I was going over front suspension design today. If any of you have read Chassis Engineering, by Herb Adams, you know he gives an example independent front suspension setup. I'm thinking of just using these geometries, as they are a proven design. Then I can go right to designing the chassis. As far as the rear suspension, I have been leaning towards a dedion with a watts link. This would be simple and easy to design in harmony with the front suspension. If you don't have the book, here's the measurements:
lower control arm........13.028 in.
upper control arm........6.942 in.
scrub radius.................4.373 in.
kingpin angle...............8.653
tie rod length..............11.634
track............................60.3 in.
wheelbase...................101 in.
CG height.....................18 in.
bump travel.................4 in.
droop travel.................4 in.
swing arm length.........124 in.
swing " " full bump.......33 in.
roll center.....................approx. at ground
camber full bump..........-4.3 deg.
camber full droop.........-1 deg.

This setup should work for any car, no matter where the engine is. My car would be much lighter in the front, so I would design the springs/shocks accordingly. With 8'' total travel, riding on the street would not be harsh and good cornering power can still be had. My car's wheelbase would be (guessing) 90-94 in. and track would be probably 56-58 in. The weight at each wheel and CG height are, of course, to be determined. Can anyone point something (or everything) I am missing to have a complete suspension design? The computer analysis is in p. 50 of the book.

Do you have a front upright in mind? That usually dictates a lot.

Well, that is just an example from a book, and it's really hard to tell what it is. Could be a Camaro stocker, for all I know. It's a big car! Really, I don't think it's a good idea to copy this car because too much is unknown about it. Better to copy the Locost, IMO.

If you want to build a one-off, there's a substantial amount of knowledge about vehicle dynamics that you need to acquire if you want to be confident in your project. It's interesting how other builders have dealt with this. User kb58 has a really good section on his website about his knowledge acquisition. Alan B initially had some help from another Locost builder in designing his suspension, then went on to do his own. Then there's user ProjectLMP, who hired Allan Staniforth!!! (No insult to Mr. LMP, who first learned plenty in order to ask the right questions.) There are many others who have taken the time to learn about suspension geometry.

I'm not trying to discourage you, but rather the opposite. It would be a lot better to pick your drivetrain and package it in a way that is pleasing to you. The suspension geometry can be worked out at that point, and I'm sure your fellow posters here would be glad to help evaluate your plans.

Keep on reading, too. I liked the Adams book, and also Carroll Smith's 'Tune to Win' and Allan Staniforth's 'Race and Rally Car Source Book'. The Holy Bible of vehicle design is Milliken & Milliken's 'Race Car Vehicle Dynamics'. I won't be arsed to spend $100 on it for my Locost, but I might if I were doing a one-off.

Best of luck,

Pete

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Originally posted by pbura
.....Alan B initially had some help from another Locost builder in designing his suspension, then went on to do his own.....

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Yes, it all revolves around the upright you choose, and the ball joints/rod ends. It's also connected to the size of the wheels and tires, and the brakes.

Everything is connected to everything else, if not literally then at least figuratively.

Do it in this order:
1. Pick a tire size.
2. Pick wheel sizes/offset, etc.
3. Pick upright to fit inside the wheels.
4. Decide on brakes, calipers and rotors.
5. Choose ball joint type or rod ends.

THEN you design the suspension, since only now do you have the outboard pickup points.

As said above (thanks for the complement BTW) in my Design link I have a more detailed sequence of steps.

I have that same book. While the suspension works for *that* wheel/tire/upright combination, it's very unlikely your situation will match close enough that it'll work for you...

To be frank, you'll have to do your own design... which isn't a bad thing.


[Edited on 24/1/04 by kb58]

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Originally posted by Alan B
Getting me mixed up with someone else Pete I think.....

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Nope figured it all myself, with the help of lots of study and research and AutoCAD....I think I am a pretty good example of people helping each other though, but more as a help giver than receiver........I don't mean that in an arrogant martyr type way...it's just that my heaps of questions (when I can't start the engine perhaps..?) just haven't cropped up yet...but they will...

I wasn't going to copy it exactly. It can be scaled down or up to any size. Since my donor will likely have a smaller tire diameter and uprights, I could scale things down. The things that are relevant: The upper wishbone is 54% the length of the lower wishbone. The swing arm angle/length, and other geometric relationships. I am not against designing my own suspension, it's just that I don't want to reinvent the wheel. I want to just use what works and modify it to my application. I do not own a computer, therefore I cannot install any suspension programs at my computer here at work.

quote:

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Originally posted by Alan B
....I think I am a pretty good example of people helping each other though, but more as a help giver than receiver........
...it's just that my heaps of questions (when I can't start the engine perhaps..?) just haven't cropped up yet...but they will...

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Such a shame he's from Lancashire....

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Originally posted by Metal Hippy
Such a shame he's from Lancashire....

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quote:

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Hey MH, don't knock it, it's a great place to come from..

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Just borrowing a popular phrase heard here in FL...often used by Nu Yawkers.....who all seem to live here instead...

[Edited on 25/1/04 by Alan B]

quote:

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Originally posted by scuzzer
I wasn't going to copy it exactly. It can be scaled down or up to any size. Since my donor will likely have a smaller tire diameter and uprights, I could scale things down. The things that are relevant: The upper wishbone is 54% the length of the lower wishbone. The swing arm angle/length, and other geometric relationships. I am not against designing my own suspension, it's just that I don't want to reinvent the wheel. I want to just use what works and modify it to my application. I do not own a computer, therefore I cannot install any suspension programs at my computer here at work.

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Thanks for the sharing of wisdom kb58. I will use whatever tire size and uprights my donor uses. Which means I need to find a donor before I can design my suspension. Still I like to have things worked out in my head beforehand. Looks like I have some more reading to do.

If someones got all the dimensions needed for the susprog3d program, a mates got a copy. Give me all the details it needs and i'll see if I can run them through.

Its seems to be a really nice bit of s/w by the way (not that i've got any experience in this but it draws nice pictures!)

Another consideration is the sterring rack you will be using.

If for arguements sake your wheel, upright and suspension dimensions are identical to your proposed figures, but your track is different then you will need to adjust the A-arms length in order to match the steering arms length.

It is a minefield trying to match parts together that is why it is safer to understand the inter-relationships between the components.

The Herb Adams book was one of my first reads and although very good was only the start of the learning process!

Terry

Good point about the steering rack... another piece in the puzzle.

A couple other things about scaling the Adams numbers. Whatever upright you use is unlikely to have the same kingpin inclination and scrub radius...

About building the suspension, keep in mind it's rare (for most builders) to actually get the suspension points within 1/8"(3mm) of where your perfect suspension numbers say to put them. Worse, the entire chassis distorts as you weld other stuff to it. FWIW, when I released the clamps holding my "perfect" chassis to the rigid table I was building it on, it went "Bong" and I could see the *real* shape of the car, when unrestrained, was like a banana, with the bottom surface warped about 1/4" front to rear. While I understand why it did that, and know what to do about it "next time" there won't be a next time. It is what it is...

You just have to do the best you can, then tune it... as best you can. If it makes you feel better, most all home-builts have some pretty "generous" tolerances, which only the builder knows (or doesn't know.)

Hi scuzzer,
Don't let the guys descurage you! All they actually saying could be summed up:
(This is all too complicated )
No offence guys!
Now if you'll let me suggest you something I'd like to first ask you some questions - can I?
1) What do you want this car to do track, street etc.
2) What tires (bias ply or radials) you're going to use? - this will influence suspension design - A LOT.
3) Have you considered using NOT donor upright - there's simply no satisfactory upright out there - that you can get cheap. Machined upright will be more expencive - but it's not THAT bad. It will worth every penny as it will be much lighter, and will suit YOUR requirements.

You can ask Paul - he did just that - I'm sure US prices are close to UK.

Now you have to take into account that if you want your car to perform nice on track than suspension geometry IS very important.

There's a modern theory of what succesful suspension layout should be like - different for different race classes.
BTW - good old Staniforth is not up to date. I love his books but if you look at what level he was/is operating you'll see that there are better sources. Still I love his books!

If you answer this questions I can help you to model suspension geometry.

BTW - susprog3D is a err.... waist of money and than waist of lot's of time for one who's using it. Pics are nice thought.

Cheers
Ted

PS: that H.Adams suspension - it's better if you give 3d dimensions rather than arms lengths etc.

PPS: Sorry for my english

Ted, the tentative answers to your questions are:
1) 98% street
2) Radial street tires
3) I would like to have a cheap build, but unsprung weight is a big deal to me. I was thinking a honda civic upright would be fairly light. Ultimately the answer to this question is undecided.

Whatever upright I choose, I will keep the original tire size, scrub radius, and king pin angle as the manufacturer. The only thing I will be designing is the inboard suspension pickup points locations, spring rates, etc. Also it would be nice if the tie rods were stock length too. Once I find exactly what donor I will be using, I can design the suspension to accomodate the parts I have. I may find a different donor for the suspension than for the drivetrain. Only problem then is the two cars may have different tracks, meaning the driveshafts (or tie rods) may need adjustment in length.

Curious by your comment about Susprog3d. I was under the impression it was loved by all......

Now having shown my ignorance by being swung by some nice pics - what do you think are its weaknesses?

I have had a trawl around for info.

Rollcentres
Latest experimental all independent Caterham7 has front at 30mm and rear at 65mm.
Original Elise had front at 30mm and rear at 75mm
I have done an analysis of roll centres and reckon that there are good reasons for a practical road car minimum of about 30mm at the front. I'd choose a bit more, about 35 to 40mm just to be sure. See Project LMP and Kimini sites for more roll centre thoughts.

KPI and scrub radius
Elise 12.0 degrees and 10.5mm
Many cars seem to have lower KPI at about 7.5 degrees. A low scrub radius is popular with ASFAIK only Staniforth actually suggesting a zero value.

The upper arm should be longer than 7 inches in that example. Possibly 8 or 9 inches would be better?

What are the latest design rules that you mentioned Tudor Miron? We'd all be interested!

"Many cars seem to have lower KPI at about 7.5 degrees. A low scrub radius is popular with ASFAIK only Staniforth actually suggesting a zero value."

I am also a proponant of shallow KP angles... I am running zero. Less weight jacking/transfer, lighter steering, less caster needed, smoother wheel movements, etc. Generally scrub radius suffers, but can be kept in check... I am at 1.25"

---rt

Hi,
I’m not sure what to suggest for road car as aims are unclear with one – it’s a bit easier with track car as main issue is making it faster. Any way I think that many things which work on a race car should work fine on a road car if comfort is not too important.
I’m not really sure that it’s necessary to keep factory KPI, scrub radius etc – thinking that if big car manufacturer made it this way it’s the only right decision is misleading. If you’ve driven a purpose build race car and compare it to production vehicle you’ll agree that most of them are dull and ill handling machines. Simple reasons (mostly economical than technical) dictates them do it this way. On the other hand if car is not meant to be driven on or anywhere close to limit (which is NOT an option on public roads) you should not worry TOO MUCH about all that mess.
Mike,
Short answer is – after I got Mitchells software I consider susprog as wasted money. Its very user unfriendly (ie time consuming) and it’s weak in some important areas – bump/roll steer etc. Honestly WinGeo is form different league.
Oh – one thing I disliked a lot - when I wanted to install program on another PC – I had to purchase another copy of it! What a joke!

Ron answered already on KPI and scrub – I’m with him. In general keep KPI less than 4° - lower – better. KPI effects steer/camber characteristics – steered wheel will lean out at top – towards positive camber. Also it will raise the chassis, some non pleasant kick back in steering wheel is because of king pin - there are many issues when you look at KPI and caster together - most of them not good. How ever I know very successful cars which had KPI in 6 - 7° area.
With low KPI scrub radius within 25% of your contact patch should not be a problem. If 6-7° than keep it less than 1”.
Caster of 3 - 5° is common on race cars.
All in all it’s more of a packaging problem – so when using a donor you limit your self considerably and take all donors shortcomings (expect many) with you...

Cymtriks – there’s no “rules” that I’m aware of. How ever there’s usually so called “current thinking” which is constantly changing. It changes rapidly with new data becoming available at higher echelons as they got resources for great deal of R&D.
As an example – with current dampers technology (I don’t mean "cheap" stuff like Koni’s 28 series etc) wheel rates and so frequencies raised dramatically.
I was lucky to have GREAT advisers/instructors – people working at highest level of motorsport - F1, CART etc. They say that “current thinking” is somewhat lower than what you mentioned – this is for not VERY high downforce cars. In F1 they have some very weird geometry resulting in weird loads. All is compromised in favor of stable aero platform. Just look at that new piggy nose Williams and it’s “upside down” geometry!

Cheers
Ted

I have SusProg3D. Which I have not used yet. I will attempt to use it on my next design iteration and I could provide some feedback on its usefulness at that time. I wanted to have a copy both at work and at home so I emailed them with my situation and Beven gave me the key to install it on a 2nd machine. Very nice of him. The software does not do more than what you can do yourself by hand with a calculator, some paper and some time.

All this talk about choosing your KPI is all fine and dandy, but if you aren't up to fabricating your own uprights, then you are stuck with whatever is built into your factory upright since the upper and lower pivots are cast into the product. An easier solution for those of us without machine shops is a modified McPherson strut upright with a balljoint adapter bolted to the top.

Here is Alan B's Meerkat front upright adapter.


Graber

[Edited on 1/27/04 by sgraber]