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Old 05-09-2017, 06:23 AM   #1
LJH
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Default School me on Diff set-ups please

Hello All,
20+ years ago I did a wee bit of racing with a TA03 but back then there was not much of any chat about setting up diffs on RC cars but I can understand the importance of setting up the diffs and it really intrigues me.

I have done a bit of full scale motorsport and understand the importance of limiting the slip on a diff but when it comes to AWD I am at a bit of a loss....which is kind of funny since I have personally owned nothing but Audi Quattro's for the the last 25 years and have wheeled them around the Auto-X course and have done a number of track days in them.

With that said I just built a TT02 that will be powered by a 17.5 Blinky set-up but I have not run the car yet. When I built the car I used some 500K weight diff fluid in the front and left the rear totally open. I did not fill the front diff completely and currently the diff will still work as it should but with a bit of resistance whereas the rear is very "open".

From my experience in full scale I know that limiting the slip of the front diff will help get power to the ground but also promote understeer in tighter turns, does the same hold true for small cars? I imagine you are looking for the right balance front/rear to get the rotation you are looking for?

Any tips would be most appreciated.

Last question, as I play with this what is the best way to clean out old silicone diff fluid?

Cheers,
Jim
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Old 05-09-2017, 06:25 AM   #2
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depending on what racing surface you are running front spool will be a better choice.
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Old 05-09-2017, 06:45 AM   #3
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depending on what racing surface you are running front spool will be a better choice.
OK....but why.

Really interested in how and why different set-ups work for different surfaces.

I may be going a bit overboard as I really do not plan on racing but chassis set up intrigues me. I feel from my years of racing full scale I have a pretty good handle on suspension set up and alignment, back when I was quite competitive in SCCA C Stock I did all my own alignments on my MK1 MR2.

With that said I will be running the car on unprepared tarmac that ranges from a tennis courts to slick roads.



Cheers,
Jim
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Old 05-09-2017, 06:47 AM   #4
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With that said I will be running the car on unprepared tarmac that ranges from a tennis courts to slick roads.

Cheers,
Jim
then you are better of with the spool
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Old 05-09-2017, 06:57 AM   #5
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Good luck trying to get a substantiated answer to that one. There will be many empirical-based ones, most of them disagreeing, though
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Old 05-09-2017, 07:04 AM   #6
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I just run ball diffs.

I run my tighter up front, and slight snug int eh rear, but I can get away with that on Black CRC carpet
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Old 05-09-2017, 07:06 AM   #7
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In the Hudy setup book you can find some explenation about the tightness of the diff and the use of solid (spool) axles and a oneway.

https://www.hudy.net/xhudy/products/...8ce373b7c7d349
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Old 05-09-2017, 07:11 AM   #8
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I always run the rear diff open (lube only). When traction is very high, adding some viscosity can help out of the corners, but it also makes the car less stable under power, and my reflexes aren't fast enough any more to compensate.

The front diff viscosity depends on both horsepower and grip. As they increase, go higher on the viscosity. This accomplishes two things: first, it keeps the inside front tire from spinning under power coming out of a corner, and second, it offsets the thrust of the front end to the tire with more weight loading. This makes the car more stable under trail braking (since the outside tire thrust counters the chassis yaw), and lessens understeer when accelerating out corners (since the outside tire thrust makes the car yaw into the corner).

I adjust the front diff oil viscosity to achieve a nice balance of stability under trail braking and on-power steering. Going too high on the viscosity will make the car understeer into a corner and oversteer coming out of a corner.

Using too high a viscosity will also cause the car to scrub off speed in mid-corner and in sweepers. It's really noticeable in VTA, since the available horsepower is low.

For VTA, I'm running 50K in the front diff; for USGT, 300K; and for 17.5 TC, 1M. The most sensitive car seems to be the TC, since the tires are stickier and the horsepower is greatest. I've gone as low as 200K on very low-traction surfaces.
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Old 05-09-2017, 07:54 AM   #9
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So Howard how does changing Ackermann do anything on a car running a spool? The inside turning tire is forced to turn the same speed as the outside tire, hence inducing more slip then you would get from having no Ackermann?
Maybe I'm over thinking this but on Real 4wd Race cars and trucks we don't run Ackermann. I was talking to a couple Engineers at the 24hr of Daytona about this, and they stated Ackermann isn't really needed in a Race Car, unless it's a high downforce, high grip car then I want Negative Ackermann.

Thoughts Howard or any other chassis setup guru's.
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Last edited by patorz31; 05-09-2017 at 08:47 AM.
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Old 05-09-2017, 08:24 AM   #10
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I run the diff that works best for the situation I'm in
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Old 05-09-2017, 09:15 AM   #11
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Quote:
Originally Posted by patorz31 View Post
So Howard how does changing Ackermann do anything on a car running a spool? The inside turning tire is forced to turn the same speed as the outside tire, hence inducing more slip then you would get from having no Ackermann?
Maybe I'm over thinking this but on Real 4wd Race cars and trucks we don't run Ackermann. I was talking to a couple Engineers at the 24hr of Daytona about this, and they stated Ackermann isn't really needed in a Race Car, unless it's a high downforce, high grip car then I want Negative Ackermann.

Thoughts Howard or any other chassis setup guru's.
I don't have much insight on this. Full-scale racing cars do tend to run negative ("wrong-way"!) Ackermann, and the reason, according to Costin and Phipps (in their book Racing and Sports Car Chassis Design), is that the outside tire twists more on the rim due to higher loading. But I don't know much this occurs on our very-low-profile TC tires.

(P.S. That book reference gives you some idea of when I was born!)
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Old 05-09-2017, 11:11 AM   #12
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Quote:
Originally Posted by LJH View Post
Hello All,
20+ years ago I did a wee bit of racing with a TA03 but back then there was not much of any chat about setting up diffs on RC cars but I can understand the importance of setting up the diffs and it really intrigues me.

I have done a bit of full scale motorsport and understand the importance of limiting the slip on a diff but when it comes to AWD I am at a bit of a loss....which is kind of funny since I have personally owned nothing but Audi Quattro's for the the last 25 years and have wheeled them around the Auto-X course and have done a number of track days in them.

With that said I just built a TT02 that will be powered by a 17.5 Blinky set-up but I have not run the car yet. When I built the car I used some 500K weight diff fluid in the front and left the rear totally open. I did not fill the front diff completely and currently the diff will still work as it should but with a bit of resistance whereas the rear is very "open".

From my experience in full scale I know that limiting the slip of the front diff will help get power to the ground but also promote understeer in tighter turns, does the same hold true for small cars? I imagine you are looking for the right balance front/rear to get the rotation you are looking for?

Any tips would be most appreciated.

Last question, as I play with this what is the best way to clean out old silicone diff fluid?

Cheers,
Jim
The stock TT02 diffs are not not intended to be filled with fluid.
No seals anywhere in there.
Will probably get really messy, really fast in your gear case.
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Old 05-09-2017, 12:53 PM   #13
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Originally Posted by PizzaDude View Post
The stock TT02 diffs are not not intended to be filled with fluid.
No seals anywhere in there.
Will probably get really messy, really fast in your gear case.
Supposedly anything above 300K fluid stays inside them pretty well according to a number of guys....there is also an easy mod that can be preformed that is working well at keeping the fluid inside the diffs.

Good stuff here, I have learned a lot from some of you.

Howard,
If it makes you feel better I have a copy of "Sports car chassis design" from back in the day of me competing in full scale. An older guy, who campaigned the first generation MR2's at a high level, kind of took me under his wing and he told me that he would offer all the help I needed if I read a couple books on chassis design and dynamics cover to cover. I did and it taught me a lot and paid dividends when I finished 3rd in the Northeast region in C-Stock.

Cheers,
Jim
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Old 05-09-2017, 01:14 PM   #14
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Quote:
Originally Posted by howardcano View Post
I don't have much insight on this. Full-scale racing cars do tend to run negative ("wrong-way"!) Ackermann, and the reason, according to Costin and Phipps (in their book Racing and Sports Car Chassis Design), is that the outside tire twists more on the rim due to higher loading. But I don't know much this occurs on our very-low-profile TC tires.

(P.S. That book reference gives you some idea of when I was born!)
The explanation I got for Negative Ackermann was that on a High Grip High Downforce car, (Like a F1 or Prototype Sportscar or any chassis that can pull over 1.5g in a corner) was that the outside tire is so loaded compared to the inside tire that it didn't matter. Negative ackerman takes into consideration the optimum slip angle for the tire at reduced vertical load (inner wheel). Also its much better and aerowise for sure as the inner tire does not block /modify the airflow as much when there is not as much lock available on the inside tire. Supposedly works awesome on the track but sucks pushing the car around the paddock.

Still all the theory in the world about Ackermann assumes the car in RWD or 4WD with a diff. With a spool it would seem; by modelling I've done on a computer, that it all goes out the window.

I really need to make some carpet plots of the popular TC tires.
Anyone know of a way to scale a way of doing this for TC size tires?
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Old 05-09-2017, 03:16 PM   #15
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Hi, from my experience, thicker oil more entry understeer, more on-power oversteer, more power transmitted to the ground, more acceleration.
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