tegwin
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| posted on 28/10/06 at 04:47 PM |
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Struggling with front suspension
I am currently converting my sliding pillar suspension over to double wishbones but am struggling with the finer points of the design.
I have chopped off the nasty sliding pillar mounts and have started putting the brackets in place...
The thing I am struggling to work out is what the optimum distance should be betweeen the upper and lower wishbone pivot points and how to ensure that
all the angles are correct to give a decent drive....
Would I be correct in assuming that the lower wishbone should be parallel with the road when the car is fully loaded?
Any help anyone has would be greatly apreciated.
Thanks.
Dunc
[Edited on 28/10/06 by tegwin]
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tegwin
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| posted on 28/10/06 at 04:49 PM |
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A picture of what I have so far
 
Rescued attachment Wishbones 011-600.jpg
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JoelP
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| posted on 28/10/06 at 05:05 PM |
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parallel to the ground is probably ideal for the lower bone, upper can range from parallel to shorter and sloping up, you can get all technical and do
sums on it, or just copy someone elses 
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tegwin
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| posted on 28/10/06 at 05:11 PM |
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Hmm, that makes sence I guess having the lower parallel to the road and the upper one at a slight upwards angle...
The last thing thats bugging me is...Should the upper and lower ball joints be inline vertically or should the lower be firther forwards? (caster
angle I think)
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JoelP
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| posted on 28/10/06 at 05:19 PM |
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yes.
Usually described as the upper backwards but of course thats all relative. An angle of 5.5 is normal for a locost, some people round it up to 7
degrees to ensure good self centering.
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tegwin
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| posted on 28/10/06 at 05:32 PM |
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Where is the 7 degrees measured from?
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JB
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| posted on 28/10/06 at 06:16 PM |
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Suspension Basics
Designing suspension is complex and you should not really build a suspension system without a basic understanding off what is required.
However here are some basics:
Lower wishbone parallel to the ground.
Upper balljoint 5 degree behind the lower ball joint. This is caster for self centering. The actual angle does not matter too much (5, 5.5, 6 etc) but
make sure they are they same on both sides. To measure the angle draw a line between the ball joint centres and when you look from the side of the car
this angle is the castor. In practice it is hard to measure.
The top wishbone wants to be approx 2/3rds the length of the bottom wishbone and run downhill from wheel to chassis by about 10 to 15 degree.
When you view the wishbones from the side of the car the chassis mounts should be on a horizontal line. This will give no anti dive or squat, keep
things simple for now.
Have -1 degree camber (wheel leaning in at the top)
Looking from the front, draw a line from your top wishbone mount on the chassis to the bottom wishbone mount on the chassis, the steering rack pivot
should lie on this line, as a starting point to minimise bumpsteer.
If you want to get more complicated then aim for a roll centre of around 3" above ground and alter the angle of the top wishbone to achieve
this.
I hope this helps,
John
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tegwin
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| posted on 28/10/06 at 08:14 PM |
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Yes, that does really help!
How much travel should I be looking to acheive? (what does everyone else have on a car like this)
And I agree with you...I shouldnt be doing this without some knowledge as its a vital part of the car....are there any simple to understand books that
explain everything I would need to know?
Thanks.
Dunc
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zetec7
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| posted on 28/10/06 at 09:29 PM |
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After doing a great deal of research, we came up with a design that allows considerable fine tuning once all is done. If you look at the front
suspension pics on my website, you'll notice two things: 1) the lower wishbones are pointing downward lightly (going from inboard to
outboard), as the suspension is unloaded...this will lievel out when fully loaded. And 2) the inboard end of the upper wishbones are attached to
plates that can be adjusted, vertically horizontally and diagonally. This allows us to fine-tune later for caster, camber, toe & anti-dive. If
you do it this way, you'll be able to get it right for sure!
http://www.freewebs.com/zetec7/
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tegwin
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| posted on 28/10/06 at 09:36 PM |
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Looks pretty good...Unfortunately due to the stupid design of my chassis it would be pretty hard to have a sliding upper mount like on your car....not
impossible, but very tricky!
I have adjustable camber from the upper ball joint and a teeny bit of caster adjustment in the dummy struts sitting in the sierra uprights...
Its just going to be a case of bolting it all inplace and playing with it for a bit whilst taking measurements 
Remind me again....why do people actualy want to build these cars again?!?!?!?! far to much like hard work 
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JB
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| posted on 29/10/06 at 05:57 AM |
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Travel & Books
Aim for as much suspension travel as you can. I can never get as much as I want and always wish for more travel. However the more travel you have the
harder it is to keep the wheel doing what you want (camber in roll and bump).
Longer travel also allows you to use softer springs which is good.
My car has 130mm of total wheel travel (saloon not a 7 style car)
Allan Staniforths Race and Rally Car Source book is a good starting point, then all of carroll Smiths books and finally Milliken.
John
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t.j.
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| posted on 29/10/06 at 02:42 PM |
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quote:
Originally posted by JB
Aim for as much suspension travel as you can. I can never get as much as I want and always wish for more travel. However the more travel you have the
harder it is to keep the wheel doing what you want (camber in roll and bump).
Longer travel also allows you to use softer springs which is good.
My car has 130mm of total wheel travel (saloon not a 7 style car)
Allan Staniforths Race and Rally Car Source book is a good starting point, then all of carroll Smiths books and finally Milliken.
John
I think by the lack of travel there is no need to know more then earlier mentioned by you, don't you think?
If the upper and lower are equal there wil be no camber-change, otherwise this will not effect really due the less travel. Only if someone uses it on
the circuit.
The effect of the tyre and pressure are not to be underestimated. These will compensated the bad designe problems.
Aim for zero bump-steer, around 5,5-7 degr. caster.
If you take a good look at the "factory" sevens you become aware of the fact that they all did something wrong!
[Edited on 29/10/06 by t.j.]
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MikeRJ
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| posted on 29/10/06 at 02:52 PM |
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quote:
Originally posted by t.j.
If the upper and lower are equal there wil be no camber-change, otherwise this will not effect really due the less travel. Only if someone uses it on
the circuit.
With equal length/parallel wishbones the camber change in bump/droop is zero, but in roll the camber change is equal to the body roll.
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tegwin
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| posted on 29/10/06 at 05:52 PM |
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Hmm...looks like I need to make some kind of jig to hold the suspension in exactly the right place whilst I weld it..
Can anyone recomend a suitable shock for the job? Raldes sell a gaz shock...is this any good or would I be better spending money elsewhere?
Thanks.
Dunc
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JoelP
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| posted on 29/10/06 at 06:00 PM |
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i have a selection of shocks for sale if you need a pair. Not sure what lbs the springs are but they are the standard ones from GTS, just need some
new bushes, zero miles but about 3 years old. Also a pair of protec alloy bodied ones with rosejointed ends!
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t.j.
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| posted on 29/10/06 at 07:41 PM |
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quote:
Originally posted by MikeRJ
quote:
Originally posted by t.j.
If the upper and lower are equal there wil be no camber-change, otherwise this will not effect really due the less travel. Only if someone uses it on
the circuit.
With equal length/parallel wishbones the camber change in bump/droop is zero, but in roll the camber change is equal to the body roll.
ok, your right but that can easly compensated by giving 2 or 3 degr negative camber and/or place an anti-roll bar
However, I just wanted to say that is not a great problem if you will design your own just by using the princepals of suspension set-up.
Or could someone tell me if i'm on the wrong track here ?
[Edited on 29/10/06 by t.j.]
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JoelP
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| posted on 29/10/06 at 07:57 PM |
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not the wrong track IMHO, a few degrees of camber helps on track apparently, at the cost of tyre life. Its been said recently that arbs dont help a
normal 7. By shortening the upper bone and moving it up through a few degrees, you balance out some of the camber change in roll without having too
much of a bad effect of bump. Its just about getting the average.
Much as some people discount roll centres, they are on my list of things to research as they are a good way of judging and comparing different setups.
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DIY Si
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| posted on 29/10/06 at 08:01 PM |
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It may be worth remembering that anideal track set up could be near useless on a bumpy back road, and vice versa. Suspension set ups are always a
compromise between different goals.
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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JB
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| posted on 29/10/06 at 08:01 PM |
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Roll Centres
As you say roll centres are important. Get them too high and you have a high jacking effect (Beetle and Spitfire). The jacking effect is caused by the
way the forces are reacted.
However I would love to drive a car where roll centres are easily changed so I could actually feel how much difference they make to how a car
feels.
John
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