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Post rigs
Posted: 04 Dec 2009, 15:16
by Belatti
Lets start quoting this:
The seven-post rig is used in racing work because aerodynamic downforce and track banking add to the wheel loads. The amount of load added can be more than the static initial weight, so it must be included in the test procedure. The seven “posts” are hydraulic cylinders. Four of them have flat pans the tires sit on and support the car. The other three are called the “aeroloaders” and attach to the sprung mass. Normally, two are mounted to the front of the chassis some distance apart while the third one is mounted at the rear on centerline. Loading on these cylinders is done to pull the car down, opposing the four wheel pans. The aeroloaders simulate other forces on the car such as the squashing from inertia loads as the car rolls through a banked turn or deflections due to aerodynamic loading.
By adjusting the load on the three downforce rams we can simulate any combination of roll, heave or pitch displacement to recreate specific conditions seen on the track and repeat that condition. Normally, wheel travels from actual test-session recordings are re-created in the lab. By using the correct deflections indicated by wheel travel with the same springs and bars as those used in the track test, the loads will be correct. Deflections are used because race teams seldom have vertical loads as a measurement.
Now that we all know what a post rig is
I would like to discuss here the whats and hows for its operation and understanding. What are the variables used to calculate the suspension optimization and how they calculate them. I guess it may be a vertical load average and its variance. But I also have heard other stuff...
Also it would be interesting to know how its the post rig built. How are the sensors and actuators attached, etc.
I know there are many teams (not F1) that have built their own 1/4 or 1/2 car post rigs with good results.
Re: Post rigs
Posted: 04 Dec 2009, 15:27
by Jersey Tom
Just a bunch of hydraulic actuators. You run a command set that puts the car through appropriate low- and high-speed motion (roll/pitch/heave and corner vibration respectively).
Average loads shouldn't change.. but minimizing RMS load variation is indeed one strategy.
Re: Post rigs
Posted: 04 Dec 2009, 15:33
by zgred
[youtube]
http://www.youtube.com/watch?v=HYmgjIWPSSw[/youtube]
It is always nice to literally see the problem :]
Re: Post rigs
Posted: 04 Dec 2009, 15:35
by noname
Belatti wrote:Also it would be interesting to know how its the post rig built.
http://www.satec.com/ist/press_releases ... 2005_s.pdf
(very) brief overview
Re: Post rigs
Posted: 04 Dec 2009, 20:07
by Belatti
Thanks noname!
So, sensors only in the car and actuators in the rig.
Its interesting that there are rigs with "X" and "Y" actuators as well. And that they are not only used to tune suspension but to also check materials strenght and chasis stiffness. Cool.
Re: Post rigs
Posted: 05 Dec 2009, 02:15
by Giblet
Are any of the teams using an 8 post rig yet?
the 8th post is to load individual points of the car (ie one wheel).
Re: Post rigs
Posted: 05 Dec 2009, 07:17
by Jersey Tom
Giblet wrote:Are any of the teams using an 8 post rig yet?
the 8th post is to load individual points of the car (ie one wheel).
You already control each of the individual corner loads with 3 DOF's (heave, roll, pitch)...
Unless you mean adding banking and warp stiffness effects?
Re: Post rigs
Posted: 05 Dec 2009, 13:11
by ackzsel
They are not rotating the tires in the video. I can't imagine stationary tires behave the same like rotating tires.
Re: Post rigs
Posted: 05 Dec 2009, 14:41
by ESPImperium
Red Bulls 7 post rig here:
https://www.redbullcontentpool.com/stoc ... 1339332536
Shows quite detailed movements of the RB4 on the 7 post rig, and the nose of the RB5 at the start on the flex rig.
Re: Post rigs
Posted: 05 Dec 2009, 16:27
by marcush.
I actually had a very revealing conversation about this with one of the testengineers of the cardevelopment at an OEM .The guy does drive those cars day after day ,and his input decides on the final development of the car setup for series production.He stated that someday he just wanted to see what the feeling was ,sitting in the car on the rig doing his wellknown testtrack profile....and he told me that frankly speaking it was very very hard for him to recognise his hometrack on the fourpost rig they used at that time .Obviously the testing is useful to detect some issues ,but it is in no way something you could even think off replaying the real thing.It is just simulating some aspects of the real situation and helps to detect problems associated with these aspects.For sure it cannot be an alternative to tracktesting.
Re: Post rigs
Posted: 05 Dec 2009, 18:09
by Jersey Tom
ackzsel wrote:They are not rotating the tires in the video. I can't imagine stationary tires behave the same like rotating tires.
Correct. This is a downfall of the 7-post. But while the results may not be exact they're also not garbage.
Re: Post rigs
Posted: 05 Dec 2009, 21:18
by autogyro
Jersey Tom wrote:ackzsel wrote:They are not rotating the tires in the video. I can't imagine stationary tires behave the same like rotating tires.
Correct. This is a downfall of the 7-post. But while the results may not be exact they're also not garbage.
I doubt the results are complete garbage but I still come to the same conclusion as I do for fluid dynamics.
If by now there is not sufficient data available to do all future work on computer, then the whole exercise is simply a way to fly a kite for the marketing departments.
Still needs a driver and test track............realy.
Re: Post rigs
Posted: 05 Dec 2009, 22:06
by Mystery Steve
It would be great if you could just easily simulate everything on the computer, but there are a lot of complexities when you start talking about the structure of the entire vehicle that are difficult to model effectively. You either have do a very complex finite element analysis which is tedious to set up and a single iteration can take a considerable amount of time to run even with parallel processing. Or you could use a simpler dynamic model, but then you have difficulties properly representing the stiffness of the entire structure. This is the same as the CFD vs. Wind Tunnel vs. Track Testing debate. Simulations, post rig testing, and track testing all have their place in the design process, and they all offer capabilities that the others do not.
Re: Post rigs
Posted: 05 Dec 2009, 22:36
by BreezyRacer
It was interesting to see the RB video showing the flex on the front wing mounting. This goes perfectly hand in hand with the cabling for the front wing they used during testing just this week.
viewtopic.php?f=12&t=6342&start=735
Re: Post rigs
Posted: 05 Dec 2009, 23:22
by Mystery Steve