Using car springs.

Hey,

First post here. I'm a student designing suspension for a university race team in Edinburgh.
It's also our teams first year and we are still learning what to do. Is there anything else I need to say in the intro?

Most teams use a pushrod system with a downhill bike dampener.

We were looking at this design because it was cheap, but now we have found cheap car suspension on this website.
http://www.avouk.com/MotorsportFrameset-84.htm

Do you think that by using car springs directly from the unsprung hub to the chassis we would be able to do this? I have no idea about normal forces experienced in FSAE so was asking for your advice. Our car will probably be around 300kg, we are literally just trying to get it to complete endurance and that would be it.

I would guess a single car suspension rod on the wheels would be too stiff?

Would car suspension work just like a long version of the downhill bike suspension, or do we need a duel spring-dampener system (I guess that would be too complicated)

Link if previous one doesn't work.
Trackday - GTX kits - Ford Fiesta

Thanks

The car will have suspension...

I'm not going to design your car for you. You should draw some diagrams and see what it looks like. Remember F=KX and how to use the trig functions on your calculator.

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Originally Posted by I_dont_know_what_im_doing

I have no idea about normal forces experienced in FSAE...

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Why on earth not?

An FBD of the hub will give you all the forces you need...

OP HERE

I'm away until Monday so will come back to calculations then, but I was also wondering about anti-roll bars. With direct suspension I guess it would require more work to put one in, almost cancelling out the point of direct suspension?

What kind of forces do you get with lighter cars. With a 300kg you get 2943N straight down so I would guess three or four times that amount going through the suspension when cornering.
Speed of say 40mph? Radius corner of like 1.5 metres?
6000-7000N?

Does that sound right?

Do you think car suspension is suitable for that or too stiff?

I'm not just worried about forces involved, I'm also worried about connection points.
So few teams use direct shock suspension and I can't figure out why. Thats why I'm really asking. I'm going to email the company to find about what forces the dampeners are designed for.

Quote:

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Originally Posted by I_dont_know_what_im_doing

So few teams use direct shock suspension and I can't figure out why.

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Z will tell you that is because they are "all stupid monkeys copying each other, monkey-see monkey-do" and fact is that he is not that wrong... Now why would an ARB be harder to implement in a direct acting suspension? Do you really need ARB's?

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Originally Posted by mech5496

Z will tell you that is because they are "all stupid monkeys copying each other, monkey-see monkey-do" and fact is that he is not that wrong... Now why would an ARB be harder to implement in a direct acting suspension? Do you really need ARB's?

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I would venture that he (Z) is right. And as someone who has designed a suspension using direct acting dampers, it is a bit more challenging to implement an ARB. Really, you're just trading what components you spend time designing, a rocker or the arb. That said, I really agree with your last question. We had one designed but not machined in time for our first test last year (it was a bending element, rather than the typical torsion element type bar, so the machining had to be pretty precise) and the car ran well enough without it that we stopped development.

-Matt

I emailed the car suspension company and he asked for general spring rates and damping forces.

Roughly what are your values from last year.

If we can get car suspension dampeners in that kind of range then we'll probably go down that route instead of push-rod.

Quote:

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Originally Posted by I_dont_know_what_im_doing

What kind of forces do you get with lighter cars. With a 300kg you get 2943N straight down so I would guess three or four times that amount going through the suspension when cornering.
Speed of say 40mph? Radius corner of like 1.5 metres?
6000-7000N?

Does that sound right?

Do you think car suspension is suitable for that or too stiff?

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First things first. You're not ready to design a suspension or select parts yet because you don't know what you want/need it to do. Granted, learning is an iterative process--the more you know the more you can know--but you're at the very beginning of your understanding. I think you need to do some reading on wheel loads, load transfer and basic suspension calculations before thinking too much about parts or throwing numbers around. (I'll suggest Milliken's "Race Car Vehicle Dynamics" as one possible resource.) Once you have a "back of the envelope calculation" level of understanding you will be able to learn even more and *then* start thinking about your vehicle design in numerical terms.

When you get to that point you can evaluate your guesses above with "engineering eyes", and decide how much sense they make. Others here may give you more direct feedback, but my message is this: While you need to start somewhere when learning this stuff please don't try to "design" anything without being able to analyze it, or without understanding the goals/needs of your design.

That book is £66 and will take weeks to arrive, quite a lot if I am still at such basic levels. Cant you give me general damping and spring values for a 280kg car so I don't waste time looking at direct suspension only to find I will have to have the springs custom made or something.

I'm not going to use the values in the calculations, I just need them for when I'm on the phone to the car suspension company. They just need to be general.

I'll ask the team captain about buying the book on Monday if we can get it delivered sooner than a week.

Quote:

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Originally Posted by I_dont_know_what_im_doing

I emailed the car suspension company and he asked for general spring rates and damping forces.

Roughly what are your values from last year.

If we can get car suspension dampeners in that kind of range then we'll probably go down that route instead of push-rod.

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What worked for my car will not work for your car, so even if I gave you my numbers, they will not be particularly helpful to your cause.

Quote:

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Originally Posted by I_dont_know_what_im_doing

That book is £66 and will take weeks to arrive, quite a lot if I am still at such basic levels. Cant you give me general damping and spring values for a 280kg car so I don't waste time looking at direct suspension only to find I will have to have the springs custom made or something.

I'm not going to use the values in the calculations, I just need them for when I'm on the phone to the car suspension company. They just need to be general.

I'll ask the team captain about buying the book on Monday if we can get it delivered sooner than a week.

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Dr. Kasprzak is correct. You need to understand what you are trying to achieve before you set out choosing parts (or even suppliers for that matter). If you don't know what you are trying to achieve, at best you will end up with a collection of parts for a car, and it will perform no better than the worst of the parts. This is most likely not what you're looking for.

If you spend 10 minutes on Google, you can probably find a thread from an autocross forum/website (or full size car forum, think about performance oriented vehicles that have a strong track presence) that can give you the formulas to get a rough idea of what spring rates you want. If I remember right, there is even a nice free Excel sheet that will calculate a lot of the important vehicle dynamics values for you. Whether or not those outputs are useful to you will depend on the quality of information you put into the spreadsheet. There are also several good threads on this forum that reference suspension design. All you have to do is look. It only takes time, and you can probably have it done before you're able to talk to your team leader on Monday (although I would suggest still buying that book).

-Matt

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Originally Posted by I_dont_know_what_im_doing

Cant you give me general damping and spring values for a 280kg car

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Yes, I can. But the idea is for you to learn, not for me to show you that I have already learned.

You have other resources. Google is your friend. In the past some teams have posted their design reports on their team websites, or have quoted suspension values on a "specifications" page. Some digging will give you rough values. Some papers on FSAE vehicles also have suspension values. And some of those can be found via links or references buried in these fsae.com forums.

You have that "I need a number now and don't want to think about it or learn anything" sound, which concerns me. At least you're persistent in asking questions. Above are a few more ways for you to find the information you're seeking. Let's refocus your persistence into a good internet search session. You can get there from here.

And note: Information isn't knowledge. The latter will take time and effort to develop. You have a very short term goal, but our educational goals are on a much longer timescale. Someone may throw you the value you seek, and that's up to them, but engineering is about knowledge and understanding. Unless you have a context for the values you find you will have nothing more than ballpark numbers.

Edit: Matt, you beat me to it! At least we said a lot of the same things.

To I Don't know what I am doing (don't worry me neither) from Edinburgh,

This could help you too

http://www.optimumg.com/technical/technical-papers/

Quote:

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Originally Posted by I_dont_know_what_im_doing

That book is £66 and will take weeks to arrive, quite a lot if I am still at such basic levels. Cant you give me general damping and spring values for a 280kg car so I don't waste time looking at direct suspension only to find I will have to have the springs custom made or something...

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For fuck sake, do some engineering... Why take part in FSAE if you are just going to buy everything off the shelf based on calculations that you expect everyone else to do for you???

BTW, what you are referring to as "wasting your time" is known in the industry as a "concept evaluation" and is the decision which underlines your whole development direction.

And you want to palm this off on someone else. Nice...

I'm back on Monday and I'll try to figure out spring rates and damping forces then.

I can do the free body diagram without a book, but need more information for springs. I'll work out exact values next week but I just needed general values for talking to the suspension guy on the phone and so I know my numbers are with in a resonable amount and not magnitudes low or high.

"Cant you give me general damping and spring values for a 280kg car so I don't waste time looking at direct suspension only to find I will have to have the springs custom made or something..."

No, no, no.... And NO!

For a moment I thought that was another Indian question but as I see simplistic and entitled questions can come from everywhere. If anybody would be giving you silver spooned solutions, it would defeat the very purpose of FS/FSAE which is for you to better search, apply and execute engineering methods.

You_better_know_what_you_are_doing

PS 280 kg car with or without driver? Even with driver and even with a 4 cylinders that is already pretty heavy...

For novices like you ... have a look at the demo version of Dynatune for Excel. With elementary data you can get a lot done ....

Cheers,
dynatune, www.dynatune-xl.com