Slimy38
|
| posted on 10/8/14 at 09:40 AM |
|
|
Positive camber, is it ever needed?
I'm just deciding the length of my rear upper wishbone, and I was wondering whether I need to have it adjustable for positive camber? I was
going to have it adjustable from zero to -2 degrees, will that be plenty?
|
|
|
|
|
bozla
|
| posted on 10/8/14 at 09:53 AM |
|
|
I would probably add more adjustability if possible. It might look good on paper but things can be vastly different once you've put it all
together. Also, you might need to adjust more than you intended in case something gets bent.
|
|
|
Sam_68
|
| posted on 10/8/14 at 10:05 AM |
|
|
quote:
Originally posted by bozla
I would probably add more adjustability if possible. It might look good on paper but things can be vastly different once you've put it all
together. Also, you might need to adjust more than you intended in case something gets bent.
^^^ This.
You won't ever want a deliberately positive setting (though zero degrees at the rear is not uncommon), but you need to build in some tolerance.
|
|
|
Slimy38
|
| posted on 10/8/14 at 10:10 AM |
|
|
quote:
Originally posted by Sam_68
quote:
Originally posted by bozla
I would probably add more adjustability if possible. It might look good on paper but things can be vastly different once you've put it all
together. Also, you might need to adjust more than you intended in case something gets bent.
^^^ This.
You won't ever want a deliberately positive setting (though zero degrees at the rear is not uncommon), but you need to build in some tolerance.
Thanks both. Considering it's only a few millimetres to dial in several degrees of camber it shouldn't be too difficult to increase it. I
just didn't want to waste adjustment in positive camber if I didn't need to.
|
|
|
drt
|
| posted on 11/8/14 at 01:41 AM |
|
|
positive camber can be found on agricultural vehicles, some quads...and even some cars.
This for driving on a spherical surface (pardon my english, can't think of a better term).
Roads are spherical for water flow reasons.
One could argue that if you want to hillclimb your car...
or visit tracks with a spherical enough surface (eg. the nordschleife)
positive camber could be of use...
|
|
|
Sam_68
|
| posted on 11/8/14 at 06:04 AM |
|
|
quote:
Originally posted by drt
This for driving on a spherical surface (pardon my english, can't think of a better term).
Roads are spherical for water flow reasons
The English term (confusingly, in the context of this topic) would be to say that the surface is cambered.
But positive (suspension) camber is not used on agricultural vehicles and quads because it is of benefit on heavily cambered surfaces. Off-road
conditions are not 'spherical': they're just plain rough!
Positive camber is sometimes seen on off-road vehicles for one of two reasons:
- Because the suspension has to deal with a large range of movement, and the tyres used are not particularly camber sensitive. Therefore there is no
disadvantage in having a positive camber setting at the static ride position if it gives the optimum range of camber across the
suspension's full travel.
- Because it can give centrepoint (or near centrepoint) steering geometry, which minimises steering kickback over the very rough surfaces that these
vehicles are designed for.
Steering weight and kickback are also the reason that vintage, beam axle cars use steep positive camber on the front, to give centrepoint steering.
You can be assured that if there were an advantage to using positive camber on British hillclimbs, someone would have figured it out by now -
it's one of the most innovative and experimental fields of UK motorsport, so far as suspension design is concerned!
|
|
|
drt
|
| posted on 27/8/14 at 12:55 PM |
|
|
AAAh a cambered surface  thanks for that.
I want to share you my views on your comment,
friendly ofcourse! and open for debate! I have been told that my writing comes over as harsh,
so apologies!
It's been a while since I had to look into these types of vehicles.
But the positive camber is really there to improve traction roads like these;
http://www.oosterik.nl/webshop/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/0/0/002.oc5070b68_70x50cm.jpg
http://basdekker.eu/afbeeldingen/fotografie/landschap/grasland-akkerland/max_031097-ovps1-geploegde-akker-polder.jpg
As the wheels contact vertical this way.
I even remember some german cars having positive camber for Autobahn use.
I can see your point on the large suspension deflection, but its more preferable to have parrallel suspension geo in
such an event?
Unless, ofcourse, crude suspension is used.
Solid beam vehicles can also be positive cambered to compensate for load (bend through).
Centerpoint steering is a term used with Ackerman geometry (when you have 'true Ackermann' ). So I'm not following you on that 
If you meant self straightening steering  ? Negative camber will give inward camber thrust and cause centerpoint equilibrium, so does a degree of
KPI.
Kickback for the best part comes from Scrub radius.
|
|
|
Sam_68
|
| posted on 27/8/14 at 06:17 PM |
|
|
quote:
Originally posted by drt
But the positive camber is really there to improve traction roads like these;
...I can see your point on the large suspension deflection, but its more preferable to have parrallel suspension geo in
such an event?
On very rough surfaces, keeping the tyre 'square' to the road surface is of vastly secondary importance to keeping it in firm and
consistent contact with that surface. You can have the most perfect camber control in the world, and it will do you no good whatsoever if you
don't have sufficient wheel travel for the tyre to stay in contact with the ground!
quote:
Originally posted by drt
Centerpoint steering is a term used with Ackerman geometry (when you have 'true Ackermann' ). So I'm not following you on that
If you meant self straightening steering ? Negative camber will give inward camber thrust and cause centerpoint equilibrium, so does a degree of
KPI.
Kickback for the best part comes from Scrub radius.
The usual definition of centrepoint steering has nothing whatsoever to do with Ackerman geometry.
Centrepoint steering is when the steering axis (also sometimes called the kingpin axis or swivel axis) intersects the centreline of the tyre contact
patch at the road surface (ie. there is no scrub radius).
See
here
As you say, kickback comes substantially from scrub radius, therefore centrepoint steering avoids it.
|
|
|
bozla
|
| posted on 27/8/14 at 06:49 PM |
|
|
Hi DRT, it's always good to have a discussion!
>I can see your point on the large suspension deflection, but its more preferable to have parrallel suspension geo in such an event?
Unfortunately I don't think parallel geo is good in any case because the camber change is equal to the roll, and it always goes to a postive
camber change - generally not good as you want to get a negative camber change to make up for the induced camber in the tire.
http://the-contact-patch.com/figure/C1114-Rollinduced-1404857332.jpg?transform=%5Brs%3A590%7C600%7Cf%5D
You always want to try for unequal wishbone suspension (top link shorter than the bottom link) as this provides the negative camber change you need as
the car rolls through the corners. Unfortunately it also has another side effect such as a change in track (parallel geo coincidentally doesn't
suffer from this), and the camber change and track change can vary wildly between all combinations of bump, droop and roll. The deisgners challenge
is to come up with the best compromise.
http://www.walker-partnership.com/Images/camber_2.jpg
>Centerpoint steering is a term used with Ackerman geometry (when you have 'true Ackermann' ). So I'm not following you on
that
>If you meant self straightening steering ? Negative camber will give inward camber thrust and cause centerpoint equilibrium, so does a degree of
KPI.
I have no idea what centrepoint steering is (do you mean the steering returning to centre naturally?). I think I know what true ackerman is but
I'm not sure. I've always considered it irrelavant anyway as I feel that ackerman should be adjustable (apologies, I always think in
terms of race cars). You'll need more ackerman (inner wheel turning more than the outer) for tighter courses and less for the faster ones. The
other factor is tyres, and this can change the ackerman requirements a lot. Since the outer front wheel is usually more loaded that the inner one,
the slip angle of the tire is greater and depending on the load on the tire (front engine, rear engine, aero) then the ackerman requirements will
change.
>Kickback for the best part comes from Scrub radius.
That might be true, but the scrub radius itself is a result of KPI, camber angle and wheel offset.
http://www.roversd1.nl/sd1web/techtalk/kingpinincl.gif
The KPI is one of the most important parts of the front suspension as you can totally ruin it if you get it wrong. It's also something that we
don't have much control over as we try and use of the shelf parts (unless you want to make your own uprights). 8 degrees KPI is usually better
for race cars but locosts use the Cortina upright (4 degrees KPI) because the have the engine in the front and no power steering - any more and the
steering might be too heavy. The MGF and MX5 use about 11 degrees. The MGF has the engine in the rear so it's acceptable, the MX5s have both
power steering and non PS versions - the non PS version has a much higher rack ratio to make up for the large KPI.
Or you could do what F1 does and decide that aero is far more important that geometry and shove anything on the car!
http://cdn.images.autosport.com/editorial/1353021418.jpg
That is probably the sum of my knowledge on geometry.
[Edited on 27/8/14 by bozla]
|
|
|
drt
|
| posted on 28/8/14 at 03:50 AM |
|
|
you're right on center point steering,
I've looked it up in 'milliken and milliken'.
Most of my books/notes/tuition was in german, so i should google before i speak.
And thanks for the nice chapter!
And you have a point on the amount of large suspension deflection.
But stating it that way is not in the spirit of the discussion :p
As the point of havving camber in the first place is to optimise your tire use.
Classicly, to get the most lateral grip in a given condition... like mid corner.
The point was if havving positive camber is needed...
Thus an a 'cambered' road would need the tire to be positive for it to be less negative
in relative to the surface ... and go vertical (or whatever angle your tire curves demands) at the right moment.
^^
BOZLA ...About the SLA suspension geo.
Most Baja/dakar/... vehicles who have large suspension deflection really use parrallel geometry.
The just dial in some static camber to overcome the camber gained by body roll.
Because it is easy , eliminates jacking forces, has no roll center migration,...
:p I challenge you to design me an SLA with 350mm of deflection with any reasonable camber compensation
your instant center will be most likely somewhere on the opposite hub,
or if your really brave, it will be intersecting the car 'mid line'...
This will cause a lot of trouble ...
> remember that SLA rarely compensates for all the body roll induced camber change...
Mostly it's only enough to make it go slightly less positive
True Ackermann is when both front wheels are making their respective 'correct' circles.
So 0° lateral slip angle. (only true for speeds up to 30kmh)
(when the lines from the steering arms intersect eachother in the middle of the rear axle.)
Having adjustable ackermann is always a bonus
Only it is hard to set up correctly... As you (indeed) need a lot of data from your tyres...
Data only Avon will give away willingly.
Also... If I may give you a tip... Ackermann setup has different outcomes on different stages of a corner.
Turn in - apex - accel out.
Some drivers even dial in some Bump steer to get different toe
(thus ackermann because in racing the toe chance is limited to corner conditions )
geo in different parts of the corner
|
|
|
Sam_68
|
| posted on 28/8/14 at 06:56 PM |
|
|
quote:
Originally posted by drt
But stating it that way is not in the spirit of the discussion :p
Sorry, but it's true, and I couldn't think of a clearer way of putting it: camber control is quite low down the list of priorities for any
genuinely off-road oriented vehicle.
quote:
Originally posted by drt
As the point of having camber in the first place is to optimise your tire use.
Classically, to get the most lateral grip in a given condition... like mid corner.
Yes, and where you use it to optimise grip for that condition, you will be using a large amount of static negative camber.
But on off-road vehicles you're almost always more interested in traction than in cornering grip. At risk of sounding facetious, again, if you
don't have enough cornering grip, off road, you take the corner slower. If you don't have enough traction, you stop moving...
quote:
Originally posted by drt
The point was if havving positive camber is needed...
Thus an a 'cambered' road would need the tire to be positive for it to be less negative
It's almost impossible to have too much negative camber when cornering. Most tyres generate high levels of grip at really very extreme
negative camber. The usual problem is that if you give them enough negative camber to maximise grip in a corner then:
a) you seriously compromise straight line grip for braking and traction, and;
b) you wear the treads on your tyres unevenly, very quickly.
Seriously... sorry to put it so bluntly, but the whole idea that positive static camber is a deliberate technique to conform to steeply cambered roads
is just totally spurious.
[Edited on 28/8/14 by Sam_68]
|
|
|
drt
|
| posted on 28/8/14 at 08:05 PM |
|
|
Depends on the tire,
lateral force declines steeply when going beyond 5° negative.
Camber thrust however is more or less lineaire
I'll look up the table I have on Positve cambered vehicles...
It will take a while, as i see dr. ingelse not very often any more.
And its not on google
|
|
|
Sam_68
|
| posted on 28/8/14 at 08:25 PM |
|
|
quote:
Originally posted by drt
lateral force declines steeply when going beyond 5° negative.
Indeed.
But 5 degrees of static negative is a huge amount.
Even two degrees is enough to compromise straight-line grip/traction and cause problems with uneven tyre wear. You're into other problems
long before there's enough negative camber for it to become an issue in terms of lateral grip.
|
|
|
drt
|
| posted on 29/8/14 at 12:15 AM |
|
|
:p I wasn't talking static :p
|
|
|
Sam_68
|
| posted on 29/8/14 at 06:16 AM |
|
|
quote:
Originally posted by drt
:p I wasn't talking static :p
If you were talking dynamic, then it's even less applicable: you will never have 100% camber gain on the outside (loaded) wheel, so at least
some of your static negative will be removed.
Sorry, but this discussion is becoming absolutely nonsensical: you're trying to argue the unarguable.
I'll leave you to continue believing what you wish...
|
|
|
