Comments on: RevoKnuckle 1

[SusTestEng]

I found this presentation when doing some research on torque steer in high power, FF vehicles.

http://www.ukintpress.com/vehicledynamicsexpo/images/vdipdfs/Frantzen.PDF

The solution presented by Ford and ZF seems to be very interesting. It allows for a more verticle King pin axis, as well as reducing the scrub radius. Although the cost penalty compared to a MacPherson strut design is significant, they say it is still cheaper than a Double wishbone. As the US market HP requirements reach the 300 hp mark, this may be a good solution to FF vehicles that have torque steer.

Anyone else have any thoughts on this?

 

[GregLocock]

Unsprung weight will increase a bit.

The bush at the top of the strut no longer rotates, so steering friction may improve, and that'll be cheaper.

It looks a bit truck-like, I think it is a reasonable solution for an AWD.

It'll still have the same camber in jounce characteristic as a MacP of course.

I'm struggling to think of any real disadvantages -I guess they dropped it because of the silly name.






Cheers

Greg Locock

 

[shanba]

The half-shaft must have more length change and angle due to the greater distance between kingpin and CV joint center.

Caster can be tuned within the knuckle, would allow the top of the strut to be moved forward for more cabin space, or allow more caster for a given package.

Interesting, thanks for the info.

 

[SusTestEng]

"The half-shaft must have more length change and angle due to the greater distance between kingpin and CV joint center."

This was my concern after looking at the setup a bit closer. If they found a way fix this, it would solve many limits with the MacPherson design as well.

 

[shanba]

In their picture, it looks like the CV boot is/maybe rubbing on the upright assembly.

 

[ubrben]

And thus the wheel was re-invented again.

I believe this system exists on the Renault Megane Sport and was also tried in the French or German touring car championship by Peugeot on the 405 back in the early 90s.

Ben

 

[MoreWing]

Thank you. I thought exactly the same thing! I remembered it being used on an Audi in the DTM. I even ran around the 'net a little to try to find it, but gave up.

I think the change in KPI is a pretty big gain for a performance car. When you put a wide tire and a lot of camber on a Mac-strut you end up having to lean it over quite a bit. This large steering axis inclination makes for some problems that the RevoKnuckle seems to address. It would be an interesting thing to play with.

You still have to deal with the bending load on the strut which doesn't do you any favors in terms of friction in the system, so I doubt if it will ever really be the choice for true performance applications.

 

[SusTestEng]

I usually don't get into the design aspects of the suspensions, so I have a question. If one of the major negative points this system is correcting is the reduction in the scrub radius, why couln't you simply adjust the wheel ofset so the center of the contact patch matches the intersection of the king pin axis? What are the negative effects of this? Or is it simply that the King Pin offset is much more of a negative effect and correcting that is more important?

 

[GregLocock]

You have to hit the KPI a lot to make much difference to scrub radius, and you have to change the scrub radius a lot to make much difference to the car. Whereas the KPI has a fairly powerful effect on steering precision feel.

So, the compromise involved in getting the scrub radius where you want it is too big, so in practice (with typical geometries) the KPI gets minimised and the scrub is a fall-out. The 'easiest' way to change the scrub radius is to change the static camber, but that is a powerful change in its own right.

This probably doesn't apply to circuit cars, but I doubt they give a monkey's where the scrub ends up.


Cheers

Greg Locock

 

[MoreWing]

I've only played with scrub radius by itself once on a car, and that was with hub spacers. We were going for track width, but we were getting it essentially with wheel offset. A 10mm change didn't change the steering effort at all. It didn't seem to affect handling either. I think we had other issues.

I did play with KPI by adjusting wishbone geometry. A 1 degree change in KPI was a noticeable change. I left camber alone, and tried to leave wheelbase/track the same. Our car was unstable on entry, and the driver was reporting it was due to weight jacking through the steering. Our caster wasn't excessive, so I attacked it through KPI. Two drivers tried it, and they both reported better rear grip early in the corner and less weight jacking at high steering angles.

I've had too much KP offset in a kart. It makes the steering very erratic on the brakes, especially over rough pavement. Although you would guess that adding offset increases steering effort, that isn't always true. Sometimes it reduces the effort by reducing understeer. That was weird the first time I felt it.

I question whether zero or very small scrub radii is a good goal. It seems to me that if you have a very small scrub radius, then as the tire deflects in a corner the torque that is applied about the steering axis could change sign several times through the corner. Intuitively, that seems wrong to me. I've never had the opportunity to monitor it in practice. When push comes to shove, the KP offset is the 'fall-out' on the projects I've worked on.

I think the reason that wheel offset alone isn't used to scrub radius is that with a wide tire, the tire will eventually hit the strut. It's just a packaging thing. As you offset the strut more, the knuckle gets heavier and it applies more of a bending load on the strut, which is a bad thing. When someone comes up with a through-shaft strut, they're going to be called a genuis.

 

[GregLocock]

This MacP+ steerable knuckle concept is in production on the Renault Megane, or at least some of the sporty derivatives thereof.

Cheers

Greg Locock