torsional stiffness

[Karam El Hady]

Hi everyone i'd like to know how is torsional stiffness is good for chassis and how is it related to suspension force and what is the optimum valve of torsion stiffness for space tubular frame
and when i know that the valve of torsional stiffness enough to resist the suspension forces ?

[Claude Rouelle]

Karam,

Based on what I have seen in many years and many competitions for a tubular chassis 2000 Nm/deg from hub to hub (what is the point to have a stiff chassis if you have uprights and wishbones in chewing gum- a stiff spring in series with a soft spring is still a soft spring) and less than 25 kg would be a good target.

For a composite monocoque 8000 Nm/deg and under 15 kg is a good target

Just to give you an idea an LMP1 is now over 100KNm/deg. Bigger than F1. Because LMP1 cars rear bulkhead is much lager (bigger m^4) and it has a roof.

Make sure you calculate and you measure everything. Most of the time FEA is too optimistic. See picture. Without validation you won't impress any design judges.

Some students measure the chassis torsion stiffness from the front to the rear bulkhead. OK, it is a number but it is not relevant. Chassis torsion comes from a different roll torque front and rear. The forces comes from the tires to the chassis via the suspension and the inboard pick up points. That is why hub to hub measurements make sense. Be aware you still miss the compliance of the rim and the inevitable compliance of the tire.

Make sure you calculate and measure the chassis torsion stiffness distribution: you can have a first half of the chassis stiff and the second part stiff or vice versa and still have a total chassis torsion stiffness the same. But the car behavior will be different. A stiffer front part of the chassis will have the same effect as a stiffer front ARB (qualitatively, not quantitatively). When you change the Total Lateral Load Transfer Distribution (TLLTD) - what I call a magic number, there will be a "sweet spot, within a few 1/10 of one percent- that will give you a good balance performance - you change the Yaw moment. On a Go Kart there isn't front or rear ARB or springs either but they change the TLLTD by adding (bolting) or removing a front of a rear chassis brace

If I was you I would measure the chassis torsion in at least five different planes along the X axis. Usually the cockpit area is the weakest.

To know what chassis torsion stiffness does on your car behavior you will need to simulate it. You need a tire model and a good vehicle dynamics model to do so.

If you want to make simulate in transient you will need the chassis torsion damping. I let you find out how to do that...

[Claude Rouelle]

A cheap, alternative solution for chassis torsion stiffness measurement

Make sure the car has dummy – rigid – dampers.

Put shims of aluminum (lets' say 200 x 200 x 1 mm) between the front wheels (or, better, the setup – rigid -wheels) and the setup pad scales.
For example, 10 sheets of 1 mm under each of the LF and RF tires

Jack the car up. Remove 1 sheet from the RF and put it under the LF wheel. You now have 11 mm of shims under the LF wheel and 9 mm under the RF wheel. Put the car down the scales and make a note of all 4 corner weights. Continue like this until the LR load is zero or very close to zero.
Let's say that this happen when you have 17 mm of shims (+7) under the LF and 3 mm (-7) under the RF wheel. The torsion angle is ATAN (7+7)/1200 = 0.668 deg.

Measure the LF and RF corner weight. Calculate the LF and RF corner weight variation. Let’s say it is + 5 Kg on the LF and – 5 Kg under the RF.
Multiply the sum of the LF and RF corner weight variation by the track; you will have a front input torque. Let's say the front track is 1.2 meter, so the torque is (5+5)*9.81*1.2 = 117.7 Nm

Calculate the rear torque which is the opposite sign of the front one. With all the rear load on the RR, the rear torque is, with a rear track of 1.1 m; - (55 + 55)* 9.81* 1.1 = - 1187.0 Nm

Make the difference between the 2 torques, front and rear: 117.7 – (-1187.0) = 1304.7 Nm

The chassis torsion stiffness is 1304.7 Nm/0.668 = 1952.0 Nm/deg.

Worth to also do the same with real tire and dummy dampers or real springs and dummy wheels of real tires and real springs

[Claude Rouelle]

Spreadsheet attached

[BillCobb]

Instead of just dealing with a trivial "chassis torsional stiffness", do a normal modes analysis in FEA and also on the actual car so that chassis, motor and steering parts don't get fastened to spongy locations.

If your stacked (all parts assembled) model is any good, so will be you car. Otherwise, this will be just sandbox play.

https://www.youtube.com/watch?v=xXCeYg969o0


The Matlab CFE looks useful, too.

https://www.youtube.com/watch?v=iWzG8Uqu_dY

[FrederikWe]

Dear Claude,
Where do you have those numbers from? Are there teams out there that really combine a stiffness of 8000 Nm/° and weight of 15 kg? I mean you would surely know and I don’t want to say that I don’t believe you. And if it is also a hub-to-hub stiffness, it would mean that the monocoque is even stiffer?

Realizing weights around and below 15 kg you need a small monocoque. If your skin aera is to big you reach a point where bigger panel heights can't be compensated by thinner ply thicknesses and your mono gets heavier. But small monocoques usually feature a lower polar moment of inertia between the hubs which doesn’t help the torsional stiffness. Even if you go for UHM fibres and maximize the amount of fibres in the 45° degrees directions, I find it hard to believe that you can archieve values like that from what i know.

-Freddy

[dr. ill]

"For a composite monocoque 8000 Nm/deg and under 15 kg is a good target". Those may be good 'targets' but not many teams are achieving those numbers. More realistic numbers are ~3000-4000 Nm/deg and 22-27 Kg include hoops.

[Claude Rouelle]

That is what a few teams have told me. ETS is an example. I did not weight or measure the torsional stiffness myself so I have to trust them. That being said, compared with the large numbers of different cars I know, these numbers seem logical and possible.
Western Australia 10 years ago was in the 6000 Nm/deg region if I remember. If I am wrong any team is welcome to give more information.

Karam asked for some numbers, I gave what I have.

What I am more interested in is the effect of the chassis weight and hub-to-hub torsional stiffness and even more torsional stiffness and damping distribution on the car performance: criteria such as lap time obviously but more simply and probably more useful, response of yaw velocity, lateral acceleration response time (LART), yaw velocity damping, etc to step steer and chirp of steering input (same amplitude but different frequencies)

That is what will help you to make a decision on the compromise weight Vs torsional stiffness Vs time and $

[Claude Rouelle]

Freddy,

"Realizing weights around and below 15 kg you need a small monocoque." Well do you think that the goal of a Formula Student team is to create big cars?

After over 15 years of judging cars in many different competitions I can tell you that what was mission impossible for many teams at year X becomes possible at year X+2 or X+3. You have now cars that are in the 130 KG range but 5 years ago many teams in many FS paddocks told me it was impossible. It is often a question of believing and daring more than anything else.

Claude

[Claude Rouelle]

If you are in the "~3000-4000 Nm/deg and 22-27 Kg include hoops" region I am not sure why you would want an expensive composite material chassis: lots of expense for not such a big difference. You may want to use your time, focus, energy and money somewhere else. Like testing more.