Ball diff vs. Gear diff.

[8rad]

Hey yall,

Due to a new track opening up close to me I have been thinking about joining the Wednesday night 2wd racing scene. However, I missed the whole gear diff revolution (I haven't raced competitively since 96). Can someone explain to me the advantages and disadvantages of gear diffs vs. ball diffs? Gear diffs seem to be cheaper and hassle free and kind of a no brainer. Am I missing something?

Cheers

[Lizardking]

Gear diffs are typically found in Tamiyas and other hobby toy grade (as opposed to toy grade) models. They are mostly bullet proof but the diff is basically free to rotate which means one wheel in the air will spin and the car will slow down or stall. Some gear diffs are now filled with oil, which limits the slip.

Ball diffs are more race oriented. They are sets of carbide ball bearings sandwiched between two metal plates. The sandwich can be compressed by tightening the plates, making the diff tighter and less slippery. That means more traction if one wheel is up. The downside is they can get dirty and crunchy and require cleaning from time to time. Also adjusting the slip is a big advantage but can be tricky.

Thats my experience. Hope it helps.

[RC10th]

Without elaborating too much....

Very high grip/ carpet = Gear diff
Low/Med grip/ dirt = Ball diff

[terry.sc]

Gear diffs were introduced back into race buggies due to the high torque of brushless motors and the high grip smooth surfaces a lot of places use these days. Most race buggies will come with a gear diff, with a ball diff being an option. Any competition on road car will have gear diffs only. The change happened because ball diffs were slipping and failing, then we found out fluid filled gear diffs were the better option for most cases. Both are adjustable, with a gear diff you change the oil which can range from being very free up to almost locked solid.

The difference is in how they work. Under power out of a corner a ball diff will initially lock up momentarily, then once it is turning it will turn with a constant load. This initial binding up helps stability on bumpy tracks and lower grip surfaces and makes the buggy easier to drive. The gear diff spins easier at low speeds but binds up more at higher diff speeds, think of the fluid in the diff like oil in a shock with small piston holes, you can push the shock in easily if you do it slowly but it locks up if you try and do it quickly. So on a low grip or bumpy surface when one wheel starts spinning momentarily through loss of traction the gear diff will try to send power to the spinning wheel while the ball diff will bind up and equalise the power making it more stable.

On higher grip and smooth surfaces when you have a more constant traction with the surface, the ball diff advantage of initial binding up works against it. Under the higher traction and torque when you accelerate out of a corner the ball diff will bind up under the heavy load going through it giving you understeer, a gear diff will still work smoothly. There's also the fact that the ball diff will need a certain amount of tension on it so it doesn't slip so you cannot run it as free as a gear diff, and as high traction puts extra load through the diff a ball diff will need regular maintenance while once you've got your gear diff tuned for your track it's a fit and forget item.

So if you have high grip in general and smooth surfaces like US indoor clay tracks or carpet tracks, you run a gear diff. If the grip is lower or you have a surface with lots of small bumps you run a ball diff.

[8rad]

Got it! Exactly the info I was looking for! Thanks.

[rc10nick]

Nothing is faster than a ball diff on dirt.

In my testing I found ball diffs stay noticeably freer under load than gear diffs. You can feel this for yourself - hold a diff with an outdrive in each hand. Twist them to feel how much resistance the diff has. Next, press the main gear against a hard solid surface. I use the edge of my workbench. Apply a torque to the main gear of the diff and then try twisting the outdrives again. A ball diff will feel just as free when there was no load on the diff. The gear diff will feel even more resistant.

A popular option for gear diffs nowadays is to vary the number of spider gears in the diff. In my testing, more gears = less lock up under load and vice versa for less gear.

[Chewbacca]

Gear diffs are typically found in Tamiyas and other hobby toy grade (as opposed to toy grade) models. They are mostly bullet proof but the diff is basically free to rotate which means one wheel in the air will spin and the car will slow down or stall. Some gear diffs are now filled with oil, which limits the slip.

Ball diffs are more race oriented. They are sets of carbide ball bearings sandwiched between two metal plates. The sandwich can be compressed by tightening the plates, making the diff tighter and less slippery. That means more traction if one wheel is up. The downside is they can get dirty and crunchy and require cleaning from time to time. Also adjusting the slip is a big advantage but can be tricky.

Thats my experience. Hope it helps.

Lizard,


You’re thinking open / unsealed gear diffs. For those your arguments are valid, for the sealed diffs with the right diff fluid is another story.

Terry.sc explained it on point.