Inertia Test Rig

[Toomey]

Posted October 24, 2012 12:00 PM
Hi Guys,

I'm looking to design and build an inertia test rig. was thinking about just hanging four steel wires from two lengths of box section. Would this suffice? If anyone has some experience with regard to this it would be much appreciated. Would consider buying one if there available.

Thanks**

[Claude Rouelle]

If I was you and I would be in the US, I would consider not reinventing the wheel (you are building a FSAE car, not competing in a inertia bench design competition) and at least consider the benefit / cost ratio of renting the SEA (SEA not SAE) inertia test bench in Columbus, OH. http://www.sealimited.com/vehicle-dynamics
Contact Gary Haydinger there. Maybe they will give you a special FSAE discount.

Good luck

[Warpspeed]

Toomey,
Google "trifilar pendulum".

[Toomey]

Hi Tony,
my initial design was based on the trifilar pendulum. However I want to work out pitch and roll as well as yaw.

Thanks for the reply.

[SNasello]

Theoretically you can use the trifilar for roll and pitch as well, except standing the car on its nose isn't too easy (or safe). For roll and pitch you can just build a giant swing.

Thin steel cables (2mm) should be able to hold the weight of an FSAE car easily. You somehow need to mount them to a pivot axis so that there is little friction between the cable and pivot.

If possible, I would suggest measuring the frequency with a sensor and data acquisition system (wire potentiometers work well for this).

[Moop]

You can determine an object's entire inertia tensor with a trifilar, as long as you can figure out 6 linearly independent ways of mounting it. You may want to do more tests than that though, so you can calculate a couple of different estimates of the inertia tensor and average them.

[Toomey]

Thanks for the info lads!
Really appreciate it

[rmk36]

Originally posted by Claude Rouelle:
If I was you and I would be in the US, I would consider not reinventing the wheel (you are building a FSAE car, not competing in a inertia bench design competition) and at least consider the benefit / cost ratio of renting the SEA (SEA not SAE) inertia test bench in Columbus, OH. http://www.sealimited.com/vehicle-dynamics
Contact Gary Haydinger there. Maybe they will give you a special FSAE discount.

Good luck

I remember getting a quote on this. Somewhere in the neighborhood of 1,000USD to measure the 3 axes.

[Claude Rouelle]

Well not only the inertia in 3 axes but also suspended and non suspended mass inertia, CGs with and without driver, with and without fuel etc.... Be smart; if you have the time and the money prepare this test, ask questions. Worst thing is to go home and one hour later tell yourself .. ouuch for the same price we should have done this of that too.... Try for example 2 very different drivers and/or driver positions.

Make a simple calculation of your zz inertia of all 4 non suspended masses around a vertical axis going though the total mass CG and compared it with the inertia of the engine or engine+gearbox assembly around the same axis. You could be surprised.

One other interesting thing is to compare the calculated inertia (difficult to take into account every nut and bold, wiring loom, bodywork parts, welding and basically every non-homogeneous material) with the reality (if you assume that the measurement is the reality) and try to find out by how much and why there is a difference.

Evaluate which car part influence the most xx, yy, and zz suspended mass and total mass inertia. Ask yourself for example why the xx inertia (xx, not zz or yy) of the suspended mass around its roll axis (yeah I know ....what and where is the suspended mass roll axis ... it is not only a question of pure kinematics... I know but I am not going to go there in this part of the forum....)and the inertia of the non-suspended mass around its instant axis is important in the weight transfer Vs time simulation and how these numbers could be screwed if compliance is taken into account.

And so on... Anyway... You thought vehicle dynamics, race car engineering and Formula Student was easy; you should have gone fishing.

One more advise; a design judge will ask you why and how you used these inertia numbers and how this did help you to make a faster and/or more drive-able car. If you can't show him some real transient simulation taking also (but only) your spring, ARB and tire stiffness as well as your damper curves for different kind of lateral and/or longitudinal G and/or steering input (welcome to system of differential equations) backed up with data analysis overlays and well described (written ideally) driver subjective comments, the judge could end up thinking you did waste your time and money.

What the fun!

[Warpspeed]

It certainly is great fun, and fun a very low dollar team can have with absolutely minimal basic equipment.