lotus shark or susprog?

[faeez]

our team have a copy of both susprog and lotus shark. The problem is both softwares calculation for the roll centre are 2mm aparts and the roll centre migration are 0.2mm apart. Is that meant to happen if i use different software, or is one more accurate than the other? cheers.

[faeez]

our team have a copy of both susprog and lotus shark. The problem is both softwares calculation for the roll centre are 2mm aparts and the roll centre migration are 0.2mm apart. Is that meant to happen if i use different software, or is one more accurate than the other? cheers.

[faeez]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by faeez:
our team have a copy of both susprog and lotus shark. The problem is both softwares calculation for the roll centre are 2mm aparts and the roll centre migration are 0.2mm apart. Is that meant to happen if i use different software, or is one more accurate than the other? cheers. </div></BLOCKQUOTE>

[J.R.]

2mm and .2 mm???? Don't worry, manufacturing won't be able to get you within those tolerances anyways. Just go with what you got and get building!

[BilletB]

Haha.. 2mm and .2mm and with regards to roll center nonetheless? Those numbers don't tell you anything anyway, so why care if they're 2mm and .2mm different? And as J.R. said, that's way outside of your manufacturing tolerances to begin with.

[faeez]

Thats what i think is well, but a couple of teams told me that 2mm roll centre difference makes huge difference in the jacking effect of the car. is that so?

[Drew Price]

EDIT: (Jesus, how do all my posts get to be so damn long!!?) http://fsae.com/groupee_common/emoticons/icon_rolleyes.gif



That is a very broad statement. Whether 2mm difference in roll center migration has drastic effect or not depends on how stiff your suspension is, how much compliance in the system, what modes you really care about controlling (stabilizing an aero platform, keeping more uniform tire temps, or some other consideration like limits on the torque-bias-ratio of the diff, etc).

For the speed ranges FSAE cars usually see, the unknown track surface we will be competing on, the amount of compliance usually found in these cars (necessitated by the weight goals most teams are setting these days), and the fact that we do not get any practice or setup time on the actual course and track surface we will compete on leads me to believe:


1) From a practical standpoint, NO, 2mm difference in migration will probably not make a difference, let alone one you could measure, or one your driver would be able to relay.

I say practical standpoint because to be at this level of roll center control you must be so on top of each and every aspect of the vehicle as a system to achieve that, that only a few teams are probably even remotely close to actually reaching the kinematics they designed into the system in the first place, whether they know it or not. Things like ambient temp, tire pressure (are you controlling it within a few 10ths of a PSI?), slightly incorrect bearing preload, stiction in moving components, subtle differences or non-linearities in the dampers, etc all will probably have more of an effect on corner weights and tire / track interation than that much kinematic variation.

And that's all assuming spot on +/- 0.000 manufacturing tolerances.

I think there are much more important things to spend time on during the design phase than getting that last 0.1% in the design, because I have seen with my own two eyes that the difference that more driver training and comfort in your vehicle will make many times over more improvement in lap time.

Or just starting out with better drivers. Or HIRING better drivers.



2) From a design tent standpoint, YES, 2mm could make a difference. You should be able to address this when the judges ask you about it, that is IF they get the impression you even understand and can address this aspect of suspension design and performance. You should be able to justify where you placed your roll centers, where you wanted them to move, how fast, any non-linearities involved, FOR YOUR SPECIFIC APPLICATION.

My design choices will be very different from yours, because I spent a lot of time finding ways to make our spool rear end work decently this year, and last year we used a Torsen, so the direct correlation is a little lacking for me, especially since this is only our team's second car.

What I'm trying to say is don't let some other team tell you that a 2mm migration is important, just the same as don't let anyone tell you what seatback angle to use, what engine to buy, or what to modify in it. You have to decide all that for yourself, and you have to decide how much of your info is based on other people's experience, how much research and testing you do on your own, and how much you guess based on the trends you figure out.

A fair amount of this business is still about intuited guessing, even though some of the PhD designers won't let on to that - they get paid more if the rest of the team thinks everything involved is a hard science, and not witchcraft like it actually is.

Learning how to approach and solve open-ended problems is a gift and a talent for some, and is purely a learned (and sometimes forced) experience for others.

This is a competition, and some people are known to spread slightly mis-informed data to give themselves a competitive edge, it's to their advantage.

Keep that in mind.

Even all the stuff I just said above!



Like J.R. said, it's time for you to go grab a few books on the subject, and then report back.



Best,
Drew

[J.R.]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> don't let anyone tell you what seatback angle to use </div></BLOCKQUOTE>

Except perhaps Michale Royce next year.... http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Zac]

If I was on a first year team I really wouldn't get too worked up about a 2 mm difference in roll center height, what kind of jacking forces will be transfered through your tie rods, etc. These cars are simple enough that as long as you don't do something completely terrible (and you can defend your decisions in the design tent) having a competitive car isn't out of the question.

However a few things that you probably should pay attention to are bump/roll steer (and how to minimize it), suspension compliance (it can have a pretty significant effect in a lot of these cars), and your static alignment settings. It's actually rather amazing how many of these cars are running around with jacked up alignments.

[satya430]

Has anyone used lotus shark here ? Wishbones or struts - they ask for four CoG's for the quarter car . Anyone has any idea what those 4 points are ?

[faeez]

there are the centre of gravity. i guess that would depend on the way u manufacture the component

[subhu]

whats the max value of camber in off road vehicles during bumps

[rmk36]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by subhu:
whats the max value of camber in off road vehicles during bumps </div></BLOCKQUOTE>

Perhaps you should ask your open ended questions on this forum...

http://forums.bajasae.net/forum/