Sim Physics and Backlash

Hey guys,
Wondering what yall think about having the sway force be opposite of the usual turn left/lean right configuration. I've been tuning the motion profiles for my 2DOF rig that uses wheelchair motors and it has a decent bit of slop(backlash) in the motor gearbox. This creates a physical sway deadzone when balanced, meaning quick left and right motions result in a sort of flopping back and forth instead of a smooth rolling back and forth. With the help of bungee cords this isn't a big deal for slow and smooth driving, but I have found it to be quite an issue in situations like the windy bit at Virginia International Raceway. As I first turn, the undesired motion of my head towards one side is quickly followed by the undesired motion of my head back to the other side as I begin the next turn, there is not enough time for the desired effect of the centripetal force to be felt because the rig is in constant motion. This isn't necessarily due to the backlash, but it makes it worse. The only force felt, at least by me, is the undesired one which is opposite of the desired(real) force and messes with my head. I switched the sway direction and did a couple of laps at VIR and to my surprise it actually felt much better. The long turns obviously didn't have the centripetal force effect, which is an important one, but the motion that was felt upon turn-in was much more realistic and helped me keep focus throughout the corner, I noticed with the conventional config I would lose focus upon turn in due to my head being confused. Leaning in the wrong direction during the turn didn't feel too weird, maybe because I'm so focused on the steering wheel feedback at that point and I'm not leaning very much. The short turns felt good, my head being jerked in the proper direction felt better, and the chicanes and windy bits felt totally different, much better than before. So when I saw a thread about how motion didn't help some people's lap times, it made me really consider reducing the total travel for sway and having it lean into the turn, possibly getting a gseat to make up for the lack of centripetal force. I understand that many people get used to the undesired head motion in time, and having zero-backlash certainly would help, but I'm wondering is worth it for me to get used to it or should I stick with what feels good to me now? I might get linear actuators or make a better gearbox eventually, but will that really help enough for the S turns to feel right to me? I think for my setup taking advantage of the side to side motion may be better than trying to use the gravitational force. Has anyone had a similar experience?

I’ve tried elastic cords in various places, it helped dampen any jittering. The dead zone is still there though when doing quick movements because the worm gear spins faster than the elastic cords can pull the rig and it hits the other side of the backlash and then comes back. Pulling down on the front of the rig is easiest for me, but I think pulling on the back should make braking smoother so I’ll try that. I might try higher resistance because I think my motors are pretty strong, but it seems like doing too much is just unnecessary strain on everything.

I opened the gearbox of one of my motors but couldn’t really get to the worm gear, lots of grease, couldn’t tell if the worm drive could be turned somehow. Couldn’t figure how to get the axle or gear shaft out which are in the way of the worm gear. I’ll tear into it further and I can post some pictures tomorrow. Motor says ElectroCraft model no. E675.

Well I didn't tear into the motors because I got sidetracked trying to get Dirt Rally 2.0 to work, I don't think its worth my time. I saw something about Richard Burns Rally and the Rallysimfans plugin, I'll have to check that out when I want to do some rally.

I drove around the LA Canyons track in AC and it felt great, just cruising around I didn't notice the backlash at all. I think I may just turn the sway way down for tracks that have multiple back and forth movements like VIR. When I'm doing serious racing in iRacing I don't need much motion anyway, since I use vr it rattles the headset a bit and can mess with my vision. I did a few laps in a Pro 2 truck around Bark River in iRacing and it was pretty epic, landing sideways off a jump is a wild ride. The whoops really buck you around, even with just a tiny bit of roll and no pitch at all, the sway and surge is plenty. Driving a huge truck with loads of suspension fit a lot better with the smoothing that I'm using on the profile to help with the backlash, the street cars feel weird with too much smoothing.

I'll post more soon about the gearbox and how I deal with the backlash, but theres a decent chance I get sidetracked again

you write about a 2 DOF and sway, how did you realize this ?

The actual motions that the seat is making is not sway, just roll and pitch, but I am using the roll and pitch to simulate the sway and surge forces of “real life”. When I say I am using mostly sway and surge with just a little roll, I mean that I am mostly simulating sway and surge and using a little bit of the roll axis to copy the actual tilt of the car in game. A little roll helps you feel the curbs, a little pitch helps you feel the uphill and downhill, but for my profile the motion is quite small compared to the sway and surge motion.

Well I got the worm gear out of the motor, not sure how I can reduce the backlash. The bearing next to the worm drive has the tiniest bit of play and allows the worm gear to move slightly, if I can push the worm gear closer to the worm wheel that would help, but I can’t think of any way to do that.

What exactly do you mean by rotating the gear in the wormdrive? I’ve read that some wormdrives have a variable pitch worm gear that can be moved to adjust backlash, I was hoping that was my case but unfortunately it is not. Do you mean turn one of the gears slightly so that the teeth fill up more of the gap in the other gear? That seems like it would work, but I don’t think there’s any way to rotate either gear along that axis, but if anyone has any ideas that would be great.

How many dof is your sim ?

Move the cog back so the motors are always loaded ?

2DOF, if the cog is moved to one side that helps, but when you need to move to the other side you still have to cross the deadzone to get to the other side

Ahh I see what you mean. I’ve hardly used my rig so I don’t think that is a problem right now but I’ll definitely keep that in mind.

I think I’ll just shorten my motor arms some and keep experimenting with the elastic cords to minimize the backlash. With enough sway smoothing it’s not that bad. Time to actually use this thing instead of tinkering with it! Thanks for the suggestions

I've just discovered the SFU gearbox that DOF Reality sells for $200(per pair I think). I wonder if it's possible to retrofit the gearbox to my motors? Or maybe make my own version? Anyone know the RPM of the DOF Reality motors? A ballscrew gearbox on a wheelchair motor seems like a great alternative to linear actuators for people with setups that use the typical gearbox with a lever arm.

What make are your gearmotors ?
What ratio are they ?
Are they worn ?

The motors are ElectroCraft model E675. Can’t find much info about them online. Not sure about the gear ratio but I will check for myself soon. They look almost brand new on the inside. They are rebuilt and refurbished and the gears don’t look very worn at all, although they were quite cheap

Your gearbox has a worm drive and then another set of straight cut gears, it is probably the second set of gears that has the backlash

Can you move them back and forth and see the play ?

Also the worm drive is only supported on one end by the motor - does it move up and down a bit ?

There is a small amount of backlash in the straight cut gears and then a slightly larger amount of backlash in the worm drive.
Yes there is movement of the worm gear while the casing is off, but I think I can adjust the angle slightly by forcing the angle of the casing when tightening.

Hi Grant
I too would be interested in those dof reality gearbox upgrades,do you know if they sell them separately, or do you have to own one of there rigs.
I have terrible backlash in my worm drive motors.
Wish I had invested the money in better motors or linear actuators to be fair

I havnt looked into yet, I’m not sure if they sell it separate or not. I think the best option for both price and performance is a wheelchair motor with a ballscrew gearbox like the SFU, I think you may have to do some custom machining to make it work though.

Yup agreed
I don't know where you are in the world but here in New Zealand items can be expensive to ship.

I almost wonder if linear actuators might be the go for a few bucks more maybe

Yeah I think linear actuators would be good, I got my motors for really cheap ($80 for 2 including shipping) so linear actuators would be quite a bit more pricey. The other problem with linear actuators is I think they take up a lot of travel space, the rig needs to be tall if you want a decent amount of travel. The backlash really is a problem though, so linear actuators is a good option if it works with your rig.