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Old 05-03-2019, 08:32 PM
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i do find this to be one of the more amusing threads to read.

popcorn done...

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Old 05-04-2019, 05:59 AM
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Originally Posted by Davidka View Post


It isnít that simple. Thatís still testing the diffs in an unloaded state. The balls are doing the job of the spider gears, but as power & load are added, friction between them and the diff rings doesnít increase. Friction does increase in the pasrts of a gear diff under load. They do the same job, but not exactly the same way.

So set the car down on your pit table and let the free tire burnout on the table.

Whatever posi you get from the ball diff, it's only there for a short bit during throttle up, then drifts off and feels like a gear diff with light lube. At this point, I only want to see ball diffs in onroad.
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Old 05-04-2019, 11:12 AM
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Originally Posted by waitwhat View Post
There is your limited slip differential that operates like a ball diff...

Tell me Horatio, when was the last time you set a lap record?

​​​​​
And still you ignore the questions we asked you! Lol.

Indoors at an 1/10th touring super stock National meeting. I was happy with the fastest lap, but I was happier with my average lap vs fastest lap being separated by only .3 of a second.
Around 2005 with a Yokomo SD SSG RS conversion.

Outdoors: My trusty Savage set lap records at club for the best part of 2 years.

My Cat 3000 set the odd lap record at regional level around up to about 2006. I often raced with some actual Schumacher team drivers.

I also raced my Mugen MBX4 RR with RB WS7. It was great fun and a fairly large percentage of our predominantly electric (pre-brushless era) racers bought nitro buggies or mt's. We hosted numerous BRCA, RRCI, and WRCA rounds.

How about you? Have you tried club/regional/national racing or anything organised?

Most of my electric race cars were ball diffs. I got on well with most of them. Ball diffs work well in 1/12th and 1/10th brushed electric classes. Now, with all the extra HP from brushless, it seems like gear diffs are gaining popularity.


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Old 05-04-2019, 11:25 AM
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Originally Posted by waitwhat View Post
Also, unless you change the fluid very frequently, your gear diff performance changes so drastically that the car can go from good to shit in the course of a few runs.

I can get a whole season out of a ball diff between services. I can't get a whole season out of a gear diff between services.
I think big temperature change can effect fluid filled diffs, that's for sure. It alters the viscosity of the silicone - same as it can the shocks. Re- filling a gear diff is a bit of a chore, but it's much easier accessing diffs on modern cars.

On the plus side, gear diffs are sealed, so unless there's an issue, the diffs should be good for a while.

Ball diffs on belt - hmmmm, not so much. Off road or indoors - even with shafties, ball diffs need tlc to keep them spotless clean and lubed.

Most people service either type of diff more frequently than once a season! Crikey.
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Old 05-04-2019, 11:37 AM
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Originally Posted by Horatio View Post
And still you ignore the questions we asked you! Lol.

Indoors at an 1/10th touring super stock National meeting. I was happy with the fastest lap, but I was happier with my average lap vs fastest lap being separated by only .3 of a second.
Around 2005 with a Yokomo SD SSG RS conversion.

Outdoors: My trusty Savage set lap records at club for the best part of 2 years.

My Cat 3000 set the odd lap record at regional level around up to about 2006. I often raced with some actual Schumacher team drivers.

I also raced my Mugen MBX4 RR with RB WS7. It was great fun and a fairly large percentage of our predominantly electric (pre-brushless era) racers bought nitro buggies or mt's. We hosted numerous BRCA, RRCI, and WRCA rounds.

How about you? Have you tried club/regional/national racing or anything organised?

Most of my electric race cars were ball diffs. I got on well with most of them. Ball diffs work well in 1/12th and 1/10th brushed electric classes. Now, with all the extra HP from brushless, it seems like gear diffs are gaining popularity.
I set a lap record this week. Not just fastest lap of the day, the fastest lap that has ever been made on this layout for this class. I've been building ball diffs for thirty years. I've run roar nationals and regionals and have had chassis motor and battery sponsors (in the days when sponsorships were not handed out to anyone with a social media profile). Ive got on airplanes to fly to races, I've raced the Reedy race. Now I race locally and regionally for fun only. No sponsors, I do it for the fun of chasing a perfect run, to push myself to be a better driver, and to make the car I campaign as good as it can be. I can post a pic of all the trophies I've collected for the last 3 years if you'd like.

Fast 1/12 scale cars on high grip carpet run spools now a days. My all time track record holding car has 1M weight fluid in the diff now and if you manually work the diff by hand it is incredibly stiff, until the fluid moves out of the way and can't fill back in between the gear teeth. At this point the diff becomes incredibly easy (by comparison) to how it feels at lower levels of differential action.

Things evolve, and unless you are still in the game of pushing the limits of lap times, your knowledge becomes dated.

Last edited by waitwhat; 05-04-2019 at 11:51 AM.
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Old 05-04-2019, 11:49 AM
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Originally Posted by Horatio View Post
I think big temperature change can effect fluid filled diffs, that's for sure. It alters the viscosity of the silicone - same as it can the shocks. Re- filling a gear diff is a bit of a chore, but it's much easier accessing diffs on modern cars.

On the plus side, gear diffs are sealed, so unless there's an issue, the diffs should be good for a while.

Ball diffs on belt - hmmmm, not so much. Off road or indoors - even with shafties, ball diffs need tlc to keep them spotless clean and lubed.

Most people service either type of diff more frequently than once a season! Crikey.
I spend 30 minutes breaking in my ball diffs. I do this to avoid cooking the grease out of the diff. It takes that long because if you tighten the diff too much (or use too much speed) the diff will get hot and the diff will dry out quickly. I fully understand how important a diffs smooth function is to the performance of a car. My diffs take a set before they ever see a track. I've got about 1/2 turn adjustment on a ball diff, and I'll make adjustments in 1/8 to 1/16 of a turn. Plenty of adjustment when you are very in tune with the performance of a vehicle.

Remember the XX4? I ran those for years without a slipper. I never had a problem with diffs or belts because i know how to drive and I know how to build a car.

Gear diffs are a pain in the ass. Best way to deal with them is a pair (or more) of diffs you can swap in. Forget that noise. I'll take a ball diff any day.
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Old 05-04-2019, 01:14 PM
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Originally Posted by waitwhat View Post
I spend 30 minutes breaking in my ball diffs. I do this to avoid cooking the grease out of the diff. It takes that long because if you tighten the diff too much (or use too much speed) the diff will get hot and the diff will dry out quickly. I fully understand how important a diffs smooth function is to the performance of a car. My diffs take a set before they ever see a track. I've got about 1/2 turn adjustment on a ball diff, and I'll make adjustments in 1/8 to 1/16 of a turn. Plenty of adjustment when you are very in tune with the performance of a vehicle.

Remember the XX4? I ran those for years without a slipper. I never had a problem with diffs or belts because i know how to drive and I know how to build a car.

Gear diffs are a pain in the ass. Best way to deal with them is a pair (or more) of diffs you can swap in. Forget that noise. I'll take a ball diff any day.
I remember the XX4. Around that time, I was racing the Bosscat Works, mainly indoors. The ball diffs and pulleys on them could be a real PITA.

Unlike you, I have no issue with gear diffs - in 1/8th Buggy, ball diffs aren't an option!

My experience with ball and gear diffs come from years of racing in different classes. As you rightly point out, my experience isn't from racing ball and gear diffs in the same class.

I no longer race, but I still run RC cars and fly large scale aircraft. Other commitments prevent me from racing in 2019.

That being said, ball diffs from 2019 are little different from those in the 90's.

Surely, I'm still qualified to play devil's advocate with you though? People have bought popcorn and everything! ;p
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Old 05-04-2019, 03:34 PM
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Originally Posted by Horatio View Post
I remember the XX4. Around that time, I was racing the Bosscat Works, mainly indoors. The ball diffs and pulleys on them could be a real PITA.

Unlike you, I have no issue with gear diffs - in 1/8th Buggy, ball diffs aren't an option!

My experience with ball and gear diffs come from years of racing in different classes. As you rightly point out, my experience isn't from racing ball and gear diffs in the same class.

I no longer race, but I still run RC cars and fly large scale aircraft. Other commitments prevent me from racing in 2019.

That being said, ball diffs from 2019 are little different from those in the 90's.

Surely, I'm still qualified to play devil's advocate with you though? People have bought popcorn and everything! ;p
Absolutely! I enjoy a good in depth discussion.

I also ran a gear diff in my 2wd buggy on carpet. This was the same car I built for running on dirt. I had to clean the mud off of it to be able to run it on the carpet. I also put carpet tires on it to race. I kept the shock package the same and made setup changes throughout the day. Even without carpet springs and sway bars I was able to set fastest lap and win that day. My friend has a carpet specific car and he was a little impressed with the pace of my dirt car. I don't know what weight fluid is in the gear diff. I suspect I could get my car much faster with the correct springs, sway bars, and known diff fluid. I just don't care to do that to my dirt car. I actually think if I tried slicks instead of pin tires I could go faster. If I wanted to get froggy I could get foam tires and let it eat. I'd rather build more on-road cars than build a jumpy carpet anything. TC's now run gear diffs and lcg shocks/rotary dampers. Last time I ran TC it was tall shocks, ball diffs, spools or one ways. Last time I ran 1/12 scale it was ball diffs. Now they run spools. My 1/10th 4wd buggy came stock with three gear diffs. The center diff makes the car too dynamic for my taste. I built my own slipper clutch because I like the feel of a full time 4wd. Made the car much easier to push hard.

I actually had a thought just the other day about making ball diffs for an 1/8th scale buggy. It would be a torturous environment for a ball diff. Last time I ran 1/8th nitro I used a buku racing clutch. Nobody could drive my car because the clutch engagement was so smooth that they thought the car felt "electric". They said they felt like the throttle was sticking, but the car had tons of traction because of how gradually the clutch engaged. Blipping the throttle was not the way to drive the car. Smooth trigger pulls, and carrying speed. When you drove like that the car was so fast and easy to drive. You had to really use the brakes because it carried so much speed. I think under those conditions I might be able to get a ball diff to survive. There is also the prospect of a slipperential in place of a center diff. I just have no desire to deal with the work and cost of an 1/8th scale nitro program right now.
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Old 05-04-2019, 06:51 PM
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Originally Posted by Zerodefect View Post
So set the car down on your pit table and let the free tire burnout on the table.

Whatever posi you get from the ball diff, it's only there for a short bit during throttle up, then drifts off and feels like a gear diff with light lube. At this point, I only want to see ball diffs in onroad.
Weíre misunderstanding each other. I think we agree much more than we disagree.

Originally Posted by Horatio View Post

That being said, ball diffs from 2019 are little different from those in the 90's.

;p

They arenít. Theyíre exactly the same.
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Old 05-04-2019, 11:50 PM
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Originally Posted by Davidka View Post


Weíre misunderstanding each other. I think we agree much more than we disagree.




They arenít. Theyíre exactly the same.
Originally Posted by waitwhat View Post
Absolutely! I enjoy a good in depth discussion.

I also ran a gear diff in my 2wd buggy on carpet. This was the same car I built for running on dirt. I had to clean the mud off of it to be able to run it on the carpet. I also put carpet tires on it to race. I kept the shock package the same and made setup changes throughout the day. Even without carpet springs and sway bars I was able to set fastest lap and win that day. My friend has a carpet specific car and he was a little impressed with the pace of my dirt car. I don't know what weight fluid is in the gear diff. I suspect I could get my car much faster with the correct springs, sway bars, and known diff fluid. I just don't care to do that to my dirt car. I actually think if I tried slicks instead of pin tires I could go faster. If I wanted to get froggy I could get foam tires and let it eat. I'd rather build more on-road cars than build a jumpy carpet anything. TC's now run gear diffs and lcg shocks/rotary dampers. Last time I ran TC it was tall shocks, ball diffs, spools or one ways. Last time I ran 1/12 scale it was ball diffs. Now they run spools. My 1/10th 4wd buggy came stock with three gear diffs. The center diff makes the car too dynamic for my taste. I built my own slipper clutch because I like the feel of a full time 4wd. Made the car much easier to push hard.

I actually had a thought just the other day about making ball diffs for an 1/8th scale buggy. It would be a torturous environment for a ball diff. Last time I ran 1/8th nitro I used a buku racing clutch. Nobody could drive my car because the clutch engagement was so smooth that they thought the car felt "electric". They said they felt like the throttle was sticking, but the car had tons of traction because of how gradually the clutch engaged. Blipping the throttle was not the way to drive the car. Smooth trigger pulls, and carrying speed. When you drove like that the car was so fast and easy to drive. You had to really use the brakes because it carried so much speed. I think under those conditions I might be able to get a ball diff to survive. There is also the prospect of a slipperential in place of a center diff. I just have no desire to deal with the work and cost of an 1/8th scale nitro program right now.
I used to run Carbon Fiorini Slider clutches. Everybody else ran Aluminium shoes and the thickest springs. Then the throttle tended to be used like an on/off switch. Running both nitro and electric classes all summer, having my Mugen have a progressive throttle response made the go-to easier. What I also noticed was everybody seemed to have clutchbell issues - overheating, knackered bearings, having to file their clutches etc etc.

Buku clutches use a proper clutch material lining on them don't they? Are those the ones you use?

Ball diffs in 1/8th Buggy?! Well Schumacher used them in their 1/10th Nitros. I'm just not convinced that they'd hold up well to the kind of HP you get from Nitro/Brushless 1/8th.

Cecil once said that ball diffs work better with belt drive than shaft drive. Never satisfactorily got to the bottom of why this should be though.
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Old 05-05-2019, 06:56 AM
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Originally Posted by Horatio View Post

Ball diffs in 1/8th Buggy?! Well Schumacher used them in their 1/10th Nitros. I'm just not convinced that they'd hold up well to the kind of HP you get from Nitro/Brushless 1/8th.

Cecil once said that ball diffs work better with belt drive than shaft drive. Never satisfactorily got to the bottom of why this should be though.
I believe the early AE 1/8 nitro on-road cars had ball diffs. Itís scaleable, but probably not practical for a 1/8 buggy, because the diameter would be too large to achieve the desired outdrive height.

i think ball diffs arenít as good with shaft drive because of the twisting torque load applied to them by the ring & pinion gear. I still preferred them to the gear diffs in my B44.
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Old 05-05-2019, 07:40 AM
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Ball diffs do bind up under acceleration and load. You can't go by how it feels on the table by lifting one wheel and unloading the other. The diff isn't really under load then.

When there is a load on both rear wheels the ball diff tightens up. It tightens in proportion to how much load is applied.

This is because the diff gear is basically floating between the diff rings and held in position from rotating by the balls. As the load on the diff gear increases it tries to spin and puts a side load between itself and the balls. So it effectively binds up and starts acting like it's tighter. Then when the load is lowered like when the car is off power it frees up. Yes a ball diff can unload just like a gear diff when you lift a wheel off the ground. But as long as there's a load on the diff driving the car forward the ball diff gets tighter. It's effectively a limited slip diff.

Thats why a lot of guys don't like ball diffs on high bite. It binds a lot when accelerating out of a corner and wants to lift the front end and the car pushes.

Gear diffs also bind up under load but to a lesser degree than ball diffs. But a gear diff is mostly dependent on the fluid viscosity. So if you want more forward bite and increase the fluid weight, you are going to compromise the ability of the diff action when off power in the corner. So you typically get a push, because there is a lot of drag from the rear of the car.

And a high bite track especially with a mod motor is super hard on ball diffs. If the balls slip one time the diff is done and needs to be rebuilt with new balls and rings which isn't cheap.

Last edited by rcgod; 05-05-2019 at 08:08 AM.
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Old 05-09-2019, 04:12 AM
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Originally Posted by rcgod View Post
Ball diffs do bind up under acceleration and load. You can't go by how it feels on the table by lifting one wheel and unloading the other. The diff isn't really under load then.

When there is a load on both rear wheels the ball diff tightens up. It tightens in proportion to how much load is applied.

This is because the diff gear is basically floating between the diff rings and held in position from rotating by the balls. As the load on the diff gear increases it tries to spin and puts a side load between itself and the balls. So it effectively binds up and starts acting like it's tighter. Then when the load is lowered like when the car is off power it frees up. Yes a ball diff can unload just like a gear diff when you lift a wheel off the ground. But as long as there's a load on the diff driving the car forward the ball diff gets tighter. It's effectively a limited slip diff.

Thats why a lot of guys don't like ball diffs on high bite. It binds a lot when accelerating out of a corner and wants to lift the front end and the car pushes.

Gear diffs also bind up under load but to a lesser degree than ball diffs. But a gear diff is mostly dependent on the fluid viscosity. So if you want more forward bite and increase the fluid weight, you are going to compromise the ability of the diff action when off power in the corner. So you typically get a push, because there is a lot of drag from the rear of the car.

And a high bite track especially with a mod motor is super hard on ball diffs. If the balls slip one time the diff is done and needs to be rebuilt with new balls and rings which isn't cheap.
The perception that ball diffs are in some way 'limited slip' stems from the notion that the balls in some way 'bind up' with the diff cage under load. Whilst the balls are always in contact with the diff pulley/cage, under acceleration the balls will tend to be more aft in the cage and conversely, more forward when decelerating. The static coefficient of friction between the shiny smooth carbide/ceramic diff balls and the nylon/delrin (which in itself has self lubricating properties due to teflon impregnation in most cases) is 0.15 - 0.2. Virtually nothing. And the Kinetic coefficient of friction is even less than that.

It's the clamping interface between diff screw, thrust race, diff ring and balls that ensure any torque transfer takes place - as Cecil himself will attest. If the diff is set too loose, it really doesn't matter how much you boot the throttle, no 'bind' and no torque transfer is going to take place. And the diff will be knackered! The adjustability of the diff will remain between the minimum tension where useful differential action can take place and the maximum tension that can be applied before diff rings get distorted and the thrust race/diff balls are damaged. Whilst only a very small zone, it's a useful tuning aid all the same and doesn't necessitate any disassembly.

The main reason why ball diffs persist - besides the fact that people still really like them - is that they are lighter, super smooth when assembled/adjusted correctly, and have zero back-lash or play in them, which is highly desirable on low grip conditions - especially with 2WD.

With 4WD this is less of a concern. And with higher power/torque applications, there is less and less desire for a ball diff - because they are inherently the inverse of a limited slip diff - ball diffs can slip before torque gets to either wheel. When this happens, a ball diff is not even functioning as a diff that is 'diffing out' - it's functioning as a very expensive single use slipper clutch! Lol.

I read somewhere that the reason why ball diffs are not used in 1/8th Buggy is because of the height of the output shaft vs the diameter of the diff gear vs the distance between the diff balls and the centres of...yada....yawn. Stop. The reasons are much simpler than that: Ball diffs have an operational limit regarding how much torque you can put through them. At the scale we use them at, that limit would appear to be 540 sized brushless motors.

In the UK racing on grass, astro or carpet, ball diffs are becoming less common. However, on slippery dusty or clay tracks, ball diffs are considered a tuning option. Gear diffs 'bind' as much as the chosen diff oil dictates. To such an extent that a gear diff can effectively be turned into a spool if thick enough oil is used.




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Old 05-09-2019, 12:19 PM
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Originally Posted by Horatio View Post
Gear diffs 'bind' as much as the chosen diff oil dictates.
The 'bind' that occurs in a loaded gear diff is mechanical and the lightest fluid weights don't make it go away. It is the reason that they cannot replace the ball diff in 2wd off road. If silicone fluids with much better film strength and hear resistance were to be created, this could probably be improved upon. This inherent bind seems to be beneficial on the high grip surfaces where gear diffs are favored.

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Old 05-09-2019, 07:46 PM
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Originally Posted by Horatio View Post
The perception that ball diffs are in some way 'limited slip' stems from the notion that the balls in some way 'bind up' with the diff cage under load. Whilst the balls are always in contact with the diff pulley/cage, under acceleration the balls will tend to be more aft in the cage and conversely, more forward when decelerating. The static coefficient of friction between the shiny smooth carbide/ceramic diff balls and the nylon/delrin (which in itself has self lubricating properties due to teflon impregnation in most cases) is 0.15 - 0.2. Virtually nothing. And the Kinetic coefficient of friction is even less than that.

It's the clamping interface between diff screw, thrust race, diff ring and balls that ensure any torque transfer takes place - as Cecil himself will attest. If the diff is set too loose, it really doesn't matter how much you boot the throttle, no 'bind' and no torque transfer is going to take place. And the diff will be knackered! The adjustability of the diff will remain between the minimum tension where useful differential action can take place and the maximum tension that can be applied before diff rings get distorted and the thrust race/diff balls are damaged. Whilst only a very small zone, it's a useful tuning aid all the same and doesn't necessitate any disassembly.

The main reason why ball diffs persist - besides the fact that people still really like them - is that they are lighter, super smooth when assembled/adjusted correctly, and have zero back-lash or play in them, which is highly desirable on low grip conditions - especially with 2WD.

With 4WD this is less of a concern. And with higher power/torque applications, there is less and less desire for a ball diff - because they are inherently the inverse of a limited slip diff - ball diffs can slip before torque gets to either wheel. When this happens, a ball diff is not even functioning as a diff that is 'diffing out' - it's functioning as a very expensive single use slipper clutch! Lol.

I read somewhere that the reason why ball diffs are not used in 1/8th Buggy is because of the height of the output shaft vs the diameter of the diff gear vs the distance between the diff balls and the centres of...yada....yawn. Stop. The reasons are much simpler than that: Ball diffs have an operational limit regarding how much torque you can put through them. At the scale we use them at, that limit would appear to be 540 sized brushless motors.

In the UK racing on grass, astro or carpet, ball diffs are becoming less common. However, on slippery dusty or clay tracks, ball diffs are considered a tuning option. Gear diffs 'bind' as much as the chosen diff oil dictates. To such an extent that a gear diff can effectively be turned into a spool if thick enough oil is used.
How can you calculate the static coefficient of friction in a ball diff if you don't know the available traction, or the load? 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.
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