8 Wheel Chassis Design

[MichaelBick]

The turn speed should not change that much, unless there is significant scrub. That being said, there might be slight differences, but nothing really noticeable. It is not worth it though.

[sdcantrell56]

Keep the transmissions centered but space the center wheels. Do a comparison between effective wheelbase between a 6wd and 8wd and you will realize the with the wheels evenly space on an 8wd your effective wheel base will already be shorter leading to quicker turns than a drop center 6wd. Try to split the difference somewhat.

[Alan Anderson]

Everyone is assuming that a 6WD center-drop drivebase necessarily rocks and has only 4 wheels on the ground at a time. This is not always the case. With just the right amount of center wheel drop, all six wheels are in contact with the field carpet at all times and there is no noticeable rock. With the appropriate center of gravity, the robot spins around its center wheels.

Besides, some 6WD systems have no drop and still work well.

[thefro526]

Originally Posted by sdcantrell56

Keep the transmissions centered but space the center wheels. Do a comparison between effective wheelbase between a 6wd and 8wd and you will realize the with the wheels evenly space on an 8wd your effective wheel base will already be shorter leading to quicker turns than a drop center 6wd. Try to split the difference somewhat.

This.

The average Long Base 6WD has an effective wheel base of somewhere around 14"-16" depending on wheel size and chassis design. The last two drive trains I worked on (10WD and an 8WD) had effective wheel bases of 16.5" and 17.25" and neither had any major problems turning and both liked tracking straight at speed. If I were going to make an 8WD again, I think I'd space the center wheels ~14" apart from each other to have a good mixture of straight line stability and maneuverability. (Short center wheelbase 8WD's have a tendency to over rotate)

[jwfoss]

I have a few other questions/comments.

What is your reason for selecting 6" wheels?
With the ground clearance, and lack of a "wedge" on the front and rear of the chassis, I don't see an advantage to a large wheel.

For a non-direct drive application, why choose the AM Supershifter over the Gen I shifters?
The AM Supershifter contains an additional reduction but is otherwise the same internally. The output of the Gen I can be modified so that the sprockets are outside of the gearbox (see out drivetrain from 2011 or 2012).

[Starke]

Silver Stream has just been added to FRC Designs! Check it out!

FRC Designs - Drives: http://www.frc-designs.com/html/drives.html

Matt

[akoscielski3]

Originally Posted by thefro526

This.

The average Long Base 6WD has an effective wheel base of somewhere around 14"-16" depending on wheel size and chassis design. The last two drive trains I worked on (10WD and an 8WD) had effective wheel bases of 16.5" and 17.25" and neither had any major problems turning and both liked tracking straight at speed. If I were going to make an 8WD again, I think I'd space the center wheels ~14" apart from each other to have a good mixture of straight line stability and maneuverability. (Short center wheelbase 8WD's have a tendency to over rotate)

Sorry for the delayed responses.

Silver Stream 1.0 (I will have multiple version that I reiterate and they change to 2.0, 3.0...) has a center wheel base of 8in. I think in my next iteration I will change the wheel base. I have never made an 8WD Chassis before, so I wasn't exactly sure where thee center wheels would go for optimal performance. After looking at other 8WD (such as 2056's This year) I believe you are correct that the Center Wheel base should be larger. Though for the part about them over turning, could you not technically change that in the program? Although I think the better solution would be to fix it in CAD.

@jwfoss
My choice of the six inch wheels was mainly just cause I like them. They arent too small, or Giant like 8in. My team has no experience with smaller wheels, which I know isnt much of a change, but we are just comfortable using 6in or 8in. I personally don't like 8in though.

Regarding the supershifters...
We have used the gen I/II's for every year since 2007 except 2011. And we are simply sick of them. We though we would try something new, and we were looking at 234's Robot and liked the SuperShifter's they had.

You can find a lot of my Designs on FRC-Design.com
I love the idea of the site and I encourage anyone to visit the site to look for ideas from other teams, and to submit their own designs

[MichaelBick]

In my opinion, there is no such thing as too small wheels, only wrong gearing.

Originally Posted by akoscielski3

Regarding the supershifters...
We have used the gen I/II's for every year since 2007 except 2011. And we are simply sick of them. We though we would try something new, and we were looking at 234's Robot and liked the SuperShifter's they had.

Why are you sick of them? Performance? Maintenance?

[ratdude747]

Suggestion: Beef up the axle mounts... they look a bit skimpy and given a hard enough ramming could warp. with an integrated design like that, such a failure could be catastrophic.

[akoscielski3]

Originally Posted by ratdude747

Suggestion: Beef up the axle mounts... they look a bit skimpy and given a hard enough ramming could warp. with an integrated design like that, such a failure could be catastrophic.

I have't had a problem with axle mounts like this in our 2012 chassis. What should I do to make them better? cross braces meeting in the middle? or make it wider? (currently 1")

@SuperNerd256
We've used them so much they have just been over used. We like the idea of having the gears closed in too. We wont have to worry about getting chips in the gears anymore.

[sdcantrell56]

Originally Posted by akoscielski3

I have't had a problem with axle mounts like this in our 2012 chassis. What should I do to make them better? cross braces meeting in the middle? or make it wider? (currently 1")

@SuperNerd256
We've used them so much they have just been over used. We like the idea of having the gears closed in too. We wont have to worry about getting chips in the gears anymore.

A supershifter is just the AM Gen1/2 with an added stage of gearing. This is heavier and lower efficiency. I would suggest sticking with the Gen1/2 unless you have a good reason to swap like needing to direct drive a wheel. Even so you could make a custom transmission for that using wcproducts gears or better yet drop to a 3" wheel and you can run a stock AM Gen1 direct drive

[Adam.garcia]

Originally Posted by sdcantrell56

A supershifter is just the AM Gen1/2 with an added stage of gearing. This is heavier and lower efficiency. I would suggest sticking with the Gen1/2 unless you have a good reason to swap like needing to direct drive a wheel. Even so you could make a custom transmission for that using wcproducts gears or better yet drop to a 3" wheel and you can run a stock AM Gen1 direct drive

If you are looking to decrease significant weight, look to 1:1 replacement gears for the gears in the supershifter. You can find them here:

http://wcproducts.net/gears-20-dp/

[AlecS]

To go along with what Sean said, why did you chose to go with the SuperShifters, AND an additional sprocket reduction to the wheels?

By looking at the pictures, it appears like you are using a 12-22 sprocket reduction after the SuperShifters. If this is in fact correct, this would yeild,

-With the Standard 2.56:1 Spread,
~2.84 FPS in Low Gear,
~7.27 FPS in High Gear,
and only ~17 Amps per motor to spin the wheels in low gear.

-With the Standard 4:1 Spread,
the same low gear
~11.37 FPS in High Gear.

Using the optional final reduction reduces speed in both cases, though I'm assuming that wasn't your intention.

In either case, you may want to think about reducing your overall reduction. 2.84/7.27 FPS is going to be quite slow for navigating an FRC field (think, FRC71 in 2002), and won't help your overall pushing power, since you're already limited by traction.

If you want to stick with SuperShifters, you will probably want to look into direct driving one wheel, and chaining the other 3, 1:1

Although you may be "sick" of the AM Shifters, using the same (assumed) 12-22 sprocket reduction, they would yield, ~6 and 16 FPS. Both very respectable speeds to maneuver the field, and would still be able to spin the wheels in low gear in under 40 Amps per motor.

The third option would be to make you own gear reductions, and use smaller wheels.

Also, one more note, this drivetrain only has an .SLDASM on frc designs. In order for Assemblies to opened on other machines, both the .SLDASM, and all .SLDPRT files in the assembly need to be present. Using the Pack and Go feature simplfies this task greatly.

-Good Luck

[flargen507]

159 did something similar to this for 2012. To create a strong, light, flexible West Coast Drive, we used 1.5" by 3" aluminum tubes. These were pocketed circularly (a lot) and the wheels were outside. It only needed one tube per side, and even with extensive pocketing it retained structure even under competition abuse. If that's at all helpful to your designs, it worked really well for us. Look up Hawaiian Cadder, he posted the design.

Your design is very interesting. Using traingular pocketing with sheet metal was a good idea, but sheet metal does require the outside wall (although this does improve your turning radius, so more power to you.) I would suggest that the gearboxes be moved to the center to keep the CG (we used the battery as the counterbalence to the shooter). Depending on what type of tread your are using, you may need to increase your center drop to 3/16". If there is any way to use 25 chain instead of 35, this could save you a lot of weight, and 159 had no troubles with the strength of 25 chain. 8WD can turn quite well, but it does need a significant center drop to do so. We used AndyMark supershifters, however I cannot comment on their goodness/badness to Gen I shifters(having never used Gen I's), but so far as I can tell AndyMarks work just fine.

Lastly, if there's any way you can incorporate multiple placements for axles, this could make modifying it very easy for next year's season.