What are the advantages of using 3 vs 4 actuators for a 3 dof system?

What are the advantages/disadvantages of using 3 vs 4 actuators for a 3 dof system? By three dof I mean the roll/pitch/heave. If weight is not a concern would 3 be more than sufficient?
Thankyou!

Sorry I'm a bit of a noob. What did you mean by 'not too top heave'? I read this somewhat lengthy reddit post https://www.reddit.com/r/simracing/.../?utm_source=share&utm_medium=web2x&context=3 , where the person basically says 3 is better than 4 (assuming load is okay). Would you agree?

Use 3 it will enhance the experience

Make sure they are widely spaced apart to keep the cog low

This is an example of why 4 is not better than 3

Sorry I should have clarified, I'm intending to use motion sim for primarily sim racing, I believe some people say you get better feel with 4 post because you can differentiate between left right suspension. But does this make sense given 3 points make a plane?

it doesn't matter what type of simulation you are trying to do - the physics are still the same

a pegleg 3dof with 4 actuators is a waste of money imo - use the 4th actuator to build in traction loss for example

there is a discussion about it here - https://www.xsimulator.net/community/threads/stuart-platform-vs-rectangular-pt-actuator.16343/

to me its all perfectly clear - but I guess not everybody is like me

I can understand that a lot of 3dof sims with 4 actuators have evolved from a static sim and were not designed with motion in the first place.

Its easier to buy a 4 actuator kit to convert your static sim than redesign it

The physics seem clear to me too. Maybe its placebo that's driving the desire to have 4 instead of 3. However if using tactile effects on the actuators that could also be a reason to prefer 4 over 3. Would you agree?

The physics seem clear to me too. Maybe its placebo that's driving the desire to have 4 instead of 3. However if using tactile effects on the actuators that could also be a reason to prefer 4 over 3. Would you agree?

no i don't agree

I have tried a pegleg sim and it felt it was not delivering a experience that reflected how the car was handling

This is interesting. The guys at Sigma Integrale also referring to Barry from Sim Racing Garage claim as follows "the result with a 3-actuator setup is 90% the same, where a three post is lacking is it cannot differentiate between hitting the curbs on the front left or front right". Now if hitting the curbs simply refers to the roll, pitch and heave of the platform, then physically 3 post should achieve the same result, assuming the software works properly.

If you dont mind can you elaborate why you say 4 pegleg does not reflect how car is handling?

" Now if hitting the curbs simply refers to the roll, pitch and heave of the platform, then physically 3 post should achieve the same result, assuming the software works properly."

correct - but they may not have been using simtools so got a different experience

"If you dont mind can you elaborate why you say 4 pegleg does not reflect how car is handling"

no sway, surge or oversteer feedback

(on my sim you can feel the changes to the suspension setup very clearly - different cars feel very different. A driver can push harder if he can feel how the car is moving beneath him)

I'm little confused why are you talking about sway and surge. A 3 or 4 leg vertical post 3dof system doesn't have that. I'm trying to compare these two system.

Yes, it was just an observation.


Tell us about your sim, do you have photos ?

3 actuators vs 4 actuators.



Can happen if the actuators are not tied to ground like the SFX100 style actuators.

For me motion-wise, 3 or 4 actuators are the same as long as there's heave motion.

Without heave, the rig can only rotate around the center. That mean if the front goes up, the rear will always go down. You cannot differentiate whether you hit a bumper at the front or get into a drain at the back, unless you get the information from the screen.

With heave, if the front goes up and at the same time the whole rig goes up(heave), the combination of pitch and heave will make only the front goes up and the rear maintain it's position. Then you can know that you went over something at the front instead of getting into a drain at the back.

Its the same effect with roll + heave or roll + pitch + heave combination. That means you can differentiate which wheel is hitting the curb. Just make sure heave is set correctly and the direction is correct.

Only problem is, heave is usually simulating vehicle acceleration in vertical axis and not position. So you only get accurate motion feedback when you are moving at high speed.