Live Axle vs Dead Axle

[DampRobot]

The correct answer to these types of questions, of course, is that it depends. Here's my take on it, and the thinking that got me there.

Direct drive live axle is in my mind the best way to go a majority of the time. It's best suited to a drive like a WCD, where you direct drive one of the wheels, and transmit power 1:1 between them. Live axle typically lets you cantilever the wheels, which introduces some frame design benefits and allows you to swap out wheels more easily (I'm speaking generally here). It also means that you have your bearings somewhere other than the wheel, which can allow you to decrease the size of your wheel. In my mind, a live axle system is easier to repair because the wheel and the chain or belt path isn't between two frame members.

Generally speaking, live axle drives are better when the wheels are smaller (so you don't need that extra reduction in the chain/belt from the gearbox to the wheel). Dead axles work well where you are going to have the wheel inside the frame anyway, or where you have large wheels that need that extra reduction. There's also a perception that it's "easier" to build a dead axle system, as usually it doesn't require a team to machine bearing holes themselves (they're already in the wheels).

I recognize that my beliefs are probably neither totally correct nor totally typical, and in all honesty, this type of thing really comes down to "to each their own." I'd encourage you to build/design a WCD, as it's widely considered to be a very high quality DT (generally). Most important, though, is that you consider these types of design tradeoffs yourself, and make informed decisions accordingly, even if you don't happen to end up with the type of drivetrain like I described.

[MichaelBick]

It largely comes down to what kind of frames you build and what your final power transmission stage is. However, there are ways of getting around the disadvantages of either.

Dead axle is nice because the axle becomes a structural member. This is really helpful on sheet drives which can easily get crushed. In non cantilevered drives dead axle is usually easier to service. Dead axles also can be easier to machine, because you don't need to precisely machine bearing holes. However dead axles can be harder to tension without losing efficiency somewhere. A nice way of getting around this is by using C-C belts, though this may be above the capabilities of some teams.

Live axles tend to be better to use in cantilevered wheel drives. Live axles also have an easier time direct driving wheels, which makes it easier to make live axles lighter, more efficient, and more reliable. Live axles are harder to machine if you don't have the ability to precisely machine bearing holes, however are easier to efficiently tension.

Regardless, speed and torque have nothing to do with which axle type you choose and everything to do with your gearing. Dead axles don't have to have more backlash then live axles either. For example 971 uses gears to direct drive their center wheel from their 1 stage transmissions which allows them to have the same efficiency and backlash as any live axle team.

[apalrd]

I see a lot of confusion in this thread between live axles with west coast drives, and dead axles with sheet metal. Neither type of drivetrain requires a specific type of axle.


Just to clarify:
-Live axle drivetrains - The wheel is attached to an axle which transmits power to the wheel. This axle can be driven by a chain, belt, or gear. Nothing mandates that the live axle be cantilevered, there can be a bearing on both sides (our 2012 robot's third wheel was live, driven via a chain on the inboard side of the chassis rail).
-Dead axle drivetrains - The wheel is riding on the axle via bearings. The axle is fixed, in some cases a stressed chassis member, and does not spin. Nothing requires that the dead axle be non-cantilevered also, I have seen cantilevered dead axles in west-coast setups.

-West-coast drive - A drivetrain design and construction method pioneered by teams 254 and 60 almost 10 years ago that is usually characterized by a welded aluminum box tube frame, 6 or 8 wheels in a drop-center configuration, and cantilevered axles (often live).
-Sheet metal drive - A drivetrain designed and fabricated out of folded sheet metal. Due to sheet metal fabrication methods and strengths, these usually use axles supported on both ends (no requirement for dead or live axle, though).

There's also some comments about tensioning being easier on one or the other. This is simply not true - One can design any drivetrain with slotted axle holes to tension chains by sliding axles, it's not unique to west coast drives and it's not really any harder to fabricate for any type of drive. The same goes with speed, torque, gearing, etc. - Any axle, live or dead, can have any type of reduction to it. We've run live axles with chain reductions from the gearbox, and live axles direct-drive from the gearbox. It's possible to do the same with dead axles and chain, or dead axles and gears.

There are really no advantages/disadvantages to each, since the live or dead axle in itself is not frame-specific. There are many advantages in frame design to use one or the other in certain frame designs, but this isn't a function of the axle choice, it's a function of the frame choice.

There are also some totally false claims in this thread. Specifically:

Live axle usually means faster, less torque, and less backlash. Dead axle usually means slower, more torque, and more backlash.

This quote has no bearing in fact. The axle makes no difference for any of these. This is dependent on gearing selected, fabrication quality, and chain tension. If anything, I've seen more backlash in keyed live axle setups as the keyway opens up under shock load.

I think one advantage of a live axle is they usually make a more rigid chassis . A sheet metal dead axle is usually a lighter setup.

The axle makes no difference. You can argue west coast vs sheet metal, but that's not live vs dead axle. You can live or dead axle both west coast (although it might not be called 'west coast' depending on who you ask) and sheet metal designs. In fact, it's significantly easier to use a dead axle as a stressed chassis member, which would increase the rigidity of a dead axle setup in the right application.



The power transmission methodology is a tool to be used when designing the drivetrain. There are really no benefits to dead or live axles in general, although one can be highly favored over the other in a specific chassis setup. Design the chassis as a whole integrated design. Don't just pick the best individual elements, since there are many cases where they won't work well together. Also, you should design around what machine tools you can use, and what fabrication methods are easy or hard on those tools. For us, we use sheet metal because we have a brake and shear in our shop, and access to CNC waterjet and plasma machines, while the small precise components of a west-coast drive would have to be made by hand on our manual mill, which we just recently got a DRO for. This is the driving factor for us to use sheet metal - If we had access to a CNC mill instead of the waterjet and plasma, we would almost certainly design everything around milled box tube.

We use all dead axles with chain now. We are very aware that a west coast design makes it easier to change a wheel, and designed and built a west coast drive in our 2011 season, but we continue to design using dead axles in our sheet metal setup because of weight, strength, and ease of manufacturing.


In conclusion, you (OP) are asking the wrong question. The proper question should be on the design of the entire frame - And to properly answer, you (and us, if you ask us) need to know your manufacturing resources. There is no single right answer.