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Old 08-09-2006, 12:38 PM   #1
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Default Diffs and you - a discussion about the magic world of diff technology

Okay something's been bugging me for a while but I'm going to embarass
myself by asking it. Please be gentle because I bite when angered.

I don't understand the mechanics of how diffs work. I'm new to nitro so
I will be referring to 8th scale gear diffs.

Let's break it down to addressing one diff only like the front diff in an 8th
scale buggy. On the car with wheels mounted (while elevated off the
ground) you can turn one wheel and the other spins the opposite direction.

What I don't understand is when do your tires ever do that while on power in
a race? Even off power as long as you are rolling forward
both tires are rolling forward meaning no diff action is happening. I've always
read and been told that you need diffs to allow the front wheels to turn at
different rates. For example the front left travels farther than the front right
tire when making a sharp right turn. How does the non-oneway geared diff
allow the tires to turn at different rates when the gears can't slip?

The only scenario my little brain can come up with is that when one wheel
leaves the ground over rough terrain the tire in the air wants to spin
backwards translating into more power being transferred to the tire on the
ground...The diff oil controls whether or not that happens fast or slow. Is
that correct?

Someone pull back the curtain for me please. I need help.
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Old 08-09-2006, 12:52 PM   #2
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Quote:
Originally Posted by Rysuleod
Okay something's been bugging me for a while but I'm going to embarass
myself by asking it. Please be gentle because I bite when angered.

I don't understand the mechanics of how diffs work. I'm new to nitro so
I will be referring to 8th scale gear diffs.

Let's break it down to addressing one diff only like the front diff in an 8th
scale buggy. On the car with wheels mounted (while elevated off the
ground) you can turn one wheel and the other spins the opposite direction.

What I don't understand is when do your tires ever do that while on power in
a race? Even off power as long as you are rolling forward
both tires are rolling forward meaning no diff action is happening. I've always
read and been told that you need diffs to allow the front wheels to turn at
different rates. For example the front left travels farther than the front right
tire when making a sharp right turn. How does the non-oneway geared diff
allow the tires to turn at different rates when the gears can't slip?

The only scenario my little brain can come up with is that when one wheel
leaves the ground over rough terrain the tire in the air wants to spin
backwards translating into more power being transferred to the tire on the
ground...The diff oil controls whether or not that happens fast or slow. Is
that correct?

Someone pull back the curtain for me please. I need help.

Try this, have the car fired up and rev it slowly, watch the speed of the front tires. Now grab one front and hold it. Watch the speed on the other tires double. On the track, it is not such an exteme, but the same concept.
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Old 08-09-2006, 01:19 PM   #3
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Diffs come into play when one tire needs to spin faster than the other, such as going around a turn. The outside wheel will travel a greater distance than the other, so it needs to spin faster than the other to do so in the same amount of time.

They will never spin opposite one another while on power, but the ability to do so is the same as what allows one to travel faster than the other.

Normal diffs do not allow the tire on the ground to recieve more power than the unladen tire such as when cornering and the inside tires spin.. The thicker fluid in the diff helps the loaded tire to recieve more power than with thinner fluid, but the car will not rotate (carry as much speed in the middle of the corner) as well with thick fluid.

As with any adjustment, there are pros and cons. Finding the balance is the key.
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Old 08-09-2006, 01:25 PM   #4
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The inside tire travels a shorter path then the outside in a turn. The sharper the turn, the more pronounced the difference. A solid axle would drag the inside wheel. An open diff without any resistance from heavy oils would allow the outer tire to spin faster then the inner tire. This allows more power to be transfered to the inner tire. In race conditions, the inner tire wants to lift. An open diff will spin the inner tire loosing any power going through the diff. To compensate for this, heavy diff oils are used. The oil, limits the diff action which prevents the inner tire from spinning as much when it looses traction.

Your thoughts are correct though with the one tire not spinning backwards durring a race, but it does allow the tires to spin at different speeds under power.
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Old 08-09-2006, 03:03 PM   #5
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Thanks for the different views. Hearing the information in a different way is what I needed.

Okay to expand on that information. Let me know if I am off base.

The diff oil really only provides resistance between the left and right wheels. So does that mean...thicker oil would theoretically minimize the potential speed differences in the wheels (i.e. left at 5000 rpm, right at 4000 RPM) ? Thick oil in the front would tend to make the car push more in a tight corner off power right?

...and so then Thinner oil would theoretically allow the wheels to have a much greater variation in rotational speeds in relation to one another while off power going thru a tight turn?

Also the thinner the oil you use in your front, the more your gear diff behaves like a one-way. Is that right?
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Old 08-09-2006, 03:35 PM   #6
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Your pretty much right on except the one-way part. Since the wheels are never allowed to freewheel from the driveshaft, they wont act as a one-way.

Diffs react differently to different traction levels. If you run a tight diff in the rear on a high traction surface, it will cause the car to push since it will get more drive off the inside tire. If you run a tight diff in low traction, it will make the car loose since the car will always want to break at least one tire loose, taking the other with it.
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Old 08-09-2006, 07:28 PM   #7
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The new issue of XRC has a pretty detailed article on tuning buggy diffs. Check that out too.
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Old 08-10-2006, 09:07 AM   #8
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Thanks again everyone. I will check out that mag article.

Back to the action. I can see how a thick center diff oil would make the car shift toward all wheel drive. The thicker the oil the closer you get to it replicating a solid axle with no diff action. So the question is: Does going lighter necessarily shift the car back toward front wheel drive or does the diff just want to spin the front wheels more not necessarily gaining front wheel drive?
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Old 08-10-2006, 07:59 PM   #9
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A lighter oil will allow power to transfer to the tires that have less grip more easily.
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Old 08-10-2006, 08:13 PM   #10
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everything i have about diff's:

Diff tuning

Assuming we are talking about standard gear diffs. Not Torsen or Spider diffs. When the diff has very light oil in it, the outdrive cups will spin very easily in opposite directions.(diff action) So when the oil is very thick the outdrive cups will spin with a lot of resistance. This is important when one side of the diff becomes unloaded. ie, when one wheel looses traction.

This is most important in, but not restricted to, cornering. When turning the outside wheels have to spin faster since they have to travel farther than the inside wheels. The rear wheels are fixed pretty much staight so easier diff action is more important than in the front. The front wheels turn with the angle of the turning radius and the inside wheel usually turns sharper than the outside so on the front you need less diff action.
In the rear the lighter the oil the less power there is to one wheel if the other lifts or looses traction. The heavier the oil the more power to one wheel if the other wheel looses traction. Most important when turning because usually the inside rear wheel tends to unload. If you have too much drive power to the outside rear wheel when that happens then the rear end breaks loose.
In the front you want both front wheels to pull the buggy around the corner. When cornering chassis weight tends to transfer to the outside front wheel. So you want it to pull hard even if the inside front wheel unloads. So in the front the heavier the oil the more the outside front wheel will pull even if the inside front wheel unloads. This makes the buggy turn quicker under power up to a point. Too heavy wt oil can cause the buggy to push if there is not enough traction to the ground.

As for the center diff. It's still pretty much the same idea. The lighter the oil the less power to either end of the buggy. The heavier the oil the more power to both ends of the buggy. This is mostly important for keeping the buggy stable. If the center diff has light oil in it and one wheel at one end of the buggy looses traction then power to the other end drops off too. This momentarily slows the buggy down and lets the loose wheel regain traction. Heavy oil keeps the power to the opposite end of the loose wheel.

So, most of the time the rear wheels will loose traction more often than the fronts. You want the rear diff very light so when one wheel breaks free the other wheel stops pulling so the buggy does not swing around. It lets the loose wheel regain traction. In the front you want the wheel that still has traction to keep pulling when the other front wheel looses traction. All of this, most of the time, works out to a basic starting point of 3000/5000/1000 wt oil F/C/R.

If there is a lot of traction and you can put a lot of power to the ground then you want to stiffen up the diffs more, like 5000/5000/1000 or even 5000/7000/1000. If the surface is really loose and there is very little traction the you want to soften up the diffs. More like 3000/3000/1000 or so. This all works out to keeping the front end in front and the rear end in the rear. This is a generic setup for standard diffs. Torsens and Spiders work a bit differently. Thats a different story.
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
versa.
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
times.

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

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
action.


and


This is some info I gathered here on the Grid a couple of years ago. I
am not sure who posted it but it is some good info on diff's.

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, i.e. least amount of weight transferred, i.e. 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, i.e. 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. This will control torque
transfer from the left to front tire and vice versa. 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 turning (which happens at the very beginning
of the turn up until the point where your car has transferred 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, i.e. 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
times.

What I said above is not the be all end all gospel but just my
experience.

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 sway bar 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
action.

hope this helps!
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Old 08-12-2006, 09:46 AM   #11
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The site at the below link has some pretty good explanations.

http://www.rctek.com/general/differe...they_work.html
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Old 09-07-2006, 09:20 AM   #12
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I just wanted to thank you guys for the contribution to the thread. I had forgotten to do that. It definitely helped me learn about setup stuff that I previously didn't realize.
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Old 09-07-2006, 06:42 PM   #13
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what about the pro drivers set up.. its look very big different with our basic setup.. seem they use very thick diff oil and the thickest is at the rear end..

Jon Dell, the mechanic of Jon Hazlewood:
"we went from 10 000 in rear to 5 000 and from 7000 in center to 4000. This made the car easier to drive and for the future of the track this is a good change"

Yannick Aigoin, Team XRAY France:
"I started with 5000/7000/10 000 oils in the diffs but changed as the track got more gripy so I have changed to 10 000/10 000/20 000. I believe tomorrow I will still have to change the front oil for 15 000 or even 20 000 because I have still plenty of steering and I need to gain more stability in the corners"
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Old 09-07-2006, 07:36 PM   #14
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The hot setup on my track for Xray buggies is 357 or 257. It sounds weird but it works.

I wonder, is it necessary to use a thicker diff oil for long mains in the pro world because of heat buildup? I've only done 20 min mains so I have no clue.
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