Hi All
I have 2 questions for you:-

1) In Rons book the wishbones are mounted in an horizontal plane which means that even though the front upright is leant back by 5 degrees the wheel still moves up and down in a vertical plane.....BUT in the Avon book the wishbones are mounted lower at the back allowing the wheel to move rearward as it rises.....
What is the point of the castor angle and which is correct if any?

2) Does it matter if the top wishbone is mounted higher on the chassis as long as the front upright is in the correct position?
My reason is purely aesthetic, I want then to be more parallel.

Cheers
Andy

Caster angle should be around about 4-5 degrees. In the RC book this is built into the top wishbone, can't speak for the Tiger book, haven't seen it

Correct setup for front suspension is bottom wishbone parallel to the ground, top wishbone sloping higher at the wheel, lower at the chassis end. This gives more desirable camber change as wheel goes up and down

John

Both will work, the RC way is the 'conventional' way for twin wishbone arrangements, the Avon is probably easier for the home builder to control and measure with accuracy. In fact, look under most road cars with Mcpherson strut suspension on the front, the strut leans back to give the castor required. Yes, the wheel will move backwards slighty, but the change would be small enough as to be insignificant.

Thanks for the replies but what is the point of having a caster angle if the wheel moves vertically anyway??

The castor angle is there mainly to provide a degree of self-centering to the steering, a car without castor would feel quite unstable and be inclined to wander

John

you will fail sva if the cars steering does not self centre, and as 'interested' said, caster is the cause of the self centering action.

HOWEVER I have heard from other builders that you need a bit more than the book suggests on some setups to get the correct action. More like 7 deg or so..

atb

steve

[Edited on 19/10/02 by stephen_gusterson]

Caster controls the stability and self centering rather like a shopping trolly wheel, Camber controls the footprint of the tyre, ie being slightly negative will allow some body roll bringing the wheel more upright giving max loading of the tyre onto the tarmac. Now add in bumpsteer, king pin inclination, brake dive and about 10 other parameters all gets a bit complicated.

Does anyone know on a 4 A arm indepedant rear what the optimum settings are for best traction and roadholding.

Also why don't we use anti roll bars rather than brick hard springs to control the front

On IRS, 0.5mm toe-in and 0.5mm neg camber should be in the ball park.
As for anti-roll bars, its very much a personal preference, but like you mentioned in your post its just something else to complicate things. With just making and fitting the bars there are so many variables.....
Also beware of going too soft with the front springs, as they also have to control the dive when braking heavily, too soft and it will be sat on the bump stops with the absence of any anti-dive in the suspension geometry.

read a few books i got off amazo at the beginnin of week if u angle the wishbones down slightly and the shock tilted bac as well then yes the whell does go bac under bump but it wouldnt be noticable but it also gices a bit of anti dive geomoetry

I'm not familliar with either the RC or Tiger set-up, but generally speaking, There are 2 reasons the wishbone pivot axis might be angled.
1/ As the upright rises, the top of it also moves rearwards. This has the effect of increasing the castor, which can be beneficial in reducing drag steer (not to be confused with bump steer) when the wheel hits a large bump or pothole.
2/ As Johnston pointed out, it can also contribute to anti-dive under braking, but only if it's been acurately and properly calculated.
I would advise, under no circumstances, alter or vary the geometry in your book/plans. Moving pivot points, even by milimeters, can have disastrous results!
Cheers, Bob.
http://www.rortydesign.com

Front suspension geometry is too complicated to write into one paragraph but I would be a bit worried if the front wishbones on the tiger were mounted lower at the back. This will increase the antidive effects. Ideally in an antidive/ squat geometry all wishbones should be inclined towards the cars centre of gravity.

As for the springs, these should be chosen for a natural frequency of 1.5Hz, something to do with what the human body is designed for and feels comfortable with, similar to walking frequency. Going higher begins to feel uncomfortable as your teeth are rattled out. I would recommend going for softer springs and an anti roll bar rather than just going for a stiffer spring.

Anyone recommend a good technical book on this subject.

For my disertation at uni i designed an irs for the Formula 27. I worked with steve porter at F27 and they actually built a prototype and displayed it at the Stoneleigh show (about 6 years ago). I designed it to incorperate 100% anti squat (car would not squat during acceleration), minimum camber change on inside wheel and positive camber increase during cornering and fully adustable toe. The basics of the anti squat was that both wisbones needed to point towards the centre of gravity, but this means that the upright has to be on rose joints to allow for the opposing movement of the wheel during bump. A trailing arm is then needed to hold the wheel straight, but by making this adustable in length allows toe in/out adustments easy. The ideal situation for the camber change is the same as on a motorbike (lean into corners) but to a leaser degree due to th profile of the tires. That is one of the reasons why when you see the front wheels turn they lean into the corner. Its a very complex issue but with a bit of research (can't remember names of books i used, i will look them up) you can improve things. If anyone is interested i could send them my thesis.

Does anyone rember seeing the suspension set up at stoneleigh? I was out of the scene for a while and didn't know if F27 put then design into production. After looking at car recently i have seen they have gone for a more conventional, easier setup. It was complicated but looked very trick. I wonder if Dax saw it?

quote:

---

Originally posted by MK9R
The basics of the anti squat was that both wisbones needed to point towards the centre of gravity, but this means that the upright has to be on rose joints to allow for the opposing movement of the wheel during bump. A trailing arm is then needed to hold the wheel straight, but by making this adustable in length allows toe in/out adustments easy.

---

John, I may be wrong but I'm sure the GT40 has a similar arrangement at the rear, can't remember if it was a lower wishbone or control arm and trailing rod. As for the books, there are couple out there, A speed pro book with a picture of a 7's front suspension is quite good but the best on I found had a picture of a Merc A class cornering. I'll need to look out the books to get the names but I'm pretty sure they have them in the CCC mag.

MK9R, I wouldn't mind a look at your thesis. Wish I'd kept mine, Active Exhaust Noise Cancellation. A modded version would be quite useful for the SVA noise check. Switch it on and off at will.

It depends where you put the piviot and the length of it. You can actually position it so to give you 4 wheel steering.
There was/is a car with a similiar arrangement because i remember taking pictures of it during my research, but can't remember what it was.

Will search thesis out, hopefully i still have an electronic copy.

quote:

---

Originally posted by MK9R
It depends where you put the piviot and the length of it. You can actually position it so to give you 4 wheel steering.

---

I know it works as it has been built. Have a look at this attachment showing the principles (not to scale)

quote:

---

Originally posted by MK9R
I know it works as it has been built. Have a look at this attachment showing the principles (not to scale)

---

The dimension would have to be crucial for it to work correctly. Is the trailing arm there to allow higher powered engines while keeping the wishbones small or primarily as a form of track rod while cornering?

quote:

---

Originally posted by Dunc
The dimension would have to be crucial for it to work correctly.

---

quote:

---

Originally posted by interestedparty
Downloaded it but don't have the software to open it, can you convert it into something else?

---

Yes, it is as I thought. I don't know why it isn't obvious to others but I will try to explain-

Consider the traing arm. It is anchored to the chassis at on end, by a rose joint. This means that the movement available at the other end is circular, with the anchored end the centre of the circle.
Add that movement to the movement of the upright and perhaps you can see that the suspension as shown simply doesn't work.

John

i'm starting to get annoyed now!!

The trialing arm is not parallel to the chassis and you get a 3d sperical motion, so the length of the trailing arm in side veiw would appear to change in length as it moved from side to side as the wishbone tarvel up and down. This side to side movement obviously rotates the upright (toe in/out).

If you still don't believe it works, make it from some pencils on you desk. I'm not claiming that the system i designed worked fantastically (as i don't know, as steve porter never gave me any feed back!), but the principle does work.

[Edited on 10/23/02 by MK9R]

I'm going to think about this some more, and get a second opinion. In the meantime, why haven't you chased Mr Porter for some feedback, and surely you must have some photos of the real thing?

John

I didn't contact him for a while, but after numerous attempts to get hold of him i gave up. He was always a bit like that.

I have got some photos in my disertation. Next time i visit my parents i will hunt it out of their loft.

John,

the motion of the trailing arm isn't circular it's spherical. As is the joints at the top and bottom of the wishbones. The wishbone motion relative to the bushes is circular. As the wishbones move up the castor changes on the upright and possibly the camber too depending on the wishbone lengths. This design is basically similar to a front suspension system but with the track rod being replaced by the trailing arm. The wishbones do not hold the upright parallel to their pivot axis.

wahey!!

I actually started to think i had got it wrong!

If I'm seeing this right then the length change of the trailing arm as it passes through it's arc will cause toe change at the upright?

Deliberate bumpsteer and rollsteer in effect?

Just seeing if I understand

I mean deliberate if the geometry is appropriate of course...it may be possible to dial out all the toe change....?

Or you may not want to?

OK, I will now accept that the suspension as shown can move through a short distance without binding. It looks to me as if that upright is going to have to twist quite a bit in order to accommodate a 50mm bump.
Do you have any figures? especially the lengths of the wishbones and the trailing arm

John

Have a look at the fifth photo down on this site http://www.pilot-odyssey.com/~hoser/web/drakart.htm this setup is pretty good and seems to get lots of grip, some consider it superior to a conventional twin A arm setup

A bit like mine but 1 trailing arm instead of 2 trailing links. Will give castor change...but so what?

simple and effective..

quote:

---

Deliberate bumpsteer and rollsteer in effect?

Just seeing if I understand

---

Yes. Obviously bumpsteer is not a good thing, but the roll steer could be used to an advantage. It depends what your car would be used for. Both of this can be minimised/maximised with the geometry set up.

With the relative small amounts of suspension movements in 7 type cars (as it was designed for) i considered the combined advatages of anti squat and roll steer to outway the bumpsteer, especially as i was intending it to be a setup mainly for the race track.

Once i have finished the MK, i might think about building a one off special with a suspension set up like this.

---

Alan B - 24/10/02 at 01:43 PM

Just curious...is it actually possible to have rollsteer without bumpsteer?

If you find the thesis in electronic form I'd like a copy please..cheers..

(edited for spelling...AB)

[Edited on 10/24/02 by Alan B]

---

Simon - 24/10/02 at 03:14 PM

Andya,

Even though Ron's wishbones may me horizontal to the ground, by their very nature, they both scribe different diameter circles.

The top, being much shorter than the lower, will naturally draw top of upright inwards during compression.

If the top w/bone is inclined, during "de"compression, this should aid keeping the wheel upright (assuming body roll).

I'll pop a doodle in the pic's later.

ATB

Simon

---

95seped - 28/10/02 at 10:55 AM

Ok, I read most of the posts, not all, so sorry if some of this was covered... but here we go.

First, the rear suspension works... it's a front suspension with the steering links (tierods) connected to the chassis. By moving the chassis position front or rear of the hub you can get bump toe-in or toe-out.

To make your car more stable, you want toe-in. (tie rod behind the hub) Just say you turn right... the left suspension compresses. As it compresses, you want the left wheel to turn to the right (toe-in).

A book I'm reading, that i suggest is: Chassis Engineering by Herb Adams ISBN: 1-5788-055-7 it's only 130pages, but is easy to understand for the beginner.

Some basics for independant suspension design for both front and rear (from the book I suggest you buy):

You want unequal length A-arms... top shorter. This provides Camber gain so as the car rolls, the tire stays flat on the ground. Lower arm as long as possible. Upper is very important... normally between 50%-80% of lower length. If you extend the lines of the a-arms to where they cross (the chassis mount of the upper A-arm should be closer to the lower A arm then the distance between them at the hub), and measure from that point to the wheel center you get swingarm length. At ride heigh 100-150 inches is good. I won't go into roll centers... too much explaining... get the book for that.

Wheels should have the most negative offset possible, this is for proper knuckle design.

wide track eguals less weight transfer. this equals more even tire grip = more turning power.

Knuckle design:
Balljoints as far apart as possible (height) for most strength to keep wheel position true.
Kingpin angle (top balljoint in farther than bottom) 5-10 degrees. This centers the steering. The distance between where this line hits the ground and the center os the tire is scrub radius... we want this to be as close to 0 as possible (this is reasn for high offset tires... gets knuckle inside wheel)

Caster angle change should be minimal during suspension movement because it causes a change in handling (very important for speedway oval cars) but should change for anti-dive characteristics. The upper Arms should be angled up at the front to provide anti-dive.

Bump steer..... toe-out in front, toe in in rear... makes understeer which is more predictable to drive.

Springs.... run soft as possible so tires absorb bumps and stay on ground. Run antiroll bars to stop roll. Springs should be stiff enough to stop most bumps. Run rubber bump stops, they are like variable rate spings, for the big bumps. Springs also need to be stiff enough to provide anti-dive and anti-squat. Good chassis design will have anti-dive and anti-squat built in (anti squat can only be up to 25% on the IRS, over 100% can achieved on a solid axle)

For more indepth study, and how to design a suspension I suggest getting this, or another book.


BTW, this is my first post... thought i'd say Hi!.
I live in Canada, and am planning to make a few 7s, one for me and a few (4) to sell to pay for the first. I think I'll make a McSorley 7+442 with mods like cymtriks mentioned in a chassis strength post. I'm researching the costs, and suspension design is a major thing I'm looking at. I am planning to design all the suspension parts to get the proper handling. I plan on IRS with common hubs (custom) front and rear to keep costs down (build 20 of the same thing). Having a full machine shop, aswell as friends in the metal supply, aluminum welding and CNC machining help alot. I also am restoring 2 1972 Datsun 510s and a 1964 Reily Elf. (see attached)
[img]a15842-mysig.jpg[/img]

---

Alan B - 28/10/02 at 05:03 PM

Good, informative post

Are you also on the Locost North America list on Yahoo?

Good resource.

---

Alan B - 28/10/02 at 05:10 PM

Oh, BTW I recommended someone on TOL to visit here too

By way of balance,,,,

---