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Old 07-24-2006, 01:51 PM   #1
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Join Date: Jun 2006
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Default Diff Oil Explanation

Apologoes if this has been covered before but I have done a search and not found what I'm after.

Can someone give me an explanation on how different thickness of oil in the diffs impact handling. I run a Jammin CRT so it has front/centre/rear diffs and I'm struggling to understand the impact of thickening/lightening the oil in each diff.

I'm not after setup advice, it's more an understanding or the theory behind the effect of different oils.

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Old 07-24-2006, 03:14 PM   #2
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Posts: 660

Here ya go!

Diff Oil explanation
I look at it this way:
1st remember and this is all you have to remember is that a diff's job
is to transfer torque to the side of the diff with the least amount of
traction, ie least amount of weight transfered, ie least amount of
traction (This is opposite a torsen by the way).

Center diff fluid:
Say you accelerate, weight transfers towards the rear (your buggy
squats - how much depending on several factors such as rear antisquat
plate, how hard you are on the gas, etc), a center diff with very light
oil will "diff" more towards the front to try to balance the load and
stabilize the buggy. This is great on a slick or maybe a tight track
where you want to feel the front diff pulling your car a little more
and keep you from "spinning out."

Now, say you put in very thick center diff oil in your center diff.
You accelerate, weight transfers towards the rear, a center diff with
very thick oil will "diff" less towards the front but will give you the
most torque at the rear of the buggy, ie more acceleration. This is
great for high bite tracks but on a slick track you may find your rear
end swinging around more.

BUT, straight line acceleration is but one aspect of a center diffs
purpose. Say you are on a track and you approach a woop section. a
car with very light fluid will transfer more torque to the front of the
buggy as soon as the front tires begin to loose contact with the apex
of the woop, hence your rear end feels less power and consequently the
front of your buggy's nose comes down nice and fast ready for the next
woop. your center diff is constantly "diffing" to keep the car
balanced through the woops. Now, if you had very thick oil in the
center, the nose of the buggy would come down a little slower because
it will not diff towards the front so fast. This can cause your buggy
to get out of shape easily because your front tires are not in contact
(think steering) as much with the ground.

Also, center diff oil can even depend on what engine you are running.
if you have an OD MOD, and you go too light on the center oil, you will
find your front tires ballooning to the point of almost flying off the
wheels down the straight away, so you may increase the center oil
weight a bit.

Ok, now for my 2 cents on front diff fluid. T
his will control torque transfer from the left to front tire and vice
Let's say you have very thick front center fluid and you exit a left
hand 180 on the gas. Your right front tire has the most traction and
hence, if you want the tightest steering, you would want most of the
torque to stay on that outside tire, and hence you would use heavier
front diff fluid. However, if you want more turnin (which happens at
the very beginning of the turn up until the point where your car has
transfered more weight to the outside tire), you could use lighter
front diff fluid.

BUT, the front diff is not just there for turning. Let's say you hit a
bump with only the left front tire of your car. With light front diff
fluid, more torque will transfer to the tire with the least amount of
traction than with thicker oil, ie the right tire starting to come off
the ground due to the chassis lifting, hence the front will stabilize
itself more than a car with thicker front diff fluid.

The rear diff follows the same principle of the front diff.

I know this was long and boring and winded, but i just wanted to get my
feelings out there as an alternative point of view. I am also a rc
photography fan and I love watching and photographing diff action on
buggies. So i am always noticing how much different buggies including
mine are diffing by watching and photographing their tires at different

What i said above is not the be all end all gospel but just my

Oh yeah, the front diff thing is also a reason you find some buggies
(at least mine) will have more front steering with a front swaybar on
some of the faster corners. The bar keeps the car from transferring
too much weight to the outside tire, thus reducing the front diff
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Old 07-25-2006, 02:00 AM   #3
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Join Date: Jun 2006
Location: UK
Posts: 18

Not long winded and boring at all. Thanks very much for that excellent post.

I'm a noobie to RC and went straight into Nitro. I did the usual thing and read a few forums and got consistent advice to swap out the grease that comes with the truggy (it's an RTR version) and replace with diff oil. Then came the minefield of what oil to put in and there seemed to be as many different setups as there are oils.

Now, I haven't touched any of the other adjustments on the truck other than setting ride height so the steering arms front and rear are level when sat on the trucks wheels. I did this mainly to stop the chasis bottoming out but to also because it was a general consensus as a sound base setup, which makes sense to me.

With that in mind I went out with a couple of diff oil settings in the truck and quickly realised that I didn't actually know what to look for when I was changing the oils. I noticed I was getting some different behaviours but not really knowing what was causing it.

With your advice I can now sit down and logically work through what to try next to improve certain characteristics until I get a diff oil base setting that I'm happy with. Then, and only then, I can start to play with other settings on the car to make up for some of the downsides of a particular diff oil setup. Eg, things like the front sway bar.

I know setup is a compromise but at least I'm compromising from an informed point of view.

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