Reasons Not to Use an X Drive?

  1. 7 months ago

    briancole

    12 Apr 2018 Utah 4270C

    I've noticed that not a lot of teams used X Drives during IN the Zone. Besides the obnoxiousness of mounting a mg intake and lift on an x drive, are there other reasons not to use one?

  2. Neil_6121C

    12 Apr 2018 Pennsylvania 6121C

    I would assume it’s pretty hard to get over the starting bar with an X-drive, although I’ve never tried it out

  3. Easton

    12 Apr 2018 GA, USA 1958A

    @Neil_6121C I would assume it’s pretty hard to get over the starting bar with an X-drive, although I’ve never tried it out

    We had one for a brief period of time at the beginning of the season. It could get over the bar but not with larger stacks.
    @briancole for me it was harder to drive, I don't know if it is for everyone but for me it seemed much less accurate then tank drive. Also, loss of torque (but at the same time a speed gain). Also, there's not much need to strafe. If it was like sack attack, where everything is in a line, it would be great. But with ITZ and cones everywhere, you don't need to strafe most of the time

  4. TheCrimsonDoubleChin

    13 Apr 2018 Appleton, Wisconsin 5062C,A,D

    The main factor (for me at least) is that it's just much more complicated for a drive train. Sure you have strafing but at what cost lost to friction? That's another thing, speed; for me, I've never seen (in Wisco or Worlds) an X-drive that can go as fast as a regular 4 motor 1:1 HS drivetrain.

  5. Easton

    13 Apr 2018 GA, USA 1958A
    Edited 7 months ago by Easton

    @TheCrimsonDoubleChin The main factor (for me at least) is that it's just much more complicated for a drive train. Sure you have strafing but at what cost lost to friction? That's another thing, speed; for me, I've never seen (in Wisco or Worlds) an X-drive that can go as fast as a regular 4 motor 1:1 HS drivetrain.

    X drives are actually faster than tank drives by a factor of sqrt(2) I believe, when using the same motor internals. That's why they don't have as much torque as a tank drive. Maybe the teams you saw were using torque motors, but if they used HS 1:1 it would be faster than a regular 4M HS 1:1

  6. Mystellianne

    13 Apr 2018 Miami, Florida 4411S

    X-drives are theoretically faster than tank drives, but they lose so much torque it’s hard to notice a difference between the two. If you don’t absolutely desperately need the strafing ability, they’re not worth the hassle.

  7. TheCrimsonDoubleChin

    13 Apr 2018 Appleton, Wisconsin 5062C,A,D

    @Easton X drives are actually faster than tank drives by a factor of sqrt(2) I believe, when using the same motor internals. That's why they don't have as much torque as a tank drive. Maybe the teams you saw were using torque motors, but if they used HS 1:1 it would be faster than a regular 4M HS 1:1

    The h*ck? I was always told it was slower lul.

  8. Mystellianne

    13 Apr 2018 Miami, Florida 4411S

    @TheCrimsonDoubleChin The h*ck? I was always told it was slower lul.

    If you do the trig, it’s actually faster. Theoretically, of course. In practice friction basically negates that.

  9. 97934U

    13 Apr 2018 Nashville, TN 97934U

    It's super slow and has tons of friction

  10. kypyro

    13 Apr 2018 V5 Beta Tester Central Kentucky

    @Mystellianne If you do the trig, it’s actually faster. Theoretically, of course. In practice friction basically negates that.

    We've found the omni wheels have so little friction going sideways on the rollers that frictional losses in an X drive are low. And yes, as you say, they're definitely faster than tank for the same motors. If the math doesn't convince someone, they can always build two chassis and race them. We did that, and it convinced those who were unsure.

    That said, the interaction between angle-mounted wheels and and the starting bar meant that an X drive didn't test well. I don't believe we tested a "+" drive configuration; it might have worked better.

    Overall, I think it takes a very particular situation to make holonomic drives rate better than tank in actual use.

  11. biglesliep

    13 Apr 2018 Menlo Park, CA 1666

    We built an x-drive for Starstruck, and it was *really* hard to build. The angles have to be *so* precise for it to work. That said, we were glad we did it, because strafing along the fence was really useful. Once we got it built, it was solid all season. I will second what the other posters have said: you lose a lot of torque.

    For ITZ, we didn’t feel that strafing was going to be that crucial, and getting over the starting bar seemed like it would be hard.

    Funny - here in CA, I don’t think I saw a single x-drive all season!

  12. Easton

    13 Apr 2018 GA, USA 1958A

    @Mystellianne If you do the trig, it’s actually faster. Theoretically, of course. In practice friction basically negates that.

    Maybe if you build it wrong, but when we did it it was obviously faster. Omni wheels don't have a lot of friction.

    @97934V It's super slow and has tons of friction

    As I already said, as have others, it's actually faster. There is more friction but from my experience, if you do it right, it doesn't really slow it down a lot

  13. callen

    13 Apr 2018 Braintree, MA, USA

    No, an x-drive is not faster than a tank drive. I wish people would stop saying that. Discussing drive types in this way is comparing apples and oranges. If you gear them up identically, the x-drive is faster and weaker than the tank drive, yes. If you really want to see which is faster as a general statement, you should set them up at roughly equal strength so you can say which is faster while move the same load. Even then there are issues of what we mean by making them equal strength, though.

  14. Easton

    13 Apr 2018 GA, USA 1958A
    Edited 7 months ago by Easton

    @callen No, an x-drive is not faster than a tank drive. I wish people would stop saying that. Discussing drive types in this way is comparing apples and oranges. If you gear them up identically, the x-drive is faster and weaker than the tank drive, yes. If you really want to see which is faster as a general statement, you should set them up at roughly equal strength so you can say which is faster while move the same load. Even then there are issues of what we mean by making them equal strength, though.

    The bold part is what I'm saying. I'm not saying which one is faster at at set forward torque (this is hard to measure anyway). But when people say X drives are slower than tank drives, you can't compare those equally without having equal motor internals. Using equal motor internals, X drives are faster. I realize they have less torque as well, I wasn't talking about torque at all.

  15. Robo_Eng_13

    13 Apr 2018 Indiana 1483

    @Easton The bold part is what I'm saying. I'm not saying which one is faster at at set forward torque (this is hard to measure anyway). But when people say X drives are slower than tank drives, you can't compare those equally without having equal motor internals. Using equal motor internals, X drives are faster. I realize they have less torque as well, I wasn't talking about torque at all.

    This a major problem in terminology on this forum about certain mechanisms or designs being "faster" or "stronger". They are all dependent on what goes in and how it is built. (Torque_Out x Speed_Out) = (Torque_In x Speed_In) x % Efficiency.

  16. [SPR] JP

    13 Apr 2018 Texas PANDA
    Edited 7 months ago by [SPR] JP

    Guys I still don’t understand how
    How is 45 degrees faster than 0?

  17. Edited 7 months ago by TeamTX

    Aura has a write up about why X-drive is faster (with same gearing):

    http://www.aura.org.nz/archives/1137

  18. Mystellianne

    13 Apr 2018 Miami, Florida 4411S

    @kypyro We've found the omni wheels have so little friction going sideways on the rollers that frictional losses in an X drive are low. And yes, as you say, they're definitely faster than tank for the same motors. If the math doesn't convince someone, they can always build two chassis and race them. We did that, and it convinced those who were unsure.

    That said, the interaction between angle-mounted wheels and and the starting bar meant that an X drive didn't test well. I don't believe we tested a "+" drive configuration; it might have worked better.

    Overall, I think it takes a very particular situation to make holonomic drives rate better than tank in actual use.

    Sure, the sideways rollers don’t have much friction when moving along their respective axis, but you have to keep in mind that the wheels are moving along an axis that’s 45* off compared to their normal axes. This does cause friction, because neither the overall wheel nor the individual rollers move in a straight line respective to their axes, they move slanted. This is what I mean by friction.

    Also, X drives traditionally require a lot more support and practically force you to use steel gussets, which doesn’t help given its inherent torque loss.

  19. mbrh

    13 Apr 2018 Pennsylvania 1267 and 1274

    My middle school team used an X-Drive for most of the season and were initially happy with it and the results (used something similar last year). On our home field, it worked fine getting over the bar.

    In January/February at competitions, they started to have trouble, but it varied by field. It appeared that between mounting differences and wear on the bar from getting pounded on, the amount of flex in the tube increased to a point that it would bend more when hit and keep it from climbing over. They made a quick switch to a tank drive and moved on, but many of the design decisions for x-drive (higher mogo lift, etc.) were too deep in the design.

    They are building a new robot from scratch for worlds. Sure hope they get done in time...

    They would love to go back to X-drive next season, but the motor restriction on V5 may make that not practical.

  20. RougeScaless

    15 Apr 2018 Bangkok 2979A, 2979C, 2979E

    I think X drives are harder to attach MOGO intakes? I don't know how you can attach intakes with the X drive since the team I saw with X drive didn't have a MOGO intake

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