Upright Mass Comparison

[mech5496]

Interesting discussion; would love to see how many teams measure their installed camber/toe compliance.

[Claude Rouelle]

Kevin,

Just to be sure we speak the same language: what in your mind is a "wheel"? A rim? A tire? Or the assembly of both?
As we speak about tire force and moments toe and camber compliance and you have minimum influence on the tire compliance (ok... a bit with pressure and camber ), I guess you are mainly speaking about the rim. Please confirm.

[Claude Rouelle]

Harry,

A few FS team have been testing their cars on a K&C test rig but most of them did not prepare the test properly so only got poor results.
And even if they were decently organized the end up not making good use of the collected data.
I guess you need to go 2 or 3 times to start knowing how to use it. Same thing for wind tunnel 4 or 7 post rig, tire testing machine, of even simple damper dyno.

Some teams have designed and manufacture their own tire K&C. It is not very difficult. Install dummy (solid) dampers.
Take 2 big rod ends at each end of a very thick tube with somewhere in the middle a strain gauge and a turnbuckle. Attach each rod end at the base of a dummy wheel. Link the LF and RF or LR and RR.
Or the LF and LR (or LR and RR) but then you need to lock the wheel in rotation with a solid link between the hub and the upright (to simulate braking - pushing on the brake pedal won't be enough) or the link between the gearbox and the drive shaft (well it depends if you have inboard or outboard brakes or solid axle of a independent suspension,.. that is another story)
Turn the turnbuckle and read on the strain gauge the lateral or longitudinal force you simulate.
Look at the camber and toe and wheelbase and track change. Hold your breath. Scary movie.
We did one ourselves with an Australian racing team a few years ago I show pictures and videos of it in our seminar.
I also show some examples of videos of a FS and of a Nascar on a K&C and on a 7 post rig.
Really scary.
In one of the videos of a FS on a 7 post rig, you see that for a wide range of wheel frequency excitation, that the spring is not moving at all but the axis of the rocker has about 12 mm of lateral deflection (and that is a top 10 FSG car)

Often, the main reason of this unexpected compliance is that most students use static FEA, they do not try it in the frequency range (which I have to admit is using much more computer CPU and memory power, not all universities have such tools)

[BillCobb]

Get Up & Down, Left & Right Tonight

https://www.youtube.com/watch?v=08psY2ILeOo

[mech5496]

Claude, thanks for the insight. Well, we do not have access on a proper K&C rig, but we do our testing by making our own rigs, using similar principles to what you described. It was really educational, especially the first time, where we also measured (or at least tried to) the contribution of each part down the loadpath to the overall deflection. IMO something that every team should do.

[Claude Rouelle]

Harry,

I an only push you (and all other teams) to include some of the compliance number you measured in
1. A simple steady state skip pad simulation
2. Two simple transient simulation; response (yaw velocity, yaw velocity damping, slip angle, yaw velocity settling time, LART lateral acceleration response time, CG yaw angle (beta) rise time, yaw velocity rise time etc...) to 2 sorts of input: step steer and steer frequency.

Bill Cobb has been pushing FSAE teams to do that and he is right; you will get major very useful information.

Try to do it in the time domain and in the frequency domain. You will have a challenge though for the frequency domain; not easy to use non linear tire (piece wise linearization could be a solution)

In the new version of our seminar we do not give away the software of those tests we use in our consulting work but we show the input and output of such simulation and such on track test. Very useful: there are some parts of the car parts design that matter and some that barely do.

If, finally, you can show data from your steering and gyro that validate your simulation. It would be better if it does first time out - we can dream - but if it doesn't no big deal it is a learning process)

[Kevin Hayward]

Claude,

To confirm; I was referring to the wheel rim (and centre). This particular component was chosen just as an example of something else in the path between the tyre and the chassis.

We were performing system stiffness tests with pretty simple gear way back when at UWA. The fancier K&C rigs are nice, and something the team found access to in the US after I had left. For my money I like the immediacy (and ridiculously low cost) of a few dial gauges and a couple of ratchet straps.

Typical FSAE wheel rims tend to deflect quite a lot for fairly obvious reasons. They pose a problem that is pretty much choose two out of the following:

- Stiff
- Low Weight
- Low Cost (time/money)

Poorly designed uprights, wheel centres, bolted connections and/or bearing arrangements can cause significant deflection. This can be readily observed at any FS competition.

...

I don't really adhere to the "it must be beautiful approach". Beauty is subjective, but the ability of an object to deflect a certain amount for a known load is not. I offer the following list of simple ideas to follow. Note there is nothing special about these points, being merely application of well known engineering principles. If you are already following these principles, please feel free to ignore the post. If you are a member of one of the many teams where your suspension is deflecting far more than it should please note that you will likely have already ignored these suggestions. By the way if you are unsure of whether your suspension is deflecting too much then you might well want to check.

1) Loads direction and magnitude changes, and in most load cases of interest is off-axis
2) Load paths should have minimum curvature (straight lines are best)
3) Structures need depth to resist bending loads
4) Bearings have slop so space them apart
5) More connections/parts means more opportunity for deflection
6) Resist machining all the way through (leaving thin webs can do wonders for resisting off axis loads)
7) I beams, and C channels are better than plates, but eggs are fantastic
8) Stress will result in strain (a few highly stressed areas can dominate your total deflection, stiff light parts have well distributed stresses)
9) Think about buckling (columns, thin sheets)

Lastly I would suggest that the material matters less than most would think. The specific modulus of steel, aluminium and titanium are all pretty similar. Carbon fibre reinforced plastics are a stand-out in this area, but should be used with caution for parts with high cyclic loads. Caution doesn't mean don't do it, it just means having good quality control practices and lots of physical validation.

Kev

[Claude Rouelle]

Kevin

"5) More connections/parts means more opportunity for deflection "

Yep. That is why until proven otherwise, ideally by simulation and, even better, by measurement validation and comparison, I have concerns about your university car steering system wit its additional rockers.

That being said Your 1 to 9 list of compliance cause and how to avoid or at least decrease compliance is excellent.

The aesthetic is not an obsession. Neither for me or I guess FS design judges as there are only 5 out of 150 points for aesthetic. Just have noticed (and was pushed to notice by former teachers / mentors) that often when it is light and rigid is is "pleasing to the eye".

A soft spring in series with a stiff spring is still a soft spring and you are right to mention the rim (what you call the wheel): the rim is the biggest cause of camber compliance a 13 " 3 pieces bolted aluminium rim has about 0.7 deg of compliance per G.

[Kevin Hayward]

Claude,

I will bite.

You seem concerned about the additional rockers, but do not consider the removal of the universal joints as being beneficial?

The majority of the deflection in the steering system (others and others) is due to slop. Assuming the angle of deflection (a) is small we can say that a=sin(a). Assume a slop of s for a given joint, and a distance d between bearing surfaces. This leaves us with a total angular deflection of a=s/d for a given joint.

Making some reasonable approximations (the forum version of the calcs) the d for the rocker is around 80mm and there are three joints per rocker. Therefore the total angular deflection due to slop is approximately 3s/70. Note the slop is directly proportional to the tolerance of the connection.

For a typical universal joint in these cars the angular deflection due to slop is approximately s/8 (2s/16). The UJ will happen before the steering gear reduction (4) so we need to include that for the rocker.

Therefore the UJ will have a angular slop of approximately s/8 at the steering wheel, and the rocker will have around s/7 (taking into account the 4 ratio). So it comes down to tolerances of the holes/pins, with the rocker having around 15% more slop for a given tolerance. Obviously removing the UJs/rockers altogether is best.

I will note at this point that the UJs (of the sizes generally used by teams) tend to wear pretty bad, and many cars that start out with low slop in their steering can develop it pretty quickly.

...

But that is not the whole story. The dominant contributors of slop are easily the quick release and the steering gearbox (rack & pinion). The former was improved by not using poorly fitting splines or hex joints in most quick releases. The latter is vastly improved with a planetary gearbox with 3 tooth contacts instead of the single tooth contact from the rack and pinion.


In order of magnitude of deflection:

1) Quick release / steering gears
2) Connection slop
3) Component deflection / rod end play

I am still unsure as to why you think making a steering rocker with low slop/deflection is significantly more difficult than doing so with a much higher loaded suspension rocker. The components are appropriately sized for the task.

Still the holy grail of low slop FSAE steering is a direct acting pitman arm with no gear reduction, universal joints, or quick release.

...

I will add that the team spent a fair bit of time in developing the system with physical prototyping. A very early version to check the kinematics used a commercially available planetary gearbox mounted near the steering wheel (such that it doubled as the upper steering shaft mount). In this version the steering shaft was required to take the multiplied steering torque post gearbox multiplication. At that stage the gearbox was a 4.5 ratio. Seeing 4.5 times the steering shaft twist was significant. Earlier versions also had the rocker mounts attached to a pedal box, which was quickly scrapped for a much stiffer separate chassis attachment. The primary concerns in initial development were slop and deflection, both of which ended up much better than our older more typical steering systems. The focus for the latest car was keeping the same outcomes, but reducing weight, which was largely accomplished by better integration.

From a conceptual level when compared to a more typical installation the system has less slop, improved steering geometry, and allows for a lower COG (by lowering vehicle's nose). The downsides were cost (time and money) and additional weight. Make your own call on whether the advantages outweigh the disadvantages.


So I have four questions for you (two from a design judge perspective, and two as a vehicle dynamicist):

1) Would you accept a process involving appropriate engineering calculations, multiple physical prototypes, and physical measurements as being sufficient for proof?

2) As a design judge do you place similar requirements of the more conventional teams to perform the same process on systems with universal joints and steering racks?

3) What is the appropriate trade-off for weight and COG height? Is it ever worth adding weight to reduce COG height, or conversely is it ever detrimental to remove weight at the same time as increasing COG height?

4) What is the relative performance advantage of improved steering geometry (as compared to say mass)?

Kev

[Mitchell]

Kev,

Surely the ECU system also has 3-5x the toe base normally seen when teams try and squeeze ~25+ degree of wheel lock into 10" wheels.