal torque to both front wheels 100% of the time. A Clutch type like
cars with Positraction/engaged lockers sends very unbalanced torque that
varies with the speed differences left and right by attempting to drive the
slow wheel with the fast one, and to a large extent, this is what a locker
also does; a Torsen style releives drive from the FAST moving wheel with
the slower moving one. Essentially the difference is an open one reacts at
the tire contact surfaces, whereas a Torsen does this reaction within the
carrier. Although an unlocked Locker would be just like stock to drive, when
engaged, it might be a real handful to say the least.
By varying the pitch of the worm gears in the carrier the Torsen can be made
more and less aggressive in its action, but the action is always VERY
progressive and linear. At rates of slippage encountered in normal driving,
it has no noticeable action, but it will never permit an unloaded wheel to
accelerate very much, and has won favour in FWD racing for this reason. You
may occasionaly see European Touring Car Racing, many cars used in this
class have Torsens, and the action is apparent on hard corners when a inner
front wheel is lifted clear of the Pavement- they dont start spinning wildly
in the air, and the car is just driven thru the corner by the outboard
wheel.
One especially nice feature of the Torsen also is they last a long time with
no deterioration in performance. The mechanical construction of the worm and
sectors means even if there was significant wear, the action would remain
consisitent. No seals, clutches etc to fail. The worm pitch is the point of
action, not the actual metal surfaces per se.
> One advantage of the ARB system is that when it is disengaged, there is
no
>drag on one wheel or the other. I'd like to try either on my GMC.
I am thinking another good possibility would be adaptation of an ABS to
brake a fast moving front wheel on one side to allow the other to drive the
coach. With the current availability of electronics, perhaps an automotive
system exists that could be adapted. Really, the basic issue is there just
isn't a sufficient proportion of the GMC's weight carried on the front
wheels for it EVER to have outstanding traction on slippery roads. However,
something like a Torsen likely would give the coaches a little advantage and
better wet and slippery road handling generally speaking, which is worth
something, and might make the difference in isolated cases. Personally, I
favour no modifications, and just driving on the crown of the road;-)
Brent Covey
Vancouver BC
