strikerbird
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| posted on 17/3/14 at 11:47 PM |
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Rake angle problem
I've been told to set the rake angle on my sylva striker 20mm higher on the back than the front.
I've also been told to keep the panhard rod horizontal to the chassis when the cars loaded.
The problem is when the panhards level the rear of the chassis is 110mm off the floor but to have the front 90mm high my sump is to low.
I'd get away with 105 front and 125mm rear but the panhards not horizontal then.
Do I alter the bracket on the axle or just leave it?
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Sam_68
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| posted on 18/3/14 at 07:03 AM |
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Don't worry too much about rake: it's next to irrelevant on a 'Seven' style car like a Striker, so long as you're not
actually running nose up.
Rake is primarily intended to give aero stability by generating venturi effect beneath the floorpan, but the aero on a 'Seven' is
so messy and inefficient it's really not worth losing any sleep over.
The horizontal Panhard rod is to minimise lateral deflection of the rear axle relative to the sprung mass by setting the rod in the mid range of the
optimum part of its arc when the car is at static ride height. The Panhard causes quite substantial lateral deflection (thereby forcing the rear tyres
to scrub sideways against the inertia of the sprung mass), so I'd say that it was a much higher priority than the rake.
Of course, if you are willing to relocate the Panhard, you can have the best of both worlds...
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sdh2903
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| posted on 18/3/14 at 07:40 AM |
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Sorry Sam but I don't agree with your statement. I'm not clued up on the theory so won't argue it but in real world experience on my
old car there was huge handling differences (better) when going from a level stance to a 1" rake. Much more stable at speed and also more
predictable handling.
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mark chandler
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| posted on 18/3/14 at 08:57 AM |
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Leave it alone, the theory behind it being level is the axle pivots on this so if level you will get least sideways movement.
In reality this will make maybe 1/4" difference sideways as the axle moves, if you can feel the difference on the road you are in the wrong job!
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scimjim
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| posted on 18/3/14 at 05:51 PM |
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someone's been following Adrian Newey - rake angle may be important for aero on an F1 car but on a road/sprint car it has much more to do with
weight transfer - launch and heavy braking for example.
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Peteff
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| posted on 18/3/14 at 06:53 PM |
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If you can get the Panhard rod as level as possible when the suspension is in the centre of it's travel you will be alright. Don't worry
about it, the car will still handle fine.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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Sam_68
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| posted on 18/3/14 at 07:30 PM |
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quote:
Originally posted by sdh2903
Sorry Sam but I don't agree with your statement. I'm not clued up on the theory so won't argue it but in real world experience on my
old car there was huge handling differences (better) when going from a level stance to a 1" rake. Much more stable at speed and also more
predictable handling.
Yes, I'm going to disagree with myself slightly, here, by pointing out that I emphasized the word 'primarily' when saying that rake
was there for aero stability.
It also has another couple of other implications:
1) It raises the height of the mass at the rear.
2) It changes the rear roll centre height, which on a live axle + Panhard rod lies where the centreline of the rod crosses the centreline of the
sprung mass.
Taken in isolation, both can have a significant influence on weight transfer and therefore stability, so yes, certainly you can expect to feel a
change in the car's handling when you alter the rake without making any other changes.
...But these factors can be tuned out by other means, whereas the 'stagecoach effect' of lateral axle deflection is a negative
characteristic that can't be tuned out by other means.
So I stick to my assertion that you're better off keeping the Panhard geometry as optimal as possible and tuning out any other rake-related
handling influences other methods.
And as Scimjim points out, it's all very well talking about static rake, but the car will be pitching significantly nose up when it's
accelerating, and nose down when it's braking, anyway, so better to manage pitch by means of springs and damping than static rake. The basic
rule is that road springs resist squat/dive but ARB's do not, so you can juggle their relative stiffnesses to vary the pitch resistance whilst
keeping the same roll resistance, or vice versa.
An extreme example is Dax's CCA&R-equipped Rush. This can run relatively soft road springs, because the trick suspension allows you not to
worry about roll resistance, hence it exhibits an awful lot of dive and squat, without any apparent stability issues. Here's
Duncan Cowper's monster at Brands Hatch doing a very effective nodding dog display (watch
how much the nose of the car rises and dips relative to the front wheels under acceleration and braking).
If you want to see the real-world effect of lateral deflection on a live axle/Panhard rod, you can do little better than watch our very own
JeffW's video of his Phoenix at Abbeville. Watch how the rear tyre moves side to side
relative to the body (sprung mass), then tell me you're not interested in keeping such movement to a minimum!
If you want another good reason to keep the Panhard level, then there's also the (asymmetric) effect of torque reaction on a live axle/inclined
Panhard rod causing jacking and so degradation of grip on acceleration out of corners, which is noticeable enough for 750 Formula racers to worry
about despite their Reliant/Fiat engines not actually having any torque worth speaking of...
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adithorp
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| posted on 18/3/14 at 07:52 PM |
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What the hell has Jeff's tyre side wall deflection got to do with it?
Sam, as ever, you spout a load of guff trying to make yourself look clever then shoot yourself in the foot.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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rodgling
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| posted on 18/3/14 at 08:04 PM |
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quote:
Originally posted by sdh2903
Sorry Sam but I don't agree with your statement. I'm not clued up on the theory so won't argue it but in real world experience on my
old car there was huge handling differences (better) when going from a level stance to a 1" rake. Much more stable at speed and also more
predictable handling.
Do you mean the GKD? AB performance set mine up with the nose 28mm higher than the rear (I seem to recall he would have preferred to get it lower but
we were running low on time) and it's been absolutely fine - highest speed was probably at the end of the straight at Brands (about 130 mph)
without problems. JeffW did comment on the rake looking questionable at North Weald but I've not experienced stability issues.
So in my limited experience it doesn't seem critical.
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Sam_68
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| posted on 18/3/14 at 08:37 PM |
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quote:
Originally posted by adithorp
What the hell has Jeff's tyre side wall deflection got to do with it?
Sam, as ever, you spout a load of guff trying to make yourself look clever then shoot yourself in the foot.
Look more closely, sweety. It's by no means all sidewall deflection.
You can judge the sidewall deflection by looking at how the edge of the 'tread' moves relative to the outer face of the sidewall (exposing
the white flash of the tyrewall markings as they rotate past in some shots).
The sidewall deflection is actually a fairly small proportion of the total lateral displacement. The remainder is geometric displacement of the
axle.
...and since you're obviously in a mood to try to nit-pick your way into a spat, I ought to clarify that this geometric deflection is only
partly due to the arc of the Panhard and is also partly due to what Arthur Mallock called the 'stagecoach effect' of the
sprung mass rotating sideways around the geometric roll centre (but obviously the arc of a Panhard adds to the 'stagecoach effect'
when rolling to one side, and subtracts from it when rolling to the other).
It's really very simple to prove this lateral axle displacement, and the detrimental effect of even a small angle of static inclination on the
Panhard, even to a complete moron, if you draw up the axle geometry on paper.
[Edited on 18/3/14 by Sam_68]
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Oddified
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| posted on 18/3/14 at 08:45 PM |
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The ideal rake depends on so many other factors, and without knowing a lot more about the rest of the car geometry and settings it can only be a best
guess.
A relatively short panhard rod is much more important to get as near horizontal as possible, the longer the rod the less deflection it generates on
suspension movement so being slightly out isn't as detrimental to the handling.
Ian
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