Modern Spindle Geometry Design Practices

[Joest]

After closely inspecting the front spindles of a new '05 Ford Mustang (strut suspension) I notice that the hub centerline lies forward of the lower ball joint by about 25mm. I was hoping to spark some dialog about what effect this geometry has in terms of handling and steering feel. Perhaps Ford moved the spindle pin centerline forward as a way to run a lot of caster (10-15 deg) without offsetting the steering axis too far from the tire contact patch center as to not make steering effort excessive. Assuming a castor angle of 14 degrees, I come up with about 26mm of mechanical trail. The benefits of caster (hence mechanical trail) are clear to me, but what I am specifically targeting is whether it makes sense to run lots of caster, yet limit the amount of mechanical trail.

Cheers,

Joest

 

[GregLocock]

Well, I'd rather start by talking about the trail, then the castor.

I think you don't want too much mech trail, because that will mask the change in pneumatic trail as the tire approaches its limit.

For whatever reason the mech trail of modern cars falls in a tight band, so even if detecting the limit is not the reason, some limit still applies.

OK, then I'd wonder about what castor is optimum. It helps a bit with camber gain in tight turns, but frankly, with radials, who cares?

It seems to add damping to the steering. That's a good thing.

However it does confuse things, vertical forces at the tire patch will become tie rod forces. This is bad. The driver doesn't need this information.



Cheers

Greg Locock

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[NormPeterson]

More +caster will jack a little more load toward the inside front tire as the car is steered, since the planes containing the arcs that the spindles swing through are not parallel to the road surface. That should shift the lateral load transfer distribution rearward and reduce the understeer slightly. Whatever amount of increased camber gain (which could amount to 1/8* additional camber per degree of added +caster during normal maneuvers at reasonable driving speeds) is at least working toward the same goal of increasing front lateral grip.

Norm

 

[Joest]

It would seem than with the obscene amounts of KPI strut cars are using to accommodate large tires and low scrub radius, running huge amounts of caster would be necessary to minimize the negative effects that a steep KPI has. Norm spelled out the benefits to lots of caster above. It would be interesting to know what the design team felt was an absolute "must have" during the design process. I know that the specific design I refered to above ('05 mustang) was closely modeled after the BMW layout if that is of any value to the discussion.

 

[SusTestEng]

I just started laughing when I read this thread, because it just hit me that maybe Ford spent way too much time designing the front geometry that when they got to the rear they ran out of time and said.."ahhh, just throw a 3 link suspension on there and call it a day!"

As for the original topic: It appears that everything they did was to help out the steering feel. More caster gives you better camber gain and thus produces better cornering feedback for a sporty car. Another good example of this is found in the Nissan 350Z/G35 vehicles (almost 9 degrees of caster I think).

 

[Joest]

Let's discuss mechanical trail for a minute without considering the caster angle. Obviously the two can be de-couple if the spindle axis (tire rotation axis) is moved forward or back (side view) effectively moving it off of the king pin axis in the side view. From the experience that you all have, what amount of mechanical trail has been shown to yield the best performance for a race-version of a production car in the 3000 lb range? Try to address only mechanical trail without bringing up the amount of caster used since we now know they can be independent.

 

[mloew]

Joest,

The Corvette (starting with the C4) and the Honda (nee Acura) NSX were early adopters of this geometry in production. Not sure what they did with their racing versions. Not sure if their specifications are available in the public domain.

Best regards,

Matthew Ian Loew


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[Joest]

The C4 corvette did have the spindle offset from the steering axis although it was in the opposite direction of the 2005 mustang. I have one of each spindle here before me right now and the mustang steering axis is about 20-25mm behind the spindle axis (reducing mechanical trail) however, the C4 spindle is about 0.75 inch in front (adding to the mechanical trail). This sounds like a complete reversal of design philosophies or am I missing something.

-Joest

 

[GregLocock]

What were the castor settings? I bet the Mustang has heaps (8-10 deg) , the Corvette not so much(3-5).

That's why I work back from a mech trail at the ground, i bet they are both 15-35mm.





Cheers

Greg Locock

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[Fabrico]

Something else that might be influencing this is that many cars and trucks now use front wheel drive geometry even if they are not front wheel drive. These often have zero scrub, rear of the axle steering, and even zero castor. Because this system offers some significant advantages, all or parts of it have been making their way into more traditional systems.

 

[GregLocock]

Can you give any examples of zero scrub, zero castor, RWD cars? I must admit, my data only covers 'image' cars, but I see no zeros there. I'm kind of interested in how you know the scrub radius.

Cheers

Greg Locock

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[Fabrico]

My point was that many FWD cars, and AWD SUV's and trucks; have at or close to zero castor and scrub because they have to. This requires a steering spindle assembly designed just for this. Chevy, Ford, Dodge, Jeep, and Toyota use steering spindle assemblies on some of their RWD vehicles that are the same ones used on FWD or AWD models. Since many of these are of near-0-scrub design, some are even swapped from side to side so front-of-the-axle steering is retained. When this type of spindle assembly is used on the fornt of a RWD vehicle, the manufacturers often do not retain the high offset FWD/AWD wheels or zero type castor. Using a more traditional type wheel and/or much castor, will cause a zero scrub type spindle assembly to again have scrub. You won't see it in the specs but you will see some odd component setups.