Why don't on-roads have center diffs?

[Lessen]

Now I could vary well be speaking out of my ass since I do not and have never had an on-road car. But from pics and stuff I've noticed none seem to have center differentials. Why is this? Are they not as necessary for on-road vs. off-road AWD vehicles?

 

[rossb]

Most (maybe all) Monster trucks do not use center diffs either (not sure about the LST). A center diff will limit 4WD operation. It will allow the front and rear diffs to spin at different speeds, while a locked center forces the front and rear to spin at the same speeds.

A center diff helps a buggy hold a good line in the turns. A buggy would probably spin out around turns if it used a locked center. On-roads get better traction because they run on pavement so they are better able to use a locked 4WD setup. Most Monsters are basically low-tech, high power brutes that are not designed for racing on tight tracks. They don't bother with center diffs.

 

[Lessen]

who brought up monster trucks?

On-roads get better traction because they run on pavement so they are better able to use a locked 4WD setup.

ok, but why does a locked center suit them better?

Maybe it's not that a locked center necessarily suits an on-road better, but than an active center suits an off-road better. Maybe because an on-road isn't supposed to slide on it's particular surface as much it doesn't need an active center to help put the vehicle in check. I'm thinking maybe an on-road wouldn't necessarily be worse off with an active center but maybe it's just not necessary to justify the extra weight/space etc. Look at your popular 1:1 AWD cars. they have active center diffs, and I doubt many people are taking their new STI off-roading.

 

[rossb]

who brought up monster trucks?

I did. They are off-road vehicles.

Look at your popular 1:1 AWD cars. they have active center diffs, and I doubt many people are taking their new STI off-roading.

You cannot run a 1:1 car on-road with a locked center diff. The tires would skip around corners and you would bind up and damage the drivetrain. You can try this in a Jeep that has a "part-time" 4WD transfer case setting.

I'm thinking maybe an on-road wouldn't necessarily be worse off with an active center but maybe it's just not necessary to justify the extra weight/space etc.

This is probably a true statement.

 

[Lessen]

Probably the only time an active center diff would have a noticeable impact is if you were racing on an a track with extremely tight hairpin style turns. I guess that makes them pretty much useless in on-road 99.9% of the time.

 

[rossb]

I can cut the wheel all the way and run tight circles with my touring car and Monster truck and as far as I can tell their drivetrains do not bind up at all. On the other hand if I lock the center diff on a Jeep Wrangler it will clearly hop around turns at less than full steering lock. It think it is related to tire size and wheelbase.

 

[Lessen]

I don't think I'm clear on what in the drivetrain would bind IF it did.

I don't think anything would bind I just think that it's a traction concern. It seems to me that it would be most apparent when turning very sharp that the front and rear tires (left or right respectively) do not turn at the same speed because they take a slightly different line. If the front/rear tires could "diff" then theoretically either one would not give up traction to the other quite as easily which in turn would lead to better overall traction in tight corners and higher overall cornering speeds. Just my 2 cents though. It just surprises me that even high end 1/8 scale on-roads do not utilize them.

 

[rossb]

1:1 cars 4WD with locked center diffs bind hard around sharp turns. This is a well documented fact that can be found by searching on the web. You can also read about it here (part-time 4wd)

http://auto.howstuffworks.com/four-wheel-drive6.htm

I am not 100% sure why locked 4WD R/Cs don't bind around turns but I am guessing it is due to tire size, wheelbase, and limited traction. A locked 4WD car is going to holeshot harder and accelerate harder. That may be one reason on-roads use this setup.

 

[Lessen]

interesting. Thanks for all your input rossb! :thumbup: