Jesus-Ninja
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| posted on 31/8/08 at 08:41 AM |
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Widening track and changes to akerman
Right, I've hit this slight niggle... did some validation and double checking. It appears the foremost upper suspension pick-up on each side in
the rear multilink desire to live somewhere I don't want them to. Namely just inside the gearbox on one side and somewhere amongst the pulleys
on the other.
This is a mid transverse engine. I reckon I'm going to have to move the suspension components out by 60mm on each side
As it happens, the wheels are all spaced by 25mm already, so I can ditch the spacers on the rear and move all the suspension gear outward 60m,
resulting in the track being moveout by 35mm.
Which brings me to the front. Everything can be moved out there too, but this will effect akerman, and the steering arms should be adjusted to
compensate...
In theory...
W = wheelbase
T = track from centre-line
The hypotenuse between them is akerman
X and Y define track rod end relative to hub ball joint (the steering arm) longitudinally and laterally,
C = change in track
I know that W = 2525mm, and for arguments sake (as I haven't measured it yet), we'll say Y is 200mm.
It's fair to say then, that T / W = X / Y
Given that to avoid weird effects on steer, the only way in which we would want to alter the position of the tie rod relative to the hub is in the
line of the steering rack - ie Y stays constant, and X is altered to account for the change in akerman. The whole point of the exercise of changing
akerman, is to allow the wheel base to remain the same, so W stays constant. So the only two values we are interested in that change are T and
consequently X. We know the change in T (although we don't need to know T itself), which at very worst is 60mm (30mm to move the bracket out,
25mm for the square tube to support the bracket, and 5mm for the heck of it). We'll use D to denote the change in X.
So, all things being proportional:
C / W = D / Y
60/2525 = D / 200mm
200*60 / 2525 = D
D = 4.75
So less than 5mm movement inboard for the end of the steering arm. How much difference is it going to make, realistically, if I don't change
akerman to account for the widened track?
If I ditch the spacers at the back, but leave them on at the front, the amount by which I need to move the front track out is about half, so the
change to the steering arm is about half - ie 2.5 mm.
The alterative is that I increase the wheelbase. Off the top of my head, the total track, from centre, is around 700mm, which would make a
proportional change to the wheel base of: just under 10%, so maybe around 200mm, that's quite a bit.... Or if I ditch spacers all round, then
again, half (ish) of that - 100m, maybe more acceptable.
The figures are rough bck-of-a-fag-packet, but hopefully might give some indication of what is important and significant, and what is not. And from
there make an informed decision, and make more precise calculations as necessary.
My gut feeling says widen the track at the rear by 60mm on each side and ditch the spacers to reduce the change in track to 35mm. Leave the spacers at
the front, and widen the track by 35mm, and to heck with the 2.5mm change to the steering arm for akerman - i probably lose that figure as
insignificant in comparison to alterations to toe and castor in the course of things anyway!!!!
Athough, If I can lose the spacers up front too, tat would be a real bonus, as it would put the scrub radius back where it should be!
Any thoughts, anyone? (other than "what a headache for a Sunday morning!!)
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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Richard Quinn
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| posted on 31/8/08 at 08:43 AM |
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quote:
Originally posted by Jesus-Ninja
Any thoughts, anyone? (other than "what a headache for a Sunday morning!!)
Yeah, I wonder what I'm having for lunch today?
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Jesus-Ninja
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| posted on 31/8/08 at 08:45 AM |
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Oh, here's a picture of what I was explaining...
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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Syd Bridge
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| posted on 31/8/08 at 08:48 AM |
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Come on, tell us the truth. You're that magazine fella having a wind up session, aren't you? 
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Jesus-Ninja
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| posted on 31/8/08 at 09:05 AM |
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quote:
Originally posted by Syd Bridge
Come on, tell us the truth. You're that magazine fella having a wind up session, aren't you?
Who? Definitely nothing to do with a magazine! Just woke up this morning thinking "akerman - how important are subtle changes to it..."
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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mark chandler
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| posted on 31/8/08 at 09:34 AM |
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Not that critical for what we are about.
Look at a go-cart, it has massive ackerman to help them bump off curbs, for us motals enough to keep the tyres relative parallel on lock should be
fine.
Other examples are citroens and merc's, old one used to have lots of ackerman, fords did not, all made at the same time.
IMHO worring about nothing unless you want to compete at a high level when this kind of stuff makes the difference between even drivers or your tryes
chirp whenever you turn a bend.
Regards Mark
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britishtrident
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| posted on 31/8/08 at 09:35 AM |
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Altering the track using wheel spacers has little or no effect on ackeman angles.
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NS Dev
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| posted on 31/8/08 at 09:46 AM |
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yep, spacers won't alter the effect.
5mm is a reasonable amount BUT,
build it first and see what its like.
How do you know that the design ackerman is perfect for the rest of the car dynamics?
I'd build it and see.
Worst case you need to modify two steering arms on your uprights by 5mm, not the biggest problem in the world.
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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Jesus-Ninja
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| posted on 31/8/08 at 09:55 AM |
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quote:
Originally posted by britishtrident
Altering the track using wheel spacers has little or no effect on ackeman angles.
I know this
Which is why one option is to leave the spacer on and move the suspension points out by 35mm.
The alternative is to ditch the spacers and move the suspension points out by 60mm.
Given that the space has no effect on akerman (just increases scrub radius), the above two options will have differing requirements on the changes to
akerman, as they move the entrie suspension out by different amounts - ie approx 2.5mm and 5mm accodingly.
[Edited on 31/8/08 by Jesus-Ninja]
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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Jesus-Ninja
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| posted on 31/8/08 at 10:01 AM |
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quote:
Originally posted by NS Dev
How do you know that the design ackerman is perfect for the rest of the car dynamics?
The entire suspension geometry has been jigged from a known setup (my S14), but as you say, widening it and moving the engine to the back will change
things anyway.
quote:
Originally posted by NS Dev
I'd build it and see.
Worst case you need to modify two steering arms on your uprights by 5mm, not the biggest problem in the world.
Yeah, I'm kind of thinking that. I could introduce a load of unnecesary complication, fabrication, error, and risk for nothing/
Leave it as is, move things to where I want them, and assess when I drive it, knowing that this is a factor to be considered if things don't
seem quite right.
The figures were for orders of magnitude, and I don't know for sure whether the original set up was even following true akerman.
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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mr henderson
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| posted on 31/8/08 at 10:14 AM |
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Very difficult to get a satisfactory wishbone rear suspension using a transverse engne/gearbox, there simply isn't room to put the pickups in
the ideal places.
I looked into this a while back and came to the conclusion that it would be best to retain the original struts or shorter struts if possible, or to
use a longitudinal set up such as from the Audi A4/6/8
John
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britishtrident
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| posted on 31/8/08 at 12:55 PM |
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Davrian built dome very sucesfull rear and mid engined cars using semi-trailing arms, far from ideal in theory but the secret was matching it to a
suitable front end
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Jesus-Ninja
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| posted on 2/9/08 at 11:49 PM |
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It occurs to me that widening the track make smy stock ARBs redundant. Not sure if they can be extended, but it's a good opportunity to redesign
them to fit the new chassis, rather than working with what I have. I may even look to make up some adjustable ARBs. The relief is that it's one
less thing to worry about in the base chassis - makeing the pick-ups for the stock ARBs which as things stood, really didn't fit with where the
other three main front points lie (arm, tension arm, top mount).
Widening the track makes for a bit more room in the driver's footwell, as wheel arch is moved outward, so that's nice
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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82 Locost
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| posted on 3/9/08 at 03:34 PM |
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1. Leave the front the same? Works for F1 cars.
Or
2. Fit front wheel spacers to retain ackerman and only have a 20mm track difference front-to-rear?
or
3. Have a beer and forget about it.
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Jesus-Ninja
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| posted on 4/9/08 at 12:56 PM |
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quote:
Originally posted by 82 Locost
1. Leave the front the same? Works for F1 cars.
Or
2. Fit front wheel spacers to retain ackerman and only have a 20mm track difference front-to-rear?
or
3. Have a beer and forget about it.
I knew an expert would turn up soon 
Of course - Option 3 
[Edited on 4/9/08 by Jesus-Ninja]
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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