Vehicle Dynamics starting points and design process

[ChristianChalliner]

Ahmad that's the beam axle car you've drawn the bar in which in our opinion didn't need the extra stiffness. The chassis guy also thinks similar for the double wishbone car but we'll see.

If i'm honest the final drive is from memory, i will check it on monday with the driveline guy, i do agree the number seems a little low but the cbr is quite a powerful and torquey unit so in our opinion it's unnecessary to be banging through the gears all the time. Our engine guy has also been working on improving the low end torque to allow us to do this. If you look ECU's car only has two gears and was rapid in acceleration at aus 2014 so...

[Ahmad Rezq]

aha sorry for that seems like i went wrong when downloading your images but i saw both cars and meant the car with double wishbones

for final gear ratio i think and especially at this time due to the narrow center distance you will suffer if you increase the sprocket size, we went for this problem and it was painful and we still scare from the situation with the current assembly being manufactured.

[ChristianChalliner]

Well your engine sprocket is only a 10 tooth so that's going to result in rather minimal chain engagement, I think to be honest you have it worse than us in that respect. According to the chain manufacturer we use they recommemd something like 120 degrees of engagement minimum if i remember correctly but i did read that a long long time ago so don't quote me on that. I also think we're running pretty close to that.

[Z]

Christian,

BE HAPPY. You have just passed through one of those magical "rites of passage", and you are now living in the real world of (ughhh...) clever and reasonable "grown ups"!

More seriously, I echo what CWA said back on page 15. You have just been bowled one of the many googlies that life will throw at you. I find that growling at the dog, kicking inanimate objects, and generally moping for a day or two is good medicine. And then thinking about how to make the best of what is in front of you. You seem to already be doing this, by thinking about Double-Wishbones. So, ALL GOOD!
~o0o~

With regard to the switch from Beams to DWs, do NOT worry too much. Although I am one of the main promoters of Beams on this Forum, I try to stress the even more important point that specific suspension type is NOT a major performance factor in these smooth-track FS competitions. As per the old saying (usually attributed to Colin Chapman), "Any suspension will work, if you don't let it.". That is, in worst case you just fit "rigid" springs to your DW car and it will perform like a go-kart ... which can be very fast!

More important than the specific suspension type is the execution of its details. Most important here is "enough" reliability to finish, followed closely by "enough" toe-stiffness to be driveable, especially at rear. Best way to figure out how much is "enough" is to finish build early, then test mercilessly! Follow with strengthening and stiffening as needed. Final paint job as late as possible.

To put further gloss on your predicament, by the time this is all finished you will be TWICE as knowledgable about suspension design. You will certainly have a lot to talk about in the Design tent, with your initial preference for the superior Camber behaviour of Beams, but then the necessary compromises you had to make because of the forced adoption of the more complicated DW design. Ahh..., "market-pressures", etc...
~o0o~

With regard to your higher-ups' views on "complicated and difficult" design, I have found that the best approach is to tell them as little as possible, and just get on with building it. Sounds difficult, but it can work.

"Well, Mr Supervisor sir, thank you for your very helpful advice about the unnecessary complication of my design. I am finding that advice fascinating ... because here is a prototype I knocked up yesterday afternoon. I guess I must have made some massive mistakes, because for some reason it all came together very easily. Not sure where I went wrong??? Anyway, the preliminary compliance testing I did showed that it has at least FIVE times the toe-stiffness of last year's design. So, err..., I'm baffled. What should I do, sir?"

OK, my diplomacy skills are crap. The above will only turn Mr Supervisor's face bright red, with that big vein on his forehead about to pop. And too late for changes now, anyway.

(But, BTW, I recall Kevin mentioning how easily their folded-sheet-steel rear-beam came together for the 2014 comps. Out of curiosity, anyone from ECU care to comment on just how "complicated and difficult" they found this design?)

Once again, best is to get on with finishing the DW build, then on to testing. As has been said before on this Forum, you might not be able to turn a pig into a racehorse, but you can surely make a mighty fast pig. Remember that Colin Chapman, et al, started out by hotting-up Austin Sevens!
~o0o~

As a final recommendation to both Christian and any other newish FSers, read Superfast Matt McCoy's take on this whole adventure. His book is now available for Free Download here.

Three good reasons for spending the day or two it takes to read it:
1. Very well written!
2. Shows that progress in the real world is rarely straightforward. Instead, mostly twisting, turning, rocky roads, that are always darkest before dawn...
3. A future reminder that the stuff you are doing now may well be the best time of your lives!

Z

(PS. I would have the front-SD-top-BJ mounted to the FRH-to-upper-SIS node (= same as upper-wishbone-rear-leg-frame-BJ), to save one node and one tube. But this is only a tiny detail...)

[ChristianChalliner]

Z, that's what everyone tells me, that I've just learnt something sooner than I would have otherwise. Yeah what you say is pretty much what happened, I announced my displeasure with the situation, argued, disappeared for a day or two and then decided to make the best of what we've got.

That's the plan on the suspension, if it works great, if not, I've got some super stiff springs lined up to help go-kart the thing up, I'm hoping it wont come to that though!

One thing I did forget to mention was that the rear RC/ front RC offset wasn't entirely a convention based decision, what I actually wanted to achieve with that was to give a helping hand in lifting the inside rear wheel so I can bin that Drexler diff and throw a spool in there. But I struggle with the maths behind it so generating a number which gives me that is difficult / I'm not confident on my answer.

Toe stiffness has been a primary consideration, both front and rear have large(ish) toe bases, 110mm rear, 80mm front (front is limited by slow speed rack).

You're probably right in telling them as little as possible, I made the opposite mistake, I tried to explain the entire system to them including built models which showed how it worked and the like but it didn't pay off. I lost it a few times which probably didn't help matters, there was a specific incident where I proposed two designs for front beams whereby one solved an issue of the other but was slightly more difficult to make and it ended with me saying "you gave me a problem, I solved it, what do you want me to do?!". I should probably work on that...

The aim now though is to just get some starting points for the DW car, it's probably not optimum but at least the starting points give somewhere to go from next year and onwards and start from easy to understand places, in my opinion if someone sees a VSAL length which is equal to track width it's much easier for them to then decide "ok so we probably want it a bit shorter, maybe 0.9 track width" than it is for them to go "well last year it was 937.325mm, I think we should move to urhmmm, 923.52mm!" but that's just my opinion on these things. Likewise, with the roll center heights, by rights there can't be jacking problems that are roll center related at the front because it's ground level so if there are then someone needs to point the figure at caster + spindle + offset, etc.

You raise a good point about the shock mount! I wish I'd thought of that now, annoyingly, I did at the back but not at the front, hmm, it might still be possible to do that since we haven't started welding up yet.

As a final point, I started reading Matt's book, in the first few pages it conveyed much of how I've felt moving through this project so I'd recommend it, also, check out his blog and his big bore Suzuki guide, that's got to be one of the funniest engine tear downs and rebuilds I've read!

Thanks,

Christian

[mech5496]

Christian, I believe Mitchell posted a picture of the 2015 UQ concept rear beam in the "beam axle" thread, I think you cannot possibly build anything simpler than that! Anyway, I know people higher up changing your concept just because, and I have found that the best possible reaction is to face it and go from there! Really eager how you guys will do in the next few years, and please stay with the team, you are the kind of engineer/person that is needed in FSAE!

A bit offtopic, but Matts' book is great indeed, so great that I recommend buying a hard copy to all FSAEers.

[ChristianChalliner]

I saw that, There's a picture of the back of the car on their facebook page, looks very similar to what we planned to do so i'm really keen to see how it turns out!

I think you could make it simpler, I'd drop the wheel hubs for a start, I see/know why teams do them but I don't like them, it makes you locked into the wheel you put on it unless you want to remachine the stub axle which in those cases looks time consuming, I also don't think it fits with a weekend racer ethos but that's just my thinking on it.

Me staying on isn't really my choice, the project is run for third year students so anyone else that is around is really only there by choice and if the next team decide they don't want me around I can't say no.

[Jay Lawrence]

The project is run (only ?) for 3rd year students? That's a fantastic way to fail every damn time haha. If you're passionate about it I'd stick around, it certainly puts one ahead of the game for future endeavours, and you might even start a trend of people persevering with it even if it doesn't directly benefit their uni marks (as was/is the case with mine and many other unis).

[ChristianChalliner]

We have mentioned this yes but it is unfortunately the way it has always run. Realistically, it should run as everyone else runs (society based) but it's not that simple to get through the transition period to that system as we found out this year.

I think to be honest the biggest problem is always ego which will continue to be a problem. The reason is that whilst it runs as a third year project it's difficult to get others on board and even when they are on board there is inevitably friction as one group of students is being marked whilst the other is not.

It seems to create a sense of entitlement for those being marked and similarly, a sense of under appreciation for those not being marked which is never going to end well.

Anyway, irrespective of the above, our current progress is this:

All chassis tubes are cut, a prototype pedal box was plasma cut this morning which i need to take a look at, we're in the process of designing the jig which should hopefully be completed either today or tomorrow so that we can order material and start building that next week.

With regards to other components we've made the decision to use steel A arms in order to save time and i've designed what I think is a reasonably quick jigging method for all the wishbones (will post pictures later). I was particularly interested in Z's suggestion of non-over constrained wishbones which I looked into but decided against since there are more operations required and time is at a premium, maybe next year?

The steering wheel and dashboard are pretty much complete now.

Lots still to do like:
Uprights
Diff hanger
Weld chassis
Intake (to be 3d printed)
Exhaust (made outside to a student design)
Dampers to finalise ordering
Springs to order
Arms to make
Pedal box to make

Basically a whole car still. There's more to say so i'll be back later with more information on things

[Tim.Wright]

I was particularly interested in Z's suggestion of non-over constrained wishbones which I looked into but decided against since there are more operations required and time is at a premium, maybe next year?

I don't see any reason to add this extra complication unless you are using the inboard chassis points to adjust the kingpin geometry which requires a wishbone that changes shape.