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Old 05-10-2019, 01:39 AM
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Originally Posted by rcgod View Post

How can you calculate the static coefficient of friction in a ball diff if you don't know the available traction, or the load?


Because a material's coefficient of friction is it's coefficient of friction - static or kinetic scalar value.

A 17.5 puts a lot less load on the diff than a 7.5
And yes there is side load on the diff balls from the gear under load. You can say it's minute but it's still there. Depending on diff ball material, gear material etc. So to make a general statement like that is misleading. You're basically saying all motor/car/traction situations give the same load.
No, it's not misleading to dispel myths, quite the contrary. I didn't ever say there were never any loads within the diff. Infact I stated that the diff balls will be fore or aft within the cage, no? Perhaps you missed that bit.

If you walk up your garden path and your trainers have a coefficient of, let's say a value of 2.0 with pavement/concrete. Imagine the same thing when you tread on a banana skin - your trainer's new coefficient is 0.1. Thus you slip. Would you find it easier if you were 10 stone? Or 15 stone? 20 stone? Or would the comedy results be the same - ninja skillz aside ;p

Same analogy with car vs lorry on black ice. Results: not good.

The materials we are talking about are the same within ball diffs. Principally injection moulded nylon/delrin, Carbide diff balls or ceramic diff balls. Steel diff rings.

Fact: There is less friction between the ball/cage interface than the ball/diff-ring interface (set by the torque held in place by the screw) which is orders of magnitude greater than any loads placed on the diff by a 1500g racing car. If this were not the case, 1) the diff simply wouldn't work as a diff and 2) the diff screw would continually be undone in use anyway.
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Old 05-10-2019, 03:37 AM
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Originally Posted by Horatio View Post
Because a material's coefficient of friction is it's coefficient of friction - static or kinetic scalar value.



No, it's not misleading to dispel myths, quite the contrary. I didn't ever say there were never any loads within the diff. Infact I stated that the diff balls will be fore or aft within the cage, no? Perhaps you missed that bit.

If you walk up your garden path and your trainers have a coefficient of, let's say a value of 2.0 with pavement/concrete. Imagine the same thing when you tread on a banana skin - your trainer's new coefficient is 0.1. Thus you slip. Would you find it easier if you were 10 stone? Or 15 stone? 20 stone? Or would the comedy results be the same - ninja skillz aside ;p

Same analogy with car vs lorry on black ice. Results: not good.

The materials we are talking about are the same within ball diffs. Principally injection moulded nylon/delrin, Carbide diff balls or ceramic diff balls. Steel diff rings.

Fact: There is less friction between the ball/cage interface than the ball/diff-ring interface (set by the torque held in place by the screw) which is orders of magnitude greater than any loads placed on the diff by a 1500g racing car. If this were not the case, 1) the diff simply wouldn't work as a diff and 2) the diff screw would continually be undone in use anyway.
Most ball diff gears holes that I have seen are not simply a round cylinder. They are a torus. When you assemble them the balls snap in, they don't fall in. This is to increase the friction when loaded because of increased surface contact. If it didn't make a difference then the holes would be simple cylinders. That would be easier to manufacture too. The torus shape isn't there to hold the balls in when building or maintaining the diff.

Different hole shapes have been tried over the years. If you change the shape of the pocket where the balls sit in the gear it affects the feel and performance of the diff on track. Square shaped, diamond shaped, cylindrical or torus. You can test this yourself by modifying a gear. That was being done years back by Schumacher owners to change the diff action. If the increase or decrease in contact surface area made no difference then why are they shaped the way they are? Again it's not to make maintenance easier.

The argument could be made that the reason for the torus shape is increased diff life. The wear on the gear would be greater if the contact area was smaller. But regardless of that, shape of the holes does change the way the diff works on track. If the side load under acceleration had no effect then there would be no difference.

I have tried changing the shape myself. Take a diff gear with torus shaped holes and drill them out so they're simple cylinders. Not saying to drill them to a bigger diameter because that would change other factors, just remove the torus shape. You will feel a difference in performance. Yes there is also the psychological effect. But I'm convinced if you did a blind study between them most drivers would prefer the unmodified gear.

Last edited by rcgod; 05-10-2019 at 03:51 AM.
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Old 05-10-2019, 04:11 AM
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I'm going to get a torque transducer and build a test rig so I can see exactly what different types of diffs do under load. That way I can prove to everyone exactly what different diffs do under load and we can end this discussion.
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Last edited by waitwhat; 05-10-2019 at 04:57 AM.
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Old 05-10-2019, 05:23 AM
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I honestly enjoy these types of threads and discussion. No one is being nasty. It's opening our minds up to other ways of thinking. Horatio and I might have to agree to disagree, but that's fine. I doubt I'll totally change his mind, and him mine. But the things that have been said here will hopefully make the readers think and make up their own minds.

Imo it's better than arguing over what car is best like 1000 other threads here.
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Old 05-10-2019, 05:29 AM
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Originally Posted by rcgod View Post
I honestly enjoy these types of threads and discussion. No one is being nasty. It's opening our minds up to other ways of thinking. Horatio and I might have to agree to disagree, but that's fine. I doubt I'll totally change his mind, and him mine. But the things that have been said here will hopefully make the readers think and make up their own minds.

Imo it's better than arguing over what car is best like 1000 other threads here.
Preach! This thread will pop up every time someone googles ball diff vs gear diff and all the information and different ideas will be right there for them to sort through.

Originally Posted by Horatio View Post
If you walk up your garden path and your trainers have a coefficient of, let's say a value of 2.0 with pavement/concrete.
I'm going to need whatever tire sauce is giving your shoes that coefficient of friction. Please.
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Old 05-10-2019, 05:39 AM
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Originally Posted by Krio View Post


I'm going to need whatever tire sauce is giving your shoes that coefficient of friction. Please.
I've been saucing my shoes for years. Reduced my corner marshaling time by .3 seconds. I thought that's why they call it Sticky Kicks. Lol.

I just had a brain storm. Selling shoes for rc racers. I'll have carpet, clay and blue groove compounds! Down side is none of the pros would use them. They don't corner marshal. 😁
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Old 05-10-2019, 06:11 AM
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Originally Posted by rcgod View Post


I've been saucing my shoes for years. Reduced my corner marshaling time by .3 seconds. I thought that's why they call it Sticky Kicks. Lol.

I just had a brain storm. Selling shoes for rc racers. I'll have carpet, clay and blue groove compounds! Down side is none of the pros would use them. They don't corner marshal. 😁
Hey, Maifield had an engine sponsor and didn't use their engine. I fail to see the problem with a pro advertising my product and not using it.
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Old 05-10-2019, 09:00 AM
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Originally Posted by Krio View Post
Preach! This thread will pop up every time someone googles ball diff vs gear diff and all the information and different ideas will be right there for them to sort through.



I'm going to need whatever tire sauce is giving your shoes that coefficient of friction. Please.
You dislike the notion that a material can have a coefficient of greater than 1? Erm, sorry dude - but that will typically be things like rubber tyres or trainers on tarmac.
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Old 05-10-2019, 09:56 AM
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....with my special oil of winter green brew, with it's combination of silicone and post it notes other secret ingredients you too can have a coefficient of friction greater than 1! Whether it be your favourite pair of trainers, or that 'speshal' ball diff that got you into the A final in 2017.

Get a grip.

Get my speshal grip.....
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Old 05-10-2019, 11:01 AM
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Originally Posted by rcgod View Post


Most ball diff gears holes that I have seen are not simply a round cylinder. They are a torus. When you assemble them the balls snap in, they don't fall in. This is to increase the friction when loaded because of increased surface contact. If it didn't make a difference then the holes would be simple cylinders. That would be easier to manufacture too. The torus shape isn't there to hold the balls in when building or maintaining the diff.

Different hole shapes have been tried over the years. If you change the shape of the pocket where the balls sit in the gear it affects the feel and performance of the diff on track. Square shaped, diamond shaped, cylindrical or torus. You can test this yourself by modifying a gear. That was being done years back by Schumacher owners to change the diff action. If the increase or decrease in contact surface area made no difference then why are they shaped the way they are? Again it's not to make maintenance easier.

The argument could be made that the reason for the torus shape is increased diff life. The wear on the gear would be greater if the contact area was smaller. But regardless of that, shape of the holes does change the way the diff works on track. If the side load under acceleration had no effect then there would be no difference.

I have tried changing the shape myself. Take a diff gear with torus shaped holes and drill them out so they're simple cylinders. Not saying to drill them to a bigger diameter because that would change other factors, just remove the torus shape. You will feel a difference in performance. Yes there is also the psychological effect. But I'm convinced if you did a blind study between them most drivers would prefer the unmodified gear.
I think changing the shape of the 'holes' in the diff cage is more likely to negatively effect the operation of the diff than give any real, quantifiable performance advantage.

With all the (many!) diffs I've built, in almost every case , the balls fit with a close clearance fit. When diffs are new, sometimes the balls snap into place. But more often, you have to use a little blob of grease to hold them in place or risk looking for balls....wait a moment, that just sounds wrong.

Square, Rectangle, Diamond - whatever - the balls are spherical and held in place by the diff rings, which often have tracks on them that match the radius of the balls for them to roll in (or on?). The balls cannot escape upwards or downwards - no matter what loads are applied. The only movement (within a healthy ball diff) is fore and aft within the cage depending on whether the car is accelerating or decelerating.
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Old 05-10-2019, 11:05 AM
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Originally Posted by Horatio View Post
You dislike the notion that a material can have a coefficient of greater than 1? Erm, sorry dude - but that will typically be things like rubber tyres or trainers on tarmac.
I don't dislike the notion of anything greater than 1 for a coefficient of friction. I was trying to bring some lighthearted banter to the thread and a garden walking path with a 2.0 coefficient of friction with any shoes sounded like hyperbole to me.

A quick search puts many athletic shoes around 0.6-0.8 on asphalt for static and 0.3-0.6 for dynamic. Passenger tires are 0.5 to 0.9. The tesla model S with Ludicrous package taps out around 1.5 with the included tires. Anything over 2 requires some primo equipment and a prepped surface.
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Old 05-10-2019, 11:32 AM
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Originally Posted by rcgod View Post
I honestly enjoy these types of threads and discussion. No one is being nasty. It's opening our minds up to other ways of thinking. Horatio and I might have to agree to disagree, but that's fine. I doubt I'll totally change his mind, and him mine. But the things that have been said here will hopefully make the readers think and make up their own minds.

Imo it's better than arguing over what car is best like 1000 other threads here.
No arguing here - just healthy debate and critical thinking. Where people are looking for answers to all their ball-diff related pseudo-science, they'll be no shortage of material.
;p

Especially if people are trying to confirm whether or not their ball diffs are limited slip!
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Old 05-10-2019, 11:43 AM
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Originally Posted by Krio View Post
I don't dislike the notion of anything greater than 1 for a coefficient of friction. I was trying to bring some lighthearted banter to the thread and a garden walking path with a 2.0 coefficient of friction with any shoes sounded like hyperbole to me.

A quick search puts many athletic shoes around 0.6-0.8 on asphalt for static and 0.3-0.6 for dynamic. Passenger tires are 0.5 to 0.9. The tesla model S with Ludicrous package taps out around 1.5 with the included tires. Anything over 2 requires some primo equipment and a prepped surface.
And my banana skin vs super sticky trainers on the other hand wasn't in the least bit light hearted?

Yes - but the point that seems to be getting lost in noise is that whether it's 1 (a normal grippy value) or 12 - when the value drops to the coefficient we see in our ball diffs, people fall on their arse! Lol.

That's the gist of it. Did you look up the coefficient of banana skins while you were at it?........
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Old 05-10-2019, 12:00 PM
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Originally Posted by Horatio View Post
And my banana skin vs super sticky trainers on the other hand wasn't in the least bit light hearted?

Yes - but the point that seems to be getting lost in noise is that whether it's 1 (a normal grippy value) or 12 - when the value drops to the coefficient we see in our ball diffs, people fall on their arse! Lol.

That's the gist of it. Did you look up the coefficient of banana skins while you were at it?........
Didn't have to look up banana skins. I saw the mythbusters episode on it.
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Old 05-10-2019, 12:21 PM
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Not sure if anyone is familiar with the RCShox center diff for the SC10 4x4, but I helped with some of the design and testing. Here's a refresher for anyone who wisely chose not to buy the SC10 4x4 and try to fix all its problems:

SC10 4x4 Center Diff installation guide - RCShortCourse

Anyways, the next design iteration had the pins shown in the post offset by 45 degrees, not right in the same radius as the diff balls show in that thread in the first few posts. This was to 'pinch' the ball against the wall to create more resistance to diffing out. You could install none or all of the pins. You could also flip the gear and have it pinch more on acceleration or braking. It worked better than the stock slipper setup on most tracks, but one issue was how much centripetal force played a roll. As you accelerated, the diff would progressively lock up as the rpm increased. This is less of a factor in the front and rear ball diffs as they are spinning ~2.5 times slower than the spur gear/center diff. Also, the 3/32" or 3mm balls in most diffs are much lighter than the ones used in the SC10 4x4 diff.

Anyways, I didn't really have a point to this post. Just more information on testing that's been done with ball diffs, good or bad.
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