4EAT Diff-Lock Switch and Handbrake Mod - "Torquemada Lite" - Page 2 - Subaru Impreza GC8 & RS Forum & Community: RS25.com
 
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4EAT Diff-Lock Switch and Handbrake Mod - "Torquemada Lite"
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Old 01-20-2009, 12:46 PM   #11
defcon5
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Quote:
Originally Posted by DunDunSkeert View Post
Holy Crap I have to try this!

Why should you feel binding on asphalt? Would this be safe to use for autocross or is it for offroad only? How long has this mod been done to your car. Is there any chance you could pre fab the resistor board for me and include the wiring and switch so all I would have to do is solder the wires to the car.?
You'll feel binding on dry pavement because the Multi Plate Transfer Clutch (MPT) is physically locked. The 4EAT doesn't have a "true" geared center differential like the 5MT, so when you activate the lockup, the transmission has no way to make up for the difference in rotational speed of the front & rear axles as you take a tight turn.
Subaru is obviously comfortable enough with the strength of their drivetrain components to allow the "fail" position of the solenoid to fully engage the AWD. If you look up "torque bind" on most Subaru sites, you'll see that it's most commonly traced to a DutyC Solenoid failure - which is exactly what we're mimicking with this mod.
Under normal driving circumstances on dry pavement, you're very unlikely to break anything unless you try really, really hard, but the wear & tear will certainly be accelerated.

As for autocross, I'd be very, very careful - especially if you're running in one of the "super sticky tire" classes. Crappy all-season tires will just skid and dissipate the load, but a nice set of R compounds will probably just transfer it on to your axles, cv joints, and the transmission itself. this mod is probably best left to situations where the wheels can slip just a little .
I also don't know how the handling would be at the limits with a locked differential on tarmac. I'd assume you'd pick up a pretty nasty push in the tighter stuff, but that's just me - and I'm a Rallycross guy.

If there's enough interest in this project, I could apply for vendor status, and sell full kits, but for now, I don't want to run afoul of the RS25 rules... I guess I could source the parts and build it - then just charge you my costs plus shipping, but I'm not sure if "non-profit" financial transactions are allowed. Maybe I'll go ask about that, or one of the mods could chime in.
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Old 01-20-2009, 01:13 PM   #12
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Originally Posted by ScoobaruRS29 View Post
this depresses me.... I got the 4eat and i dont have AWD?WTF? And i sure aint mechanically talented to pull this off. damn. Nice write-up by the way
Naw man, you've got AWD - just not all the time. Your car has to sense a bit of slip before it reacts and transfers power to the rear. In other words it's a "reactive" system. That's not a huge deal - lots of other car makers have similar systems. Done right, it's a very capable, and intelligent way to control traction.
How long that reaction takes can be influenced by many factors, including overall transmission health and age. My transmission can take up to half a second or more to react, your's may react instantly. If so, you may not need my mod - test it out and see.
My mod is fairly crude and takes away any intelligence that the system has, but since it's only for occasional/competition use, brute strength is just fine.


Glad you like the writeup
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Old 01-20-2009, 02:06 PM   #13
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Nice write up. Yes, it's allowed.

I think it needs more details and photos though.

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Old 01-20-2009, 04:05 PM   #14
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Originally Posted by defcon5 View Post
Step 1 - Build the dummy load:
...
Let's build it -
First I like to check each of the resistors to make sure they are working properly. Set your multimeter to check impedance (ohm symbol - looks like a horseshoe) and test each resistor by touching the probes to the leads of the resistors (polarity doesn't matter here). Ideally you'll see a number close to the rating of the resistor - in our case 50 and 100 ohms.
If everything checks out, grab your perfboard, and mount your resistors by pushing the leads through the holes and gently bending them to hold everything in place. Now would be a good time to add some little pieces of heat shrink tubing to the exposed leads on the resistors (see later pics). Mock everything up and make sure you're happy before moving on.

...
One observation - If you had put one 50Ω and one 100Ω resistor side-by-side, you would have more even power dissipation. As it is, the two 50Ω resistors are dissipating twice as much power as the two 100Ω resistors. Have you noticed that heatsink is warmer?

Take Care,
David Baldock
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Old 01-20-2009, 06:38 PM   #15
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Originally Posted by RS25.com View Post
Nice write up. Yes, it's allowed.

I think it needs more details and photos though.

Yeah. Me + keyboard = epic reading for everyone else.
Thanks for clearing up the "non profit" question.

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Originally Posted by dbaldock9 View Post
One observation - If you had put one 50Ω and one 100Ω resistor side-by-side, you would have more even power dissipation.
Dave - can you please expand on this? Are you saying that by pairing a 50 ohm with a 100 ohm rather than both 50s and both 100s together, the resistor would run cooler overall? I know the same impedance can be obtained using fewer resistors, and wiring them in series, but I figured the greater surface area would aid in heat dissipation - Am I mistaken?

EDIT: I just re-read your post Dave, and now I see what you mean. I read "even more power dissipation", instead of "more even power dissipation". Got it...and yes, you're absolutely correct. I'll monitor each side the next time I use it heavily and report back. If it needs to be rewired, I'll note that in the DIY - Thanks.

Last edited by defcon5; 01-20-2009 at 08:24 PM..
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Old 01-20-2009, 08:04 PM   #16
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defcon--I assume the idea for your handbrake turn mod is to duplicate inserting the FWD fuse when the handbrake is pulled, thus not torturing the center diff when you lock up the rear wheels?

Looking forward to the write up on that as well. Great stuff so far
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Old 01-20-2009, 08:17 PM   #17
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Quote:
Originally Posted by defcon5 View Post
...
Dave - can you please expand on this? Are you saying that by pairing a 50 ohm with a 100 ohm rather than both 50s and both 100s together, the resistor would run cooler overall? I know the same impedance can be obtained using fewer resistors, and wiring them in series, but I figured the greater surface area would aid in heat dissipation - Am I mistaken?
The Total Power Dissipation is determined by the voltage across the total resistance. The formula is: W = V^2 / R

Changing the pairing of the resistors physically won't change the Total Power Dissipation, since they're all still wired in parallel. All it will do is distribute the power dissipation evenly across the two pairs of 50Ω and 100Ω resistors and their heatsinks.

Maximum Power Dissipation @ 12 VDC would be:
12V^2 / 17Ω = 8.64 W

It can be divided evenly on the two heatsinks by pairing a 50Ω and 100Ω on each one.
12V^2 / 33.3Ω = 4.32 W (First Pair)
12V^2 / 33.3Ω = 4.32 W (Second Pair)

The way you've got the resistors paired is like this:
12V^2 / 25Ω = 5.76 W (50Ω Pair)
12V^2 / 50Ω = 2.88 W (100Ω Pair)

Hope this helps clear up my first comment.

Take Care,
David Baldock
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Old 01-20-2009, 08:34 PM   #18
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David - we must've been typing at the same time - I was just editing my above reply when I saw your new comment.

Great stuff man. You're absolutely right, and I feel dumb for not realizing it. I even did all the calculations to make sure my resistors were up to the task.
You'd think that once I saw the resulting power figures for each of the resistors, it would have occurred to me to mix 'em up. Guess I was too busy taking pictures .

Thanks for the expert opinions and hardcore formulas - it'll give the thread some much-needed "electronics street cred"


Quote:
Originally Posted by obsolete View Post
defcon--I assume the idea for your handbrake turn mod is to duplicate inserting the FWD fuse when the handbrake is pulled, thus not torturing the center diff when you lock up the rear wheels?

Looking forward to the write up on that as well. Great stuff so far
You're very clever....thanks for the kind words

Last edited by defcon5; 01-20-2009 at 08:41 PM..
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Old 01-23-2009, 12:35 PM   #19
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I'm going to give this a go next weekend. Looks simple enough to do.

I can't wait for the "mythical mod".
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Old 01-23-2009, 06:41 PM   #20
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Love the write up, going to be putting in a MT soon so its a little late but maybe this will prompt me to try and shoe horn the damn thing into one of the gl-10s i got.
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