Len

I have noticed some strange behaviour from the traction control system on wet roads. When starting from a stop, if I give it too much juice the rear tires can spin and hop for a couple of seconds before the TCS kicks in. In contrast, on dry pavement the TCS only allows a brief chirp before it cuts in.

Has anyone else seen that? Is it right? This is the only car I've driven with traction control.

Magnesium

TCS and DSC are only active above 5 mph

Literatii

I was in heavy rain the other day and didn't notice any problems. In fact, I tried to spin out to see what would happen... and power to both wheels was well balanced.

Len

Literatii, I wasn't talking about DSC or spinning out, but specifically about TCS - just straight-ahead wheelspin.

Magnesium, if TCS only works above 5mph, then how does it prevent wheelspin at about 0 mph on dry pavement?

zoom44

just how exaclty would you get wheelspin at 0 mph on any type of pavement?

Len

*sigh* I wrote "about 0 mph". "About" meaning "approximately". How fast are you usually going when you chirp the tires starting from a dead stop? I could be wrong, but it seems to me the rear wheels start to spin at the same time the car starts moving, not after.

How about instead of nit-picking what I wrote, you tell me what your car actually does when starting on a wet road? That's really what I was hoping to hear from other people.

jdaled

0 mph is exactly what you get if you're at a dead stop, and your drive wheels are spinning with no traction. Not going anywhere = 0mph. Of course, if the speedo measures speed from the rear wheels, then the system would think you were going faster than 0mph, and would engage. I imagine it's not too difficult to make the wheels spin fast enough for the system to think you were at least going 5mph. Hell, I can push the car that fast. (not that I'd be able to maintain that long enough to actually travel 5 miles in an hour, mind you :p )

My last car had TCS too, but no DCS, and it took a second or two to engage at start up in wet conditions also. I read in an article (in Car and Driver, maybe?) that described the TCS and DCS functions on the RX-8 as being programmed to be very inobtrusive... they don't kick in at the least little thing, but they're there and ready when you really need 'em. Maybe it just takes you a couple seconds to get enough spin to "really need 'em" when starting out on a wet road.

I also imagine (notice how imaginitive I am :D ) that the two systems might treat the difference in wheel speed from front to back differently than the difference from left to right wheels.

Reeko

My guess would be that on dry pavement, you get to 5MPH very quickly, even spinning the tire. It might seem like a brief chirp, but the car might launch quickly.

However, on wet, I know that on many other cars, I have spun the tires considerably before the vehicle even moves. I haven't tested in the Rain on my RX8 yet.

zoom44

pardon me len for my smart *** remark. the car uses wheel speed to measure mph, not air speed like a plane. so if the wheels are spinning you will be showing more than 0 on the speedo. it's been raining for about a week now here in the "great" northwest so i have some experience with starts on wet pavement. basically the car just goes. with the RELATIVELY low torque of this car easing the clutch out at 2500-3000 rpm produces no wheel spin, at least on the slightly wet roads we have had here recently. no TCS reaction at all. the LSD does the work here i think, so the TCS has not had to engage. now if i were to drop the clutch at that rpm or start at like 6k rpm on the wet pavement i am quite sure that the TCS would kick in. but it is hardly noticable. i jammed out of my warehouse parking lot awhile ago at about 6 or 7k rpm into a hard left turn and the TCS only blinked for a moment before i finished the turn. i didn't even feel what it had done.

DisneyDestroyer

From what I've seen, when I disable TCS/DSC and peel out, the speed quickly jumps up to 20+MPH, then settles down to my real speed. In other words, it's measuring the speed I would be going based on the wheel's spin, rather than the actual forward velocity, like with giros (sp?) or something.

So perhaps the TCS cuts in when your wheels are spinning as fast as they would when you are going 5 MPH, even if your true velocity is zero.

MrWigggles

Isn't the speedo linked to the non-driving wheels, i.e. the front wheels? Most rear wheel drive cars I know of are this way.

I am pretty sure Mazda has accurate speed sensors for each wheel, but only the front wheels determine what is shown on the speedo since they reflect true motion of the car.

According to Disney the rear wheels are effecting the speedo and that seems like the wrong way to do it.

-Mr. Wigggles

zoom44

there are speed sensors on all 4 wheels. remember this is why mazda says it is difficult to dyno the rx-8. difference in wheel speed front to back cause faults in the engine management software which causes the car to change timing and air/fuel maps among other things. if the tcs were "armed" it would also kick in at a pre-determined "spin threshold" to try and correct the spin. i think i also read from canzoomer that there are some senors that track the difference between steering angle input and the actual amount the car is turning, so that at some pre-determined difference threshold the TCS and DSC kick in to do their stuff. i am not sure what the speedo reads when on a dynometer, anybody got that one? but differences in those speed sensors are what trigger the TCS and again you hardly notice it doing it's job. it is very smooth like the rest of the car. it is probaly not programmed with a certain velocity but rather a certain diference in wheelspeeds.

sferrett

I believe most modern cars use a sensor/gear/gizmo in the transmission to drive the speedo...

Simon.

rxevolve10

Simon is right ..most of the modern cars do use speedo in the transmission (most ABS vehicles) However VSC DSC and all other advanced systems actually read from each indivdual wheel but has a sensor in the tranny to detirmine actual vehicle speed.

Magnesium

I just state the facts as said on the Highlights CD. Don't kill the messenger.

Len

Interesting info and ideas. I'll try to figure out more when it starts raining again. Unless it snows first.

jdaled

:D