Ackermann angle an suspension theory.

[sergei]

I decided to check the basic angles on my ST165 (because I use ST202 hubs).
What seems to me is the ST202 is running high Ackermann angle to start with (even if you account for wider track).
Is running high Ackermann angle is normality in Japanese cars?
Or I am not measuring it right?
How I measured it: from the centre of the of the tie rod (knuckle end) ball joint to lower ball joint to the axle centre.
The wheel base is 2450mm and it was falling short by about 300mm.
The S165 runs 1440mm track, while ST202 runs 1510mm.
From my measurement I get 71 degrees (between knuckle, lower ball joint and front axle line). With this measurement the Ackermann "point" falls short at ~2130mm. If I compensate for track difference (I put another 50mm towards ball joint to ball joint distance), it gets to 2200mm, still short off 2450mm by 250mm.

[Lloyd]

You're asking the sort of questions on here that you're usually the one answering

Good luck


Seems to be about the best read on it
http://www.rctek.com/technical/handling ... ciple.html

[RedMist]

Surprising, but not unexpected. Not many, if any cars use true Ackermann. It simply doesn't allow for sidewall flex, loading of the outside wheel, and a host of other factors.
My racecar wheelbase is 3175mm and I'm getting no where near that in Ackermann. At the same time I've seen others run negitive Ackermann (and I understand it's very good for pavement).
I suspect running a little under true Ackermann is good for tyre wear or perhaps may have some benefits to turn in on loose surfaces. I certainly believe in dragging the inside tyre on dirt.

[RomanV]

Did the car with the crazy ackerman run superstrut?

Could be relevant.

[sergei]

Did the car with the crazy ackerman run superstrut?

Could be relevant.

Nah, McPherson type.

After googling for a while it seems a lot of car run more Ackerman.
Having 110-120% Ackerman is apparently quiet beneficial towards reducing understeer.

Interesting info here:
http://www.auto-ware.com/ortiz/ChassisN ... er2002.htm

[gt4dude]

if celicas have high ackermann , why the hell are the turning circle so sh*t!!!?

is it because high speed cornering stability and low speed maneuvering are at odds or what?

[wde_bdy]

KP Starlet has virtually no ackerman and a sweet turning circle.

Callum

[VR-4Squid]

if celicas have high ackermann , why the hell are the turning circle so sh*t!!!?

is it because high speed cornering stability and low speed maneuvering are at odds or what?

Probably down to the lack of steering angle itself, at least partially due to the limitations imposed by having cv joints in a front/4wd setup.

[KinLoud]

I'm feeling the need for some MS Paint action to help me visualise what you're measuring.

If you put your car on a modern wheel alignment machine I think it can measure the ackerman steering factor - someone will be able to confirm or deny this.
Then (if you have a standard st205 and st165) you could compare the results.

Ackerman is a funny thing - only really needed at low speeds/tight turns (parking etc) where you want low steering wheel effort and minimum tyre scrub.
At higher speeds the slip angle and loads on the tyres (as well as smaller steering angles) mean that much less ackerman is needed.
The manufacturer will use a compromise to get good handling and minimise tyre scrub while parking.
Power steering means the steering wheel effort will be less at low speed/tight turns.
Also, they offset the effective KPI versus the contact patch so the tyre tends to roll around the pivot point - reducing steering effort and tyre scrub. This had led to modern cars having less ackerman than cars of earlier generations.

Ken

[KinLoud]

Just read your link about ackerman - i now have a better idea of your original post comments.
There are so many factors that affect how the wheels turn (when steering wheel turned) that there is probably no effective way of comparing one car steering behaviour with another (even if closely related like the ST165/ST202).

Does the ST202 use a different offset wheel from the ST165?
Same/different turning circle?

Ken

[sergei]

This is how I measured it. Specifically on the right side (with knuckle zoomed in): that is how I measured the angle (by constructing an right angle triangle and measuring two sides and inverse cosine the ratio to get the angle between green line and the centre line). Then using half of the blue line length (ball joint to ball joint) and appropriate trigonometry I have calculated where the green line will intersect the centre line (basically it forms a right angle triangle between blue line, green line and centre line).

The green dashed line shows where true Ackermann should be.

As far as the turning circle both ST165 and ST202 have 5.2m.

[gt4dude]

KP Starlet has virtually no ackerman and a sweet turning circle.

Callum

that means nothing to us,

any small car with a short wheelbase will turn sweet. nannas love hatchbacks, nomatter how badly u aim for a park there is no conceivable way to miss it.



also, shouldnt high ackerman help reduce understeer? we got alot of that too

[sergei]

also, shouldnt high ackerman help reduce understeer? we got alot of that too

It definitely should as inner wheel will be dragging more.

[RedMist]

also, shouldnt high ackerman help reduce understeer? we got alot of that too

It definitely should as inner wheel will be dragging more.

And that's exactly why it doesn't.

A slipping tyre doesn't produce the same grip as a tyre with a solid contact patch. The entire idea behind Ackermann is to stop slip, to make the tyres track correctly given the wheelbase.

The reason why large Ackermann angles work for my offroader, is I don't really have tyres, I have two front tillers that scrape through the dirt and are constantly in slip. Presenting a larger angle on the inside tyre just attempts to redistribute some of the steering load onto an inside tyre. It is however most certainly not applicable to pavement.

[Bazda]

I've been playing with this on my car.
I have 2 settings on my hub as they made the GTZ hub with some extra beef and I was able to drill another hole 15mm off.

I tried with extra ackerman, it makes the tight turns better but does feel a little odd. Odd as in steering input vs where the car goes. IMO I didnt like it on my car.

So i've ended up back in the stock position. Also i've added 2 deg extra caster + reset the RCA & bump steer points. Along with 2.5 deg camber.