User 121721

About 9 months back I had my C4 emission tested in CA. Without my knowledge, they put the rear wheels on rollers and left the front wheels sitting stationary, then tested the exhaust at I think 5 or 10 mph. The car wasn't restrained in any way. My thought is that this is like the rear wheels being stuck in a snowbank. I always expected the AWD would pull the thing out. Why didn't the traction allocated to the front wheels pull the car off the setup?

I was worried that the shop might have screwed up my AWD clutch or something so I took it to the dealer right away after this, and he put it up on a stand and tested it somehow, told me that everything was fine but I should never let anyone touch the car again without watching them.

If anyone can enlighen me I'd appreciate it.

1998 C4 Cab

Phil

Great question?.......what is the answer?. I had never even thought about it...But I am do for smog test at some point this year! <img src="graemlins/crying.gif" border="0" alt="[crying]" /> <img src="graemlins/c.gif" border="0" alt="[ouch]" /> <img src="graemlins/crying.gif" border="0" alt="[crying]" />

B-Line home

Hey Phil,

Would you believe that down here in Florida, they don't even do smog checks anymore..

Phil

Seth,
You people in Flordia have way to many beaches!!!.You probably have "good" octane too!, in CA, (or the land of fruits and nuts) we have the 91 octane stuff!!!!!!

Bins

Hey Mark. Am I being silly if suggested you ask them???
Regards

User 121721

Bins,
They really couldn't explain it. They said that the process stressed the hell out of my viscous coupling, though, and could easily have broken it. I had always thought that in the C4, that whenever the rear wheels turned, the fronts did too, even up on a lift.

Mark

laurence '97 C4S

I do not know the theory behind it; however, you should always flatbed a four-wheel drive vehicle. To do otherwise, is risking significant damage. In NJ, our emission test is on the roller as well. The inspection person immediately recognized my car should not be tested in the rolling mode. Sounds like you got a stupid state employee.

Bins

In that case sounds like you got lucky there was no damage.

mags993tt-home

I am guessing a little here but FWIW this is what I think happened... Although the car is 4 wheel drive all the drive power is normally only to the back wheels. In real life that means around 95pct to the back and 5pct to the front through the mechanical inefficiencies of the mechanical diff between front and back axles. When a wheel is stopped from rotating all power transfers to wheels that can rotate - that's how differentials do their job. Because the rear wheels to all intents and purposes (to the vehicle) did have traction - after all they were rotating the rollers and the car 'thought' it was doing 5-10mph so there was no extra transfer of power to the front wheels. Hence I think all that happened was the differential between the front and back would have been under a little more stress than usual (maybe heated up a little) because the natural transfer of power (5pct) through the coupling would have had to be dissipated in the differential. I stress I'm no expert so this may not be the case - litteraly just my 2cents worth.
M

laurence '97 C4S

mags-993tt wrote:
"Although the car is 4 wheel drive all the drive power is normally only to the back wheels."

I do not think this statement is correct. I believe the 4WD system, at the extreme (ie front wheel slippage) is 95-5% (R v. F). Under normal driving the split is more like 70-30%. Where is Viken? I am sure he will have the exact range.

Rohan Nath

I believe the power split is 95% (rear) 5% (front) under normal conditions. If the rear wheels have inadequate traction, upto 40% of the power can be diverted to the front wheels.

Viken

Rohan is correct. A minimum of 5 percent of the torque is transferred to the front wheels at all times, but a maximum of 40 percent is transferred in situations of extreme rear-wheel slippage.

Rohan Nath

I talked with Sonnen Porsche/Audi/BMW (they did my smog test about 1 week ago). The guy who worked on my car confirmed that they did NOT put the car in gear for the smog test. I can confirm that this was also the case when I had my emission test done a year ago in Salt Lake City (I watched while the guy conducted the test).

Mark, I'm somewhat curious why you believe that your shop put your car in gear and tested emission.

Rohan

Ray Calvo

[quote] The car wasn't restrained in any way. <hr></blockquote>

Mark, I find this incredible - this goes against all safety concepts. I really doubt they would throw a car on what is essentially a dynamometer and NOT strap it down somehow; this is a union-controlled OSHA-approved gummint facility - right?

If test was done without your knowledge, how do you know it was unrestrained?

Now, if it IS strapped down and rears are on a "dyno", it would try to force the power to the front wheels. And if the car is restrained, then you WILL be putting a large amount of force on that viscous coupling!

My speculation, anyway.

(note - here in the Keystone state, as I remember they exempted AWD vehicles from a dynamometer emission test)

SundayDriver

in CA, the inspections are done by private shops, not the gov't. The test is a chassis dyno test and Full Time AWD cars are exempt from the dyno part of the test. They do idle and unloaded at some higher rpm. Because of the location of the equipment, many shops would pull an AWD vehicle on the dyno to hook it up. If that is the case, they would not need to tie the car down in any way. That is my guess as to what happened, because I can't imagine how you could put the rears on the dyno and run it without the fronts getting power.