Today we had a members only AutoX practice. It rained all day and therefor did not get much of a turnout. The good thing about this was that we were able to get a ton of runs in. This was the first time I was really able to compare running in Tarmac mode vs Gravel mode. After several runs in both modes, I was able to run about 2.5 seconds (on average, 60 second course) faster in Gravel mode vice Tarmac mode.

When I was in Gravel mode, I was able to drive the car more like the pavement was dry and was able to put down much better times. One other thing I did notice was that in Gravel mode the car understeered a lot more as some people have stated in this thread: https://www.evolutionm.net/forums/sh...%22acd+mode%22

I am curious to what others have found out. It could be entirely possible that once I have some more experience under my belt in the wet that the Tarmac mode could be just as quick or quicker.

I was driving a stock IX MR with the stock yokos, I have done about 150 autoX runs, but less than 10 on a wet/kinda wet track before today.

 

i might be testing that same theory today and tomorrow at the San Diego Tour (SCCA), since we have a 40-70% chance of rain at varying times. I plan to use my slicks and Gravel mode if it's wet, or street tires and gravel mode if it's heavy rain with standing water.

 

Look in the manual. I asked the idiot sales ***-clowns when I bought the car about the three modes, but of course they were useless. Could of sworn I then read in the manual that gravel was for rain.

 

The sticker that comes on the door when you buy it new says that gravel mode is for wet/sandy roads. It was just the first time I really got to see how the car handled driving it on the limits using this setting.

 

the problem with the other thread is that they were getting way too technical. i have never tracked my car but do an awful lot of canyon driving though never all out in the rain. i have noticed that the car is a lot more free in tarmac mode... tail loves to slide out. mind you my car is entirely stock. in gravel the car sticks to the ground and tends to understeer. this is my personal experience. does this sound correct to anyone or am i crazy?

 

^^When you are autoXing tail out in the rain is not something you want IMO. Maybe in the dry where it is somewhat controllable...ain't that right Karl To me from how I interpret it there is naturally going to be more understeer as there is more power sent to the front wheels. Basically the car is acting more FWD but much more predictable in the rain. I can tell you in the rain in Tarmac I can kick sideways in 2nd gear...that would be too hairy for me at the good ol track especially in tight turns. Hell I spun today and it was barely drizzling I wonder what Snow mode would do

Btw I have an old issue of SCC where they tested a stock MR in bad rain vs 10 other highly modified cars (STI, 300ZX, 350Z, TSX, SRT4, Silvia, etc). They ran theirs in the Gravel mode and said it was far easier to drive in the rain at the limits vs Tarmac. The stock evo actually ran faster than every car there in the gravel mode.

 

Here's the really simplified version of how the ACD control works:

The ACD clutch packs are engaged or disengaged by hydraulic pressure. When fully engaged, the front and rear output shafts are locked together, and the front and rear tires have to spin at the same speed. When fully disengaged, the front and rear output shafts can spin at different speeds. The ACD pump creates the hydraulic pressure to cause this lockup and can vary that pressure to cause different degrees of lockup.

In Tarmac mode, the ACD fully unlocks in turns and relocks quickly once the steering wheel is straightened. In Snow mode, the ACD reduces pressure in turns but never fully unlocks. Once the wheel is straightened, it waits longer to relock the diff. The Gravel mode is between those two.

What does this mean to the driver? This may get a little technical. As long as both axles have grip, power is distributed 50% front and 50% rear no matter the state of the ACD. In a straight line this means that the force on all four wheels is equal. Now assume you enter a turn. As long as you don't exceed the traction limits of the tires, you are still distributing power equally front and rear.

Now take the case in Tarmac mode, where the ACD opens in a turn. You're applying 50/50 power until you exceed the limits of traction. Once that happens, with an open diff you only apply the amount of power that the wheels with the least grip have. You can therefore only apply the amount of power that the axle with the least amount of grip can take. Because the car is front-heavy and likes to roll, this is usually the front end. So what happens in low-traction situations is that if the car pushes, applying power only makes the push worse. If the car is set up to be loose, applying power makes it looser.

Next let's look at Snow mode, and assume for the sake of argument that the diff is completely locked (its not). This is a little more complicated. The front and rear axles are now must turn at the same speed no matter what. This means that power will go to the axle with grip until you run out of power or you apply enough to slip all four wheels. So what's the tradeoff? The front tires travel a longer distance than the rear tires through the turns. This means that if they are locked together, one or the other set of tires will have to slip some to get through the turn. In reality, the diff is not fully locked and the tires don't necessarily slip, but the amount of force required to overcome the diff lockup is transferred from the front to the rear wheels. This force is separate from the power delivered. It totals zero in terms of thrust, so it doesn't buy you anything, but it adds to the total force on each tire, which decreases the available traction. In short, the car gets looser and loses ultimate grip. If the car was loose and you have enough power, this may actually feel like understeer because you can now spin the front wheels where you could only spin the rear ones before.

There is one hidden advantage to Gravel and Snow modes, which is that lockup is slower and softer on corner exit. This is important in low-traction situations where a fast, hard lockup can make the car do unexpected things.

 

^ That is quite possibly the best ACD description I've read in a long time.

 

Where most people go wrong is that they think the ACD somehow connects and disconnects the rear wheels from the drivetrain. In reality, if you can lock the front wheels from turning and force the ACD pump to turn off, the car will sit there and smoke the rear tires. Shortly after this the center diff will explode and drop bits of mangled metal all over the track because the spider gears aren't designed for that kind of abuse, but you can do it as a mental exercise to help think about how AWD works.

 

Damn that is the best writeup ever!!!! In every setting its always 50/50 power split, its just how long it takes to lock is what changes? I wasn't sure on that.

 

Yeah, that part (constant 50/50 split) has been well known for a long time, but it is still misunderstood by many.

 

Power is actually somewhat undefined when you're looking at this. Power is a measure of work over time (i.e. force * distance * time). What's easier to look at is force at the contact patch, which translates to torque. (Once a tire loses traction, the Power = Torque * RPM/5252 relationship is no longer valid.) An open diff will put equal force to all four wheels, but what defines the force delivered is the resistance of the side that's spinning. Any extra torque goes into increasing the rotational momentum of the spinning wheel. A locked diff puts equal force to all four wheels, but what defines that force is the total traction of both sides of the diff.

You can see why an AWD system with LSDs (with different lockup characteristice) on each axle plus a computer controlled central diff is ridiculously complicated to analyze. Then factor in that frictional losses are different for each wheel and it gets almost impossible to analytically say what it will do. This is why some people say the car gets looser in Gravel mode, and some say it gets tighter in Gravel mode. It really depends on the setup and the surface.

What I haven't figured out yet is how you build a diff that has a constant 35/65 split. Is it done in the spider gears, or with the clutch packs? Or are they all viscous diff setups?

 

You left out the part about pump pressure and pressure curve though.

Tarmac is 2bar increasing to 10 (system max is 16 I believe)

Gravel is 4bar~10

Snow, I cant remember...I have the GEMS manual somewhere that has all of these. I can do the first two from memory.

Like you said though, the release based on steering angle is also different.

 

There's a very good reason I left it out: I didn't know any of that.

Interesting that the system is only programmed to go up to 10 bar, yet it maxes out at 16. Have the drag racers figured out how to make it go to full lockup?

 

YUP...full lock can be achieved with a maneuver that hasnt been fully researched as to what it does over time. Rather than have the pump operate off variable voltage to pressure output its wired 12v (system full 10bar +/-) and is "locked".