UTAS seat & driver position design

[Jonny Rochester]

On our spec sheet from last year it says 180mm for CoG. Obviously just a made up number. I decided to tip the car up to balance point with a driver in it, and got to 70.5 degrees, or 19.5 degrees from vertical. (Driver was both amused and concerned). Noting the car was sitting on the tyre sidewall I added 20mm and calculated our center of mass was about 345mm from ground with current setup. (We have more ground clearance than we need). Since we are reducing the tracks this year we should also reduce CoG, but just as important I want the driver looking good, not obviously sitting too high. I note the drivers weight could be 25% of the car, so drivers CoG has a significant contribution so I want to push the driver down a bit. Thinking how to design a new car, I came up with this:

Flow chart to establish low driver position and design chassis cockpit.

Establish a floor or reference plane as the lowest point possible.
Draw rollhoops 800mm apart like our old car for a starting point.
Find the steering rack height, used to define heal position. (Our rack is at the bottom, but just above the lower wishbone).
(Heels maybe 100mm above bum).
Set minimum front rollhoop height for 350mm template rule and to clear steering rack.
Set seatback angle (try 45deg, 2014 car was 51deg) or until a 175cm persons visor is above the front hoop.
Set steering column height and angle by drawing a line from toes to chin.
Steering wheel distance ~340mm from chest, or elbows at 90deg.
Set main hoop height using Percy template (95%ile male) or tallest driver, 2“ rule. Allow extra height.
(Shortest driver will need foam to see over front hoop).
Pedal box adjustment is XXmm (5%ile to 95%ile).
Set harness bar height from rules.
Check steering wheel is not higher than front hoop.

Also, I am fishing for comments on my current theory that the steering column should be on a line drawn from the toes to the chin, or near enough, regardless if your driving a F1 or a truck. The theory holds for the sedans I drive.

[mech5496]

Jonny, just a few comments on the CoG height measurement procedure:

1) Put an iron angle on the side of the tires, so that you have a sharp edge to rotate the car about
2) Be extremely cautious when measuring the angle. Calculated height deviates significantly even with small angles deviation (do a spreadsheet and play around a bit)
3) One possible method to overcome (2) is to do a lot of different measurements: both sides and nose-to-tail pitch
4) A possibly better way would be to put 2 scales under the tires on one side (or end) of the car and gradually lift the other side (or end) to predefined angles/heights (the latter is more accurate). Then use weight transfer to calculate CoG height.
5) Remember to lock the suspension. Using solid rods instead of spring/damper units is the easiest way.

[Jonny Rochester]

We had zero time to lock the suspension or change the car in any way, but that is a good idea, and we have solid rods. I just came up with this very fast way to calculate something rather than have nothing to go on. It was a new concept to the team members present so they didn't feel the need to spend time with accuracy. I even had to ask someone to look at the tyre sidewall, as I was busy holding the car. It took several attempts just to explain what I wanted them to look at!

[Jonny Rochester]

I also did it without driver. ~278mm (balanced tilt at 24 degrees from vertical)

[Claude Rouelle]

Jonny,

Here is a few advice

1. Make the steering column axis "hitting" the driver face just between the eyes. Honestly I have no theory or documentation for that, I heard from an older guy when I was still as student but it seems to be working with the small race cars I drew and manufactured many years ago

2. Consider the compromise of CG height (driver leaning low) and the yaw inertia (driver seating up straight)

3. Most students design their car with a Solidwork or Catia driver but forget that the driver needs to turn the steering wheel and there is issue with the elbows hitting the side of the cockpit in corner

4. Make sure that there is enough space between the steering wheels and the leg for the driver inside hand (I mean inside side of the corner) when he steers more than about 60 degrees

5. In my mind there is no need for a seat. Do not waist your money and time and just use the usual expandable foam (2 components) in a big plastic trash bag and make one seat from each driver. Finish it with a nice, light velour that you glue (there are glue that you can spay from a pressurized can)

6. Make sure you have should good shoulder support, ribs and hips support. That can make a difference in the driver ability to feel the car. You "wear" a cockpit like you wear a suit and good tailors design a specific suit for each customer.

7. Pay attention to heels support and side feet support, for the same reason explain in 6. Imagine that the lateral G and longitudinal G will make your foot move and you won't always have enough muscle strengths or reflex to keep your feet where you want or need them to be.

8. The biggest issue I have sees in FSAE and FS cars as far as driver comfort is concerned is the steering torque unusual high efforts; That could screw all the attention and fine work you made on all other parts of the cockpit and seat design. Try to limit your steering to 10 Nm (a passenger car steering torque is about 3 to 5 Nm). You can calculate that torque with both front tire Mz, your estimate tires Fx and Fy (or combined), your caster trail and scrub radius, the toe base (or upright steering arm) and the distance between the steering rack axis and the pinion axis). Here is the ultimate test: put your car on the ground in the workshop, tire at hot pressure, put your driver in the car with all gears: helmet, fire suit, gloves on, belt tight etc... ask him to turn the steering wheel from one side to the other until he reaches the steering rack maximum movement. He must do it 20 round trips (so for example fully left to fully right to fully left) in less than 30 seconds. When I ask this to students at FSAE / FS competitions about 80 % of the FSAE / FS drivers fails. Sometimes because they do not have enough arm muscle or worse they smoke a lot but most of the time it is because they did not care about steering torque numbers.


There is also a lot to be said about steering rack design (or choice) and steering column design and support and their installation but that is another story.

[Claude Rouelle]

Harry,

"A possibly better way would be to put 2 scales under the tires on one side (or end) of the car and gradually lift the other side (or end) to predefined angles/heights (the latter is more accurate). Then use weight transfer to calculate CoG height."

Yeah I know; it is in the Milliken book. Been there, down that with many different cars. Theoretically it should work but practically it doesn't. Simply calculate how much error there is in in your CG height calculations if your scale has a 0.5 kg error if you lift your car on the side (Or by the way from the rear or the front) 30, 45 and 60 degrees. At 60 degrees the error will be smaller that at 30 degrees but it still will be many mm.

In any case, whatever method you use repeat the measurement several times. If all the measurements do no give you a CG height within 1 or 2 mm there is something wrong with the method.

[Jonny Rochester]

2. We are taking 200mm off the front of our car.

5. Expandable foam will be for the 5 percentile female. To see over the hoop.

6. I have looked at F1 seats as I don't think it's too different from what I want. Generous cutouts for the arms, but some shoulder support.

8. We have considered a power steering rack from the Caterham F1 team auction that is current... but seriously that would create more problems.

There was a discussion on the other UTAS thread that I read. Steering effort is a main concern. We have a fast rack. To be honest I'm not confident to calculate it, too many factors.

As head of Ergonomics, I have requested from the Suspension team some different angles. (How arrogant of me!)
I am asking for very small KPI and scrub, of maybe 1degree and 5mm. I don't care exactly, just small numbers for low effort. And caster of 6 degrees maybe acceptable to steer, or trail of 10mm if that is fair. And steering arm will be increased from 70mm of last year to 75 or 80mm if that fits.

For 2014 we had a steering wheel angle of about 100-110 degrees or 215 degrees total, with the wheels turning about 30 degrees. I'm not planing to change this, but we are buying a rack so there is opportunity to.

I got in our 2014 car and checked the steering effort. It is about 19 or 20Nm with my very old torque wrench on the steering wheel. Steering is heavy. The current specs of this are
KPI 12 degrees
scrub ~27mm
caster 6 degrees
camber -1 degree

[ChristianChalliner]

Don't forget to check for assembly errors, I noticed that much of the steering effort in our car comes from crap assembly more than anything, UJ binding + rack bar scrubbing on components + bolts for tie rods in rack clevis's catching on the frame = substantially increased steering effort!

Drop the UJ if you can, I noticed it creates some interesting non-linear feeling but that might just have been the poor install causing this? although it's worth remembering a UJ isn't a CV joint.

There's more but it's basically already covered I think.

Also a fast rack gives the chance to use large steer arms which is a plus! but one can cancel the other in terms of steering effort if you see what I mean.

[Claude Rouelle]

Jonny,

"To be honest I'm not confident to calculate it, too many factors"

Oops. Come on! isn't that what this competition is about? If you don't even try you won't even realize how simple it is. It takes a bit of research on the internet then 5 minutes to build it in simple Excel spreadsheet,

20 Nm steering torque is definitely way too much. The number you give are in the ball park so the issue is somewhere else.

What is your steering rack trail?

Also as Christian suggest try to locate any unusual friction? if you steering torque is nearly the same with your car of or on the ground the issue does not come from the tire - ground friction. Disconnect the left toe link an measure the steering torque. Reconnect the left toe link and disconnect the right one and measure again. Release the torque in the LF top wishbone ball joint and measure again....etc...

Too many FS cars have too many ball joint friction.

Remember that you should machine the cylinder in which you will put your spherical joint joint AFTER you have welded your wishbone. This cylinder will most probably be ovalized during the welding phase and create friction in your spherical joint. That means you need to create a jig with the ball joint cylinder a bit smaller that the the real ball joint outside diameter.

[DougMilliken]

Yeah I know; it is in the Milliken book. Been there, down that with many different cars. Theoretically it should work but practically it doesn't.

Yes, this is a difficult measurement to make with ordinary scales. In our defense, 20 years ago we wrote (page 671):

"Because the CG is often close to the height of the wheel centers (RsubLF and RsubLR) the change in load as the car is jacked is small. To get the best results from this test it is best to take data at several angles and average them."

Jonny -- as long as you have a defined fulcrum, tipping the car up to a balance point is sensible...but this is much harder to do with a full-sized car! I've also seen cars hung from a crane (two cables or chains) to determine CG height. Requires some kind of surveying equipment to extend the lines of the cables "through" the car.