Center diff setup question.

[BarnFabRC]

Last Friday while running I was really seeing the front tires ballon on hard acceleration. It was still taking off pretty quick though. Currently it’s got 200k in the center diff. I’ve seen several people saying they’re only running like 7k. That’s got to be like an open diff compared to the 200k.

When someone says they’re running 5-7-7 or something that I’m assuming it means 5k front 7k center and 7k rear. Or is the center diff always multiples of 100k & I’m just late to the party?

I want to put something thicker in but I’m already almost 30 times thicker than where they’re at. I feel like if I dropped to 7k this thing would just pull like a front wheel drive car doing a burnout.

Am I right? Or am I off by a multiple of 100k? Help make this make sense.

For reference, I’m running the AE 200k that came with my B74.1 kit. So I have to assume that’s the right stuff at 200k

 

[DavidB1126]

I use 500K for indoor carpet. If I remember correctly and understand, thicker the center diff oil, more the power to the rear wheels than the front? I found this chart in my files.

This is a @bill_delong question. Lol

 

[bill_delong]

FRONT-CENTER-REAR

That's typical reference

On my wheeler I'm currently running 10-60-5

I used to run a center slipper but learned it was way too twitchy where 60K in the center has been the sweet spot for me on both turf and clay.

It's perfectly normal for front tires to balloon on dirt, it comes down to driver preference on how much you want to car to respond. If the back end is breaking loose on you and you're having to over correct frequently then it may help to go down on your center diff.

For reference

my 1/8 eTruggy I run 10-7-4
my 1/8 eBuggy I run 7-7-3

There is no right/wrong, just a personal preference of how you want the car to respond to your driving style

Following from the HUDY Tuning Guide:

Following photo taken yesterday where my 1/8 eBuggy with 7K in the center will still lay down power and wheelie no prob:

 

[BarnFabRC]

This is a @bill_delong question. Lol

Indeed it is! and he delivered!

Bill thanks that was very helpful information. I’m going to have to change it up and try some different weights before the outdoor season is over. I think I’ll go up a step first and see what happens, it handles quite well now that I got some softer sprinters I just know it can get out of the corners quicker. If I like it I’ll stick with it. If it makes it too twitchy I’ll jump way down and try some looser stuff just to see. There was a pretty noticeable blue groove forming as we were coming into the mains. Something I’ve not really seen there before.

I think I’m running 15k in the 4x4 Slash and it’s okay. The rest of that truck is a mess though. Haha so I can’t base anything off of it.

 

[Ramp addict]

If you think 200k is too thin then I think you should up a bit. If you have too much resistance in your center diff it will make your truck prone to spinning out. Like everything else RC there's a compromise somewhere.

 

[Lumikko]

The setup reference says that thinner oil in the center causes the front wheels to unload more during acceleration. How is that possible? As soon as the front wheels start to slip, the driving force drops at the rear wheels. Wheelies are practically impossible.

If anything, I’d imagine thicker oil in the center to unload the front tires more – even to the point of popping a wheelie.

As for the original question: people generally use thinner diff fluids on rough tracks and/or low grip surfaces. The cars often require softer suspension, more droop, and can’t accelerate as quickly so the load transfer from front to rear is delayed and lower. As the grip difference F–R is smaller, you don’t need such thick oil in the center to reduce front wheel slip. Thinner center oil helps the rear wheels maintain grip and make the vehicle easier to control at the cost of maximum push. It also allows all tires to maintain better grip when cornering as all wheels travel a different distance. The viscosity can sometimes be quite low in these cases.

Rule of thumb: thin diff oils help stability on-power and don’t keep the vehicle on railroad tracks off-power. Exactly as you want when the grip is low.

Everything changes when you have a very high grip and perhaps a smoother surface. Stiffer springs, thicker sway bars, harder acceleration available, higher cornering speeds… all of these increase load transfer between the front and rear and left and right. Thicker diff fluids become necessary to prevent the wheels with less load from diffing out. And you have plenty of traction so the thicker fluids fighting against speed differences of the wheels off-power is less of a problem.

Thicker fluids make the car more difficult to control on-power but increase stability off-power. Again, this is exactly the right combination on high grip because controlling the power is naturally easier and there’s plenty of grip available to rotate the car into turns.

On high grip, it’s not unheard of for 1/10 buggies to use a million cst in the center. Some have a slipper which is basically a spool. So if 200k seems thin because you have lots of grip, don’t hesistate to go thicker. It’s the kit setup fluid for the B74.2D.

 

[RustyUs]

The setup reference says that thinner oil in the center causes the front wheels to unload more during acceleration. How is that possible? As soon as the front wheels start to slip, the driving force drops at the rear wheels. Wheelies are practically impossible.

If anything, I’d imagine thicker oil in the center to unload the front tires more – even to the point of popping a wheelie....

It's all about resistance between surface, tires, and fluid. Power will always want to go to the part with the least resistance. If you want equal power going to the front/rear tires, than a center spool/locked diff is what you want.

My first truggy/three diff RC, that I ever got, had weak front CV axles. I got fed up with replacing front axles all the time, and decided to attempt in making the truggy a 2wd platform for poops and laughs. Well, I thought it was just an easy, take out the front center driveline, and I'd be good to go. Wrong! When I went to give the truggy some throttle, all I got out of the truggy (center diff 10k fluid in it) was a useless spinning center diff. Only after putting earplugs in the center diff was I able to get power to the rear tires.

 

[Lumikko]

If you want equal power going to the front/rear tires, than a center spool/locked diff is what you want.

I think this is a good summary of the principle.

You need more of this ”equalizing” (thicker fluid) when the grip difference between the tires is large. When one tire breaks traction, the others still have lots of it: we want to keep pushing to go ahead and the goal is to prevent the slipping wheels from diffing out.

When the grip difference is small, a thinner fluid is necessary. When one tire breaks traction, you want things on the other side of the diff to calm down because it’s very close to slipping as well. This helps the car utilize the little bit of grip still left under the other wheels.