Vehicle Dynamics starting points and design process

Just a quick advice on the brakes issue: The usual way to do this is designing a brake system that already has the desired balance set via the components (piston diameters, rotor sizes etc...) and just using the balance bar to fine tune it. I doubt that the adjustment range of a pressure regulator is big enough to offset the balance from 50:50 to what you need. Should be somewhere around 75:25 front to rear... But you need to find that out via CoG height and weight transfer. Doing this, you can then calculate the required line pressures and see if your regulator has enough range to drop the pressure to the desired value... but I highly doubt that.

Luniz, 75:25 is much higher than I expected, I was thinking more in the region 60:40. 75:25 to me seems like you will be locking the fronts up long before you're making full use of what you have available although it makes the car more stable on the brakes.

I understand that usually you would work it out that way but you're forgetting the strange environment I'm in, unless cast iron evidence that we need more front bias is presented it isn't accepted. So put simply if in testing the regulator is wound all the way to the front and the rears are still locking first I win as I've got justification to purchase a new rear master cylinder (cheaper and easier swap than rear calipers). It's possible to run higher mu value pads at the front to change the bias but it's doubtful that the pads front to rear will maintain the same mu value ratio across a run and wandering bias isn't something conducive to driver confidence.

Christian,

FYI the ideal DYNAMIC weight distribution in braking is 50 - 50, that is if you have the same front and rear tires That will tell you what your ideal static weight distribution should be for a given wheelbase, a given CG height and a given longitudinal deceleration ....which is dependent of your tire longitudinal grip. That will guide you to your front/ rear braking force distribution.

Beware that the ideal weight distribution in braking is not the one in cornering neither the one in acceleration; you will have to make compromise depending of the shape of your circuit.

Christian,

"75:25 to me seems like you will be locking the fronts up long before you're making full use of what you have available although it makes the car more stable on the brakes"

Based on what calculations?

How do you define stability in braking?

"as I've got justification to purchase a new rear master cylinder (cheaper and easier swap than rear calipers)" Or another front. But you should not go through multiple trail and errors sessions: simple calculations should tell you what master cylinder size you need.

Christian,

Easiest first.

BRAKES - The Rules mandate two independent circuits for F/R. So tossing the bias-bar+2-MCs means you would have to use a larger "tandem-piston-MC" (ie. the type with two equal diameter pistons in series). And you would then still have to add the pressure limiter. So NOT much simpler than the bias-bar arrangement. The two-pedal + hand-clutch is a good idea though, and gives plenty of room for the bias-bar.
~o0o~

STEERING - Good ackermann comes from having the correct angles between the Steering-Tie-Rod and the Steer-Arm, and also between T-R and Pitman-Arm, when using one. Try angles of 60, 70, 80, and 90 degrees, in the directions I indicated in my last post. Then, from the observed pattern, home in on better angles. Start with the SA as long as possible, and make the PA about 70 - 80% as long. So, if PA = 100 mm long, => SA = ~130 mm long, AT LEAST.

"Why do I want the crown gear above the pinion?"

So when you move top of steering wheel to right, the car goes to right! :) (Ie. it allows rearward-PA, forward-SA, and generally good ackermann.)

"Springloading gears ... not sure how I'd achieve that through a 90 degree turn..."

Firstly, the idea of springloading the gears (as is done on all production car R&Ps, EVER!) is to eliminate backlash. Especially for the teeth that contact near straight-ahead driving, which wear down faster than the other teeth. The idea is to provide a "suspension" for one set of teeth that allows them to ride up-and-down as they roll over the worn-down teeth.

With BGB, my suggestion (in earlier post here and on "Beam-Axle..." thread +) is to have the pinion-gear-shaft mounted in the housing in the normal fashion, say on two deep-groove-ball-bearings. Thus it is fully constrained, except for rotation. But (!), the crown-gear-shaft is mounted in two bronze-bushes (easiest), or perhaps two needle-bearings (maybe next time), so that it CAN FREELY SLIDE VERTICALLY. So, only radial constraint, but free to rotate, and FREE TO MOVE AXIALLY (ie. up-down).

The last step is to provide a circular pad (preferably bronze, but steel will do) that sits on top-centre of the crown-gear-shaft, and has a spring between it and the housing that pushes the pad and shaft down (with force of ~10 - 30 kg?). The crown-gear is now always in tight mesh with the pinion-gear, but it can ride up-and-down as it rolls over the pinion-gear's worn-down teeth.

The usual "adjustment" process is to screw down the spring until it is coil-bound (ie. solid), then back-off about 1 mm. This allows ~1 mm of "suspension travel", but stops the teeth separating any more than 1 mm, so they don't slip over each other. And, BTW, choose gears with fewer but BIGGER teeth. This much better than many little teeth! I would like at least 10 mm pitch (ie. centre-to-centre of teeth).
~o0o~

MODEL-T BEAM - Engineers vs Artisans...

"... my main concern is how I would make a model T style beam? It's easy to bend some tube to the shape ...
... but it's a different story to start flattening tubes and still argue it's easy to fabricate!"

[Mini-Rant :)] IMO the "Meccano Set" of recent times has SET BACK Engineering much more than it has advanced it. Nowadays, and even back in my day, far too many young Engineers think that "engineering" is about collecting together large numbers of prefabricated parts, such as shafts and wheels and plates and brackets and things with lots of holes drilled in them, and then assembling said multitude of parts with another multitude of nuts and bolts and washers and other bits and bobs...

On the other hand, ... on the other side of the world, in any remote village in Africa, the approach is very different. There the young boys don't have the luxury of selecting from a multitude of prefabricated parts. Instead, if they are lucky enough, then, perhaps, maybe..., they might have some fencing wire. Maybe even a single pair of pliers, in the whole village. Although fingers usually suffice.

From said raw materials, namely a piece of wire, these young boys, maybe 10 years old, craft a small man about a foot high. Their creation has fully articulated arms and legs. He sits on a little bicycle, complete with rotating pedals and wheels. The young boys attach a long thin stick to the bicycle seat and push their creation around in front of them, with the rider's legs going up and down, and the wheels turning around and around...

Quite brilliant, really. From almost nothing, a small mechanical-man. Most ingenious!

But back in the civilised world, I suspect that most student Engineers, 20+ years old (and also too many Senior Engineers!), think that hammering a nail into a piece of wood is too difficult a "fabrication" step.

"You want to do what!!! How the (bleep!) are we going to write a Matlab program to calculate the optimum kinetic energy needed from that "ham-whatever" thing you're proposing... Geeezzz ... ya gotta think about the practicalities here!!!"

Anyway, (IMO :)) good Engineering is more about being able to do good ART, than it is about being able to do good calculations. When the "art" is right the product works. But good calcs with bad fabrication is just junk. [End Mini-Rant]
~o0o~

Soooo..., the skills you need are more black-smithing, tinkering, panel-beating, etc., and less Matlab-ing, Solidworks-ing, etc. As mentioned before, sheet-metal working skills for ~0.3 - 3 mm thick steel will get you most of the way. I find 1.6 mm, = 1/16", = 16 g, particularly easy and useful.

The main part of the beam is just a tube, as you already have. The two tapered "torque arms" can be done like this. Cut two wedge shaped pieces of 1.6 mm thick steel that are ~600 mm long, ~200 mm wide at one end, and ~100 mm wide at the other end. Use a folder or press to put multiple small bends in it, along its long axis, so that its cross-section resembles an angular C-shape.

Next, use blocks of wood and a biggish hammer, or a big vice, or a small hydraulic press, to squash the C into a taller-than-wide polygonal O-shape. IMPORTANT! Evict all Academic staff from the area while you are doing this last step, lest they have kittens. They are terrified of hammers! Seam-weld the joint with TIG. Best to have this joint on a corner of the polygon section for easier welding.

Lastly, weld the two tapered torque-arms to the main tube, and to a gubbin that carries the main apex-BJ (which should be generously sized, say 10 mm bore). Add some nice gussets at the triangle's three corners for a true craftsman-like job. Done!

Using only "Cardboard-Aided-Design" to get the right shape for the sheets before bending, I reckon about one craftsman-day of work to make the whole beam. Students will take longer, a few days or maybe a week, and should practice first on offcuts of sheet.
~o0o~

LATERAL CONTROL OF BEAM - For this, and given the frame layout you now have, I again suggest a Ball-in-Tube joint at the centre-bottom of the front-bulkhead, with a wishbone going back to connect to the beam via 2 x BJs roughly where the torque-arms attach to the main tube. The B-T could just be a rubber bush, because very little fore-aft movement there.
~o0o~

Ahhh... this would be so much easier to explain with a very simple sketching facility here...

Enough for now. I am heading down to Melbourne in the next few days, so may not be able to add much in the next week...

Z

Quote:

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Originally Posted by ChristianChalliner

I understand that usually you would work it out that way but you're forgetting the strange environment I'm in, unless cast iron evidence that we need more front bias is presented it isn't accepted.

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Wouldn't a very simple weight transfer calculation at 1g of deceleration be "cast iron" enough to convince the decision makers of the need to run different size master cylinders? This kind of calculation should be understood even by the most idiotic white collar decision making tie wearers... Also, don't nail me down to that 75:25, could also be 66:34 or whatever. It's been a while since I have designed a brake system for my team and I can't remember the numbers any more ;-)

I just found an online calculator for the brake bias and punched in some ballpark numbers (13" tires, 25.4mm caliper piston diameter with 2piston calipers...) , and this is what I got:

http://brakepower.com/

But as a word of advice: Try to put your own spreadsheet together for this calculation so you can actually understand how it is done...

I don't have much time so I can't write a full reply but thank you all for the input (good to see you adding your thoughts again Claude :) ), I ran the numbers through that online calculator assuming a 1g stop and modifying the numbers until the front master cylinder came out at ~14mm (to match the current MC size) I ended up with the following:

http://i1296.photobucket.com/albums/...ps29142621.png

Numbers speak for themselves I suppose.

Will write a full response later.

You may have forgotten to add the driver weight, 220kg seems a bit optimistic for a first year, 4cylinder, spaceframed car on 13" tires, and I think you might have forgotten to factor the driver into the static longitudinal weight distribution as well, and I'd question the CoG height... Also, the rolling radius of the tires is a bit high, the hoosiers have 21" OD. But as far as the bias is concerned, yes, you are likely to end up somewhere between 60:40 and 75:25 depending on the numbers.

Luniz - Yep all correct! In my haste I made quite a few mistakes there, the radius should be divided by 2 and the driver weight should add ~60kg to the car weight. CofG has been calculated via the CAD models plus actual measure component masses but I do agree it appears optimistic.

Edit: actually i have no idea where that 305mm came from, should be 254mm.