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Diff Height Plunge

Old 12-09-2009, 04:43 PM
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Default Diff Height Plunge

Has anyone changed there diff height to angle the dogbones or keep them straight? I have messed with different heights and dont see a difference? Anyone notice a difference or the purpose really?
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Old 12-09-2009, 05:23 PM
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A higher diff will take away traction from that end of the car. Not sure why, but it does. I used to run the diff high in the rear and low in the front of my RDX. Haven't played with it in the Photon, because I don't have any rotation issues with this car.
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Old 12-09-2009, 05:32 PM
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I believe the theory is.. When you have more angle in the dogbones as they rotate they are trying to straighten back out. This pushes the tires into the track surface..
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Old 12-09-2009, 05:35 PM
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Originally Posted by smoke81 View Post
I believe the theory is.. When you have more angle in the dogbones as they rotate they are trying to straighten back out. This pushes the tires into the track surface..
I'll buy that.
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Old 12-09-2009, 06:32 PM
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To the OP:
As I'm assuming you have an Mi4, high diff in the rear and low spool in the front seems to be the hot ticket. Lots of rotation off power, but not too twitchy on power.
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Old 12-09-2009, 07:58 PM
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I don't buy any of this. My guess is that the possibility of having the diff up or down came about as a by-product of the eccentric belt tensioner adjusters and then the manufacturers had an idea and run with it without too much testing. Anything to boost sales. I will be convinced when I see a double blind experiment reproduced in controlled conditions by an independent institution and a peer reviewed paper published.
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Old 12-09-2009, 08:02 PM
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Originally Posted by niznai View Post
I don't buy any of this. My guess is that the possibility of having the diff up or down came about as a by-product of the eccentric belt tensioner adjusters and then the manufacturers had an idea and run with it without too much testing. Anything to boost sales. I will be convinced when I see a double blind experiment reproduced in controlled conditions by an independent institution and a peer reviewed paper published.
So you would also consider different length done bones as rubbish as well? Same theory.
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Old 12-09-2009, 08:03 PM
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Download the JRXS Type-R manual from losi.com, there is a good explanation in the tuning section about diff height.
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Old 12-09-2009, 08:15 PM
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Mechanical dog bone plunge or "bind" does indeed change handling characteristics.


High Diff; The car will stay flatter and have more responsive feel.

Low Diff; The car will roll more and increase on power steering.

Basics are:

Low front / high rear = maximum steering
High front / low rear = least steering
Low front / low rear = decrease overall traction
High front / high rear = increase overall traction

Last edited by Racecrafter; 12-09-2009 at 08:30 PM.
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Old 12-09-2009, 08:49 PM
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Think of it like roll center height and you'll get good results.
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Old 12-09-2009, 09:02 PM
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Originally Posted by Racecrafter View Post
Mechanical dog bone plunge or "bind" does indeed change handling characteristics.


High Diff; The car will stay flatter and have more responsive feel.

Low Diff; The car will roll more and increase on power steering.

Basics are:

Low front / high rear = maximum steering
High front / low rear = least steering
Low front / low rear = decrease overall traction
High front / high rear = increase overall traction
I may be wrong here, but there seems to be some contradictions here.

If lowering the diff makes the car roll more, would it not create more overall traction?
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Old 12-09-2009, 09:18 PM
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Originally Posted by mrrcguy View Post
I may be wrong here, but there seems to be some contradictions here.

If lowering the diff makes the car roll more, would it not create more overall traction?
This is described as "on-power" being the changes in dog bone plunge and bind. The diff won't actually let the car "roll more" than the suspension is set up for but in reference terms, will let it "roll more" than a higher diff set up.
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Old 12-09-2009, 09:22 PM
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Originally Posted by smoke81 View Post
So you would also consider different length done bones as rubbish as well? Same theory.
Don't get your facts mixed up.
There are different things which have obvious reasons (or it is obvious they actually change something) whereas others are not that clear. The latter need to be properly studied and tested before a conclusion can be drawn.

A longer dogbone can actually bind in the diff joint or the outer CV joint as the suspension (and steering on the front wheels) work up and down and side to side, so there's an obvious benefit to choose one over the other. Beyond that there may be other benefits/changes in the car's handling but again, that needs to be demonstrated as I explained above. Unfortunately, I don't think anyone has the time, money and resources to do it, so I will reserve judgement until someone does. "Feel" is not a quantifiable measure and can not be reproduced therefore it doesn't allow any reliable conclusions.

Here's a quote from the Losi manual suggested above:

Originally Posted by Losi JRX-S type R manual
Diff Height: Caution! When adjusting the differential heights, rotate the eccentrics as to loosen the belts, rotating the belts in the
opposite direction, with the belts installed can severely damage the belts. Diff heights in the Type R are also adjustable by rotating the
eccentrics that position them. The diffs can be adjusted from a full low position to a full high position. The low position will allow the car to
roll more and keep the car in the turn longer, increasing on-power steering. The high position will give the car a fl atter and more responsive
feel. It is also possible to change the balance of the car quite drastically by offsetting the heights of the diffs from front to rear. On asphalt
tracks with rubber tires, testing has shown the best results with the rear diff in the low position and the front diff in the high position. On
carpet with foam tires running the diffs in the low position has yielded the best results.
This is not an explanation. It is just a receipe (kinda like cooking recepies: mix this with that, cook, bake, etc, and you'll like it, goes with white wine). There's no actual explanation of what happens and why, no experimental data presented, and it doesn't satisfy any of the criteria explained above for a proper experiment (i.e. independent observer, double blind experiment, etc.). It does however use some of the trademark racer jargon words such as "feel" and other unquantifiables. As far as I am concerned this counts at best as anecdotic evidence, i.e. it may be true, but it needs to be examined properly before it can be considered demonstrated.
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Last edited by niznai; 12-09-2009 at 09:55 PM.
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Old 12-09-2009, 09:51 PM
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Raising and lowering the diff is just changing that bind. Maybe i'm wrong though...
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Old 12-09-2009, 09:57 PM
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The "bind" being refered to is the friction from the loading of the dog bone pin against the cup as its sliding across during plunge as the suspension moves.

Raising and lowering the diff effects the plunge characteristics.
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