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Adjusting Torque Split

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DrFranz

DrFranz

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So I want to direct more drive to the front axle on my 4WD cars (Traxxas Fiesta Rallye and Stampede). What's the best way to do this? Alter the diff fluids? Or?
 
heavy in front .lighter in rear. then a weight in the middle of the 2 for mid
 
DrFranz said:
So I want to direct more drive to the front axle on my 4WD cars (Traxxas Fiesta Rallye and Stampede). What's the best way to do this? Alter the diff fluids? Or?
Click to expand...

Does your car have a center fluid filled diff or a slipper?

If it's a slipper then it might have loosened on you and may need fresh slipper pads if they are slipping too much and it's already tightened all the way.

If it's a fluid filled diff and you haven't rebuilt the car within the last 2 hours of use, then chances are the diff fluid has failed and/or leaked causing the car to "diff out".

Generally center slippers at the loosest setting are close to 100K diff fluid in comparison.

My starting preference is 10K Front - 7K Center - 3K Rear with the following tuning guide to help you make changes to suit your preference from there:

https://storage.googleapis.com/sodialed-help-files/manuals-and-reference/hudy%20off-road%20setup%20book.pdf

Also a good idea to check this thread out as well:

1722005746379.png
 
These things don’t exactly have torque splitting. The diffs simply allow the side with less traction to spin faster on power and we slow down the difference with thicker fluids.

What do you want to achieve when it comes to handling? More on-power steering, better rotation in corners? Punchier acceleration?

Some vehicles allow you to set different gear ratios for the front and rear to get an overdrive to either end. This arrangement actually does drive one side more than the other but is not done within the diffs.
 
I just want to send more drive to the front, it seems the rears are spinning and doing too much work on loose gravel. Seems a little help from up front would be nice.
So, that means heavier fluid in the front?
 
DrFranz said:
I just want to send more drive to the front, it seems the rears are spinning and doing too much work on loose gravel. Seems a little help from up front would be nice.
So, that means heavier fluid in the front?
Click to expand...
Sending more drive to the front is not a handling thing you can achieve. It might be a means to get a certain kind of handling.

Lack of rear grip during acceleration is a much better description of a handling problem. When you experience a handling issue, it’s extremely important to take note the situation where you notice it: on the straights or in corners? If it’s in the corners, does it happen during corner entry, mid-corner, or corner exit? Is it on power or off power, or braking?

It’s rare for a 4WD buggy to maintain grip in the front and have the rear tires slip when you accelerate. It’s usually the other way around: front tires slipping and the center diff allowing them to do so. On a loose surface this is desirable to maintain control. You only go to heavier center diff fluids on high traction when the front wheel spin is caused by the extreme load transfer from accelerating hard. You can’t accelerate that hard on a loose surface.

If you have the rear wheels spin when you punch it on a straight, there’s a serious grip imbalance between the front and rear tires. The diff fluid weight in the front won’t do much about that.

Back to the original issue: if your rear tires slip too soon and you want a little help from the front tires, you could try adding anti-squat to the rear. This change pushes the rear tires harder into the track and unloads the front tires more quickly.

Another thing you can do is move the weight bias to the rear: rear hubs forward, battery in the rear position, and so on.

Third, you can reduce the amount of negative camber in the rear. This adds rear drive without taking anything away from the front. Enough toe-in in the rear is also important.

Yet another thing to experiment with is harder springs in the rear or softer in the front. Either change adds rear grip when you accelerate, but you must note that it’ll shift the grip balance during cornering the opposite way: harder springs in the rear reduce overall grip in the rear and increase it in the front. Softer front springs, same effect. You can compensate for this with the choice of sway bars: e.g. if you got the acceleration right with harder rear springs but the car now oversteers in corners, a softer rear sway bar helps to get that rear cornering grip back.

Aw sheesh, I’m rambling again. Better post this reply before things get out of hand.
 
The description of what the OP has now and what he wants sounds to me like there’s a center diff installed already. If there was a spool in the center driveline, the front wheels would work as hard as the rears.

DrFranz said:
I just want to send more drive to the front, it seems the rears are spinning and doing too much work on loose gravel. Seems a little help from up front would be nice.
Click to expand...
 
Lumikko said:
The description of what the OP has now and what he wants sounds to me like there’s a center diff installed already. If there was a spool in the center driveline, the front wheels would work as hard as the rears.
Click to expand...
I agree with your previous post. My comment about heavier diff fluid in the front was because the only way I can see the front having more drive at all than the rear is locking the front diff down more, allowing the rear diff to have more slip. That way both front tires are always turning, and the rear allowing one tire to slip under certain conditions. Or put an overdrive gear in the front diff housing. This whole question the OP has posed here is basically impossible otherwise. And even changing the diff fluids is only going to have a slight effect, if any, and only under certain low traction conditions. 1:1 cars only accomplish this through computers interacting withing the drivetrain.
 
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FWIW: Those two vehicles that the OP has (Traxxas Fiesta Rally and Stampede) don't come with a center diff from the factory. They are an accessory.

I only mention it because most times someone will ask how to adjust the center diff when they go to install one or how to adjust right after installing one.
 
WickedFog said:
I agree with your previous post. My comment about heavier diff fluid in the front was because the only way I can see the front having more drive at all than the rear is locking the front diff down more, allowing the rear diff to have more slip. That way both front tires are always turning, and the rear allowing one tire to slip under certain conditions. Or put an overdrive gear in the front diff housing. This whole question the OP has posed here is basically impossible otherwise.
Click to expand...
Goes to show how important it is to describe the symptoms in a way that helps others help you. The advice you gave looks solid to me.

Heavier fluid in the front should help the front wheels drive more when cornering. I visualize it like this: at the limit of traction, the inside wheel will be the first one to slip. When that happens, the outside wheel loses a lot of driving force if the diff is light. Like a 1:1 car with one wheel on an icy patch; the other wheel doesn’t move you anywhere.

With all this going on in the front, the rear is still pushing forward with full force and both wheels maintaining traction. A center diff would reduce the driving force of the rear wheels when the front loses grip but you don’t have that with a slipper. So the car understeers.

Switching to a heavier fluid in the front prevents a diff out when the inner wheel is unable to maintain grip. This means that the outer wheel still pulls the car forward -> goes where the front tires are pointing.

Because the front wheels travel a longer distance when cornering, they have what I would call ”geometric underdrive” (just came up with that one) since the rear wheels are basically taking a shortcut through the corner.

You can counter this effect with overdrive gears in the front, as you suggested, or a center diff to turn the push from the rear more into all wheels actually driving.

Darn, rambling again and I have no idea who made it this far – and if any of this makes sense.

So, in a nutshell, use a high enough temperature to properly fry the surface of the beef and leave some red inside.
 
Lumikko said:
Goes to show how important it is to describe the symptoms in a way that helps others help you. The advice you gave looks solid to me.

Heavier fluid in the front should help the front wheels drive more when cornering. I visualize it like this: at the limit of traction, the inside wheel will be the first one to slip. When that happens, the outside wheel loses a lot of driving force if the diff is light. Like a 1:1 car with one wheel on an icy patch; the other wheel doesn’t move you anywhere.

With all this going on in the front, the rear is still pushing forward with full force and both wheels maintaining traction. A center diff would reduce the driving force of the rear wheels when the front loses grip but you don’t have that with a slipper. So the car understeers.

Switching to a heavier fluid in the front prevents a diff out when the inner wheel is unable to maintain grip. This means that the outer wheel still pulls the car forward -> goes where the front tires are pointing.

Because the front wheels travel a longer distance when cornering, they have what I would call ”geometric underdrive” (just came up with that one) since the rear wheels are basically taking a shortcut through the corner.

You can counter this effect with overdrive gears in the front, as you suggested, or a center diff to turn the push from the rear more into all wheels actually driving.

Darn, rambling again and I have no idea who made it this far – and if any of this makes sense.

So, in a nutshell, use a high enough temperature to properly fry the surface of the beef and leave some red inside.
Click to expand...
The last sentence 😅

Yeah, you describe it in a lot more detail, but that is basically what I was getting at. And the center diff explanation is nailed there. The front and rear diffs start reacting to side to side weight transfer, while a center diff would react to front to rear weight transfer. That allows for a lot of tuning options as far as braking, accelerating, and cornering. But as far as the original question of this thread, the only real way to achieve that is with overdrive/underdrive diff gearing of the ring and pinion.

But my mind wanders...
This basically explains how anti-roll bars are important on a track car. And it gets me thinking, if they are so effective on keeping both left and right tires planted, why not implement a front to rear anti-roll bar on an RC with a center diff? 🤔😉
 
WickedFog said:
But my mind wanders...
This basically explains how anti-roll bars are important on a track car. And it gets me thinking, if they are so effective on keeping both left and right tires planted, why not implement a front to rear anti-roll bar on an RC with a center diff?
Click to expand...
I’ve also thought about a way to link the front and rear suspension together in an elastic fashion to reduce squatting and diving. Citroen did something like this decades ago to keep the body more level over bumps.

About the sway bar keeping the tires planted on each side, it sounds a bit confusing to me since sway bars increase the load transfer rather than reduce it. But they do reduce overall grip so if your car is lifting the inner wheels in a corner, you can make the wheels slide out instead. A car that drives flat through corners has a deceiving appearance of balance as the wheels are less equally loaded most of the time.
 
Terrific insights, guys. Amazing to me that all this applies no matter what the scale, cuz these are the discussions we'd have when I was racing full scale. (Note I did not say "real cars".) The only difference is now we're talking 4wd.

I think I'm gonna go with the center diff, as that will have the greatest impact and be the most tuneable.
 

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