8 wheel drivetrain?

[safiq10]

I recently read team spectrum article "Mason Markee - This Is How I Work" (Thanks to Allen and Team spectrum for writing this amazing article! ) in the article Mr.Mason says that the best advice the received was to use and 8 wheel drive. I was wondering what are the benefits in using 8 wheel drive. Also what are the benefits in using small wheels over big wheels. Any info would be delighted. Thank you and I love that shirt. Have a nice day

The main advantage with 8 wheel drive is little to no rocking, since the robot rests parallel to the ground on its middle four wheels. It also helps getting over large obstacles (read: 2010).

Small wheels are amazing. You don't need nearly as much reduction to get good speeds, they're light, allow a lower center of gravity, and are generally cheaper than large wheels. In fact, I've yet to hear a solid argument as to why any wheel size above 6" diameter is needed. 4" works great, but 5"-6" wheels are sometimes preferred to get over game pieces.

[EricH]

There are potentially--emphasis on potentially--a number of benefits to both a smaller wheel and an 8WD individually, or as a pair.

8WD is more stable than a 6WD drop-center configuration; it has 2 wheels on each side carrying most of the weight, instead of just 1. This only works well if the center two wheels on each side are dropped; otherwise, it can turn fairly badly.

8WD can have a lower ground clearance, due to the extra wheels; this gives a lower CG, which generally means less tipping.

Smaller wheels mean less weight per wheel, and less gearing to get to a given velocity (which may mean less weight in the gearboxes). On the other hand, you do get less ground clearance, which may mean more wheels.

[Cash4587]

Smaller wheels = more torque

[ttldomination]

Originally Posted by Cash4587

Smaller wheels = more torque

...no...just...no.

At a given gear ratio, smaller wheels do yield more torque.

However, most teams optimize their ratios to work with their wheel size in order to obtain their desired torque/speed numbers. Given the number of gearbox/chain options available in the FRC market right now, even your average team can cater its DT ratio to meet its specifications.

- Sunny G.

[EricH]

Originally Posted by Cash4587

Smaller wheels = more torque

Not necessarily. There are other factors in play

Torque = Force * Distance, or T=F*D. If T= constant, then a smaller D gives a larger F, which would roughly translate into available pushing force (given your frictional coefficients being sufficiently high--maybe they are, or maybe it's 2009 and they aren't), so yes, you are correct in that sense.

On the other hand, remember that I said earlier that smaller wheels allow you to use less gear reduction to get to a given speed. If you were to take a 6" wheel and swap in 4" wheels, with no other changes, you'd see a reduction in speed and an increase in torque--but most teams with smaller wheels tend to run at least as fast as teams with slightly larger wheels, if not faster, which means that they've used that lower reduction to go faster, sacrificing some of that available torque for speed. This can actually lead to lower available torque if the reduction is taken beyond a certain point (determined by drivetrain design as a whole).

Just a case of a general statement being true most of the time, but be careful in implementing it to make sure that either it is true or that you've accounted for the possibility that it's false.

[Cash4587]

Whoops. I was thinking of steam locomotives. And here is my reasoning, steam locomotives built to pull passenger trains had larger wheels to have a higher top speed to get to places on time. If the same passenger locomotive was then later retrofitted with smaller wheels because it needed to pull freight, it now had more torque to pull that extra weight that passenger trains didn't have. Sorry, that is why I said "Smaller wheels = more torque".. I will be clearer next time

Originally Posted by Cash4587

Whoops. I was thinking of steam locomotives. And here is my reasoning, steam locomotives built to pull passenger trains had larger wheels to have a higher top speed to get to places on time. If the same passenger locomotive was then later retrofitted with smaller wheels because it needed to pull freight, it now had more torque to pull that extra weight that passenger trains didn't have. Sorry, that is why I said "Smaller wheels = more torque".. I will be clearer next time

Good idea, though, to connect real world knowledge with your knowledge of physics. That's the right mindset to have.

[nathannfm]

What would you guys say the ideal ground clearance is for an 8 wheel bot with 4 inch wheels? (assuming "flat" field, like 2013)
And would the wheel type mater for your number ei. Colson, Versa, Performance with rough top?
Along the same lines what is the ideal center drop? I have heard 1/8 inch, is that pretty much the consensus?

[thefro526]

8 wheel drives (drop center) share a lot of the same benefits as 6 wheel drives (drop center) along with a few key differences that may or may not be something that you can use to your advantage. The biggest difference, or at least the one most relevant here, is that an 8 wheel drive will have a tendency to rock much less often than a 6 wheel drive, assuming that the center of mass is reasonably close to the center of the robot. This creates a much more stable platform to build precision scoring mechanisms (think shooter) and can work to your advantage if 1*-2* of drive train rock will make a difference between scoring and not.

There's also the added benefit as mentioned before about 8 Wheel drives handling obstacles better than a similar 6WD, but there are ways around this. As a rule of thumb, if you're concerned with your robot potentially having a high center of mass, it's worth looking into an 8 wheel drive as it'll handle the crest/apex of most obstacles a lot more smoothly.

Small wheels have some pretty significant benefits, the most obvious of which being that they're lighter than a similarly constructed wheel that's larger in diameter. Some of the additional benefits come from how they can be integrated into your overall drive system, specifically that a smaller wheel will require less gear reduction for a target performance parameter when compared with a larger wheel which allows you to use smaller and/or fewer gears in your gear box, which obviously reduces system weight, but can also increase system efficiency - which is an added bonus.

The biggest downside to small wheels is probably increased wheel wear, since the wheel is going to have less tread (circumference) than a larger wheel. This may or may not be an issue considering the design and your intent, some teams like to be able to get an entire season out of wheel tread, while others have no problem replacing wheels per event, or even per match. Another notable downside is that a drive using smaller wheels (say 4" in the FRC context) can/will require a trick or two to handle significant obstacles, especially things like 2012's Barrier, but it's not all that hard.

Originally Posted by nathannfm

What would you guys say the ideal ground clearance is for an 8 wheel bot with 4 inch wheels? (assuming "flat" field, like 2013)
And would the wheel type mater for your number ei. Colson, Versa, Performance with rough top?
Along the same lines what is the ideal center drop? I have heard 1/8 inch, is that pretty much the consensus?

A lot of this is based on preference. In 2013, 341's Drive Train had ~.75" of clearance at the drive rails, and then ~1.875" at the belly pan. The lower clearance at the rail helped to minimize the chance discs being stuck in the drive, although it happened once or twice, and the higher clearance at the belly pan allowed us to drive over discs without noticing. More often than not, assuming the field is flat, you can get away with 1/8" to 1/4" more than the largest field 'bump'. (In 2013, the field bumps were ~1/2" at the pyramids, and in 2011 they were something like 3/8" - 1/2" at the towers)

Wheel type does effect the static (built in) drive clearance a bit, a wheel like a colson doesn't 'sink' into the carpet all that much at it's OD, but something like a versa wheel will sink in about 1/8". In my experience it's usually something best decided on a case by case basis, and if you're not sure it's easy enough to figure out by taking a wheel and pushing it into carpet by hand.

Drop is a bit of a preference number as well, but as a rule of thumb, somewhere between .090" and .125" is probably the safe minimum, although, you can usually get away with running 'more drop' than a comparable 6WD before you start having stability problems. On shorter wheel base 8WD's, I prefer to go with less drop, since it helps to make the drive track 'straight', but on long drives, I'll run a bit more drop to help with turning.

[Brandon Holley]

Originally Posted by Cash4587

Whoops. I was thinking of steam locomotives. And here is my reasoning, steam locomotives built to pull passenger trains had larger wheels to have a higher top speed to get to places on time. If the same passenger locomotive was then later retrofitted with smaller wheels because it needed to pull freight, it now had more torque to pull that extra weight that passenger trains didn't have. Sorry, that is why I said "Smaller wheels = more torque".. I will be clearer next time

What you are describing has the exact same application to FRC (and life...)

The steam locomotive has a given power output, and a given output RPM at the wheel. Unless you change the locomotives power source somehow (bigger furnace? I dont know how steam engines actually function) you have to work with that given amount of power and speed output.

If you have a given RPM, and you throw 'bigger' wheels on the output, the train will be faster (more distance traveled per each rotation of the wheel). If you take the same output shaft and put smaller wheels on it, the train will be slower (less distance traveled per each rotation of the wheel). However- the speed is being traded off for a proportional amount of torque gain (like you stated).

The same concept applies to FRC- it just happens to be most teams design the RPM and torque of their gearbox outputs to be optimal for the specific size wheel they are using.

-Brando