Vehicle Dynamics starting points and design process

[Jay Lawrence]

Christian,

I don't believe there is anything stopping you from running you steering link from upright to upright, though I imagine you will have a hard time packaging it. Also on steering: the ADFA car shown above had issues at comp (may have been 2009) with a very similar design wherein they had to run their wet tyres in order to meet ground clearance rules due to the location of their steering rack. At least that is what I was told, and they were certainly running wets when the weather was fine. Also, there's obviously some linearity concerns with those ADFA steering rods, but you're probably already aware of that. Just some things to be aware of with this type of design.

[ChristianChalliner]

Thanks Jay, good point on the link running across the car, I hadn't picked up on that somehow, there are ways around it I suppose but it would result in multiple links and presumably lots of system play. That's a strange one on the wets as unless the car didn't meet the 25.4mm minimum bump travel or the rack was touching the ground I can't see how it could have failed to meet the ground clearance rule.

With regards to the steering I've been going through a few rough ideas, it seems fairly simple to make a telescopic linkage which actually physically connects and steers the wheels but the tricky part is actually altering the gear ratio whilst turning the shaft through the 80-90 degrees. It seems obvious to say bevel gearbox but finding one that isn't just a 1:1 ratio or enormous has actually been quite hard (I haven't found one yet). Another issue with the bevel box is the positioning of it in the car and the fact it reverses the shaft direction, seems easy enough right? just flip the inboard steer arm around... But that creates another problem in that then the shaft will be coming down rearwards of the FRH and it makes it harder to achieve good ackerman (at least with rearward pickups on the uprights). So I looked at another solution, a double UJ to turn the column through 80-90 degrees but then you start to end up with stupidly small steering wheel angles creating very high wheel angles unless you make the inboard arm very small which creates YET ANOTHER problem in that then the outside wheel wont turn at all in comparison to the inside wheel, in short, this is a complete pain and there seems to be no way of having your cake and eating it!

I'd rather not have to make my own bevel gearbox but I suppose that would be a possibility if I can find somewhere to cut the gears for me.

I think there must be a few arrangements I haven't yet tried which I need to which will solve the above, I'm sure it's possible, I've just not hit on it yet.

[Mitchell]

Have a look at the ECU steering setup, would work well for beam axles

[JSR]

We also had this idea for a pedalbox...



Which removes the excessive length of this current one

Maybe something like this then:

[ChristianChalliner]

Mitchell - Thanks! will do

JSR - YES! Exactly like that, even the tilting of the cylinders is how I was going to do it but perhaps even more aggressively and then tuck the reservoirs away in a frame gap covered by a thin non porous skin/bag (no heavy plate or 'shield').

I've been looking at the beam axles thread, some (lots) of interesting stuff in there. The Cylindroid concept helped my understanding somewhat, or at the very least affirmed my view (perhaps mistakenly?) that movements about the roll axis will have an effect on the pitch axis location, movement, rotation, etc. I'm also fairly sure that the compromised anti-dive geometry in favor of lower axle roll steer is the better of my two posted ideas, it's still not ideal but it does have the advantage that it would be rather easy to integrate into a chassis design. I also considered the implications of hanging hard points from the front bulkhead and main hoop, I came to the conclusion that it will/should work well when it comes to jigging the chassis since they are both 'main' interface locations on the car (and the FRH for the front shock mounts). This would be helpful since I've been told that the last chassis was awful to jig up.

The beam axles have been received favorably by team members especially after a quick chassis concept was drawn up demonstrating how simple the frame can be made and was somewhat hesitantly received by the supervisors but they've pledged their support to the idea if we decide to pursue it.

[Z]

Doug,

Silly me, with my old-fashioned views that exploding electrical appliances were somehow unusual. I have now been reliably informed that they are, in fact, everyday occurances.

Ahh..., progress..... Soon we can look forward to exploding Jumbo Jets..., every day...!!!
~~~o0o~~~

Christian,

Lots of new stuff now posted on the "Beam-Axles..." thread. Take your time reading through it, and feel free to ask for any clarifications (it was bit rushed, so I probably missed some bits...).

And photos now back in "... Adventures in Africa." thread (Gallery section).

Z

[ChristianChalliner]

Thanks Z,

I had a quick look just now in the thread, my rear beam is very similar to your layout 4 (the lower arms converge almost to an A frame aside from the fact they have to skirt around the inconsiderately placed oil pan :P ), I'll write a bit more later/tomorrow and show some more sketches along with a preliminary pedal box design (it appears I've acquired a second project).

[ChristianChalliner]

Hi guys,

Just a quick update on something I've been working on. It's not VD related (at least not in a suspension design/kinematics manner) as that will come later with many drawings plus ideas/explanations. It is however, my first step into braking systems and I believe some background information is necessary.

Last year at the competition we had a hard time passing the brake test because the front wheels struggled to lock, in the end they did but with high tyre pressures and some rather suck it and see balance bar adjustment. So for this year I've made up a spreadsheet using the formulas found on the StopTech website and I'm convinced I must have made a mistake somewhere. This is because the calculations show that the current brake system is way over spec'd with even a modest applied force resulting in what would be a wheel locking situation at best and neck snapping at worst if the tyres were able to generate the required mu values.

So as far as I can see this means either one of two things:

1. My calculations are incorrect

2. There's something wrong with the system on the car

I have attached my excel calculations for you all to look over. I tried to make it easy to understand what was doing what and all information is accurate and measured from the past car. I appreciate that there is no consideration for the weight transfer or the tyre mu values but that is because I reasoned there was no point continuing until the numbers were verified.

Thanks,

Christian

[Z]

So as far as I can see this means either one of two things:

1. My calculations are incorrect

2. There's something wrong with the system on the car.

Christian,

I assume that EACH of your (four x) calipers has FOUR x 1" pistons? That is, TWO pistons on EACH side of caliper?

If so, then my rough calcs give:
O/A Mechanical Pedal Ratio = ~3,
O/A Hydraulic Ratio = ~26 (= Area-All-Calipers/Area-Both-MCs),
O/A Wheel Ratio = ~1/6 (Mu x Rc/Rw),
for
Total Ratio = ~13.

(All good numbers, BTW.)

So,
1 kN Foot-Force = 13 kN Braking-Force,
which is pretty close to what you have...
~~~o0o~~~

So ... my guess is ... [drum roll ...] ... air-bubbles in the front brakelines!

This is a very common problem in FS/FSAE. Perhaps a result of students spending too much time doing "calculations", and then thinking it just has to be bolted together and it will work just fine?

So (if old car still working?), get a BIG bottle of brake fluid and preferably a "suction system" to fill the lines, or be prepared to turn the car every-which-way-up, and do a lot of shaking, and jiggling, (and cursing... ), to get ALL the bubbles out.

(Note that bubbles in the front lines can let the front-MC bottom-out, so it stops working...)

Z

[ChristianChalliner]

Z,

Yes that is correct, they are actually these calipers: http://www.apracing.com/product_deta...p4227-2s0.aspx

I think I'm with you aside from you've taken the pad material mu to be 0.55 whereas my selection from the list was 0.39 because I don't actually know the pad material so I assumed worst case, I will be ordering new ones at some point anyway and most likely the 0.55 ones.

That's quite possible as we did have a line failure at the competition and as the caliper has two inlets and two vertically opposed bleed nipples for each piston pair it makes it somewhat difficult to bleed (we found the best thing to do is actually remove it from the car and stick some plate in the caliper to act as a 'disc' for the bleeding process). We do have a suction system which we will be using this year Unfortunately, the old car is no longer in a running state but the brake system is still intact.

My next brake system quandary is bias bars! I understand the principle in moving the force application point along the bar so simple moments tells you that it splits the force as such however, the operation itself is somewhat mystifying, especially when the bar is not just moved laterally but then set so that it is not perpendicular to the master cylinders!

Some rough CAD ideas coming tomorrow along with comments on the front and rear beams, problems and thoughts But put simply, I think I'm in trouble at the front and that I've backed myself into a problem which will take significant work to resolve.