A major innovation related to McPherson struts

[sqrtminus1]

A new type of struts: Renault PerfoHub, Ford RevoKnuckle, or GM HiperStrut look as a promising innovation solving torque steer problems in FWD cars, as well as reducing bump steer with both FRD and RWD.
It works by adding an independent steering axis.
The only major disadvantage - a bit of extra unsprung weight.
Can this system replace the conventional McPherson struts in most cars (not only high power ones)?

 

[golfpin]

squirtimus1,
sounds fascinating where can we get more info please.
thanks golfpin rsa

 

[TC3000]

well, just google (search engine) for some of the "buzz words"
I think, more generally, these concepts are also known as "Super Strut", and IIRC Toyota used such a concept on their last Celica WRC car in the mid 90's.
some graphics, which will help to understand the general idea - I think/hope:

 

[TC3000]

I just double checked what I seem to remember, and yes the ST205 Celica (GT4) used a "SuperStrut" front suspension in the WRC

 

[dynatune]

The idea of having steering axis as close as possible to the wheel center line is as old as the front wheel drive itself (Reference Center Point Steering).And the solutions that you guys have collected are a few of the examples during time created. Being the key inventor of the famous Ford Focus RS MrkII Revo-Knuckle (called Advanced Revo-Knuckle), I can tell you that there are a few negatives to this suspension type:

The obvious one is weight which has already been mentioned, the most "limiting" one is "costs" since the suspension costs a lot more compared to a standard McPherson suspension. Then the key issue that these suspension is the problem of how to avoid "auto-rotation" of the spring strut. One needs either to put a link from the body/chassis to the strut (like a second track-rod, as on the Renautl Megane RS Mrk I, II) which causes a unique body/subframe) or one needs to but a "revolute joint" at the lower A-arm to block the rotation as on the Buick which is a) in breach with a previous patent of Ford (The Revo Knuckle I, named to the revolute joint, which is on your picture and is NOT the suspension that is in produktion), b) causes a unique lower A-arm c) prescribes a "fixed" 90° angle between "caster axis" and lower A-arm rotational axis which obviously is senstive to tolerances and can cause significant strain in the revolute joint leading to d) a higher risk to damage in the revolute joint possibly creating fatique issues and e) possible performance losses from suspension kinematics.

The production version of the Focus RS suspension you can see on

and with some imagination you can see that the rollbar is attached directly to the knuckle guaranteeing in that way the non-rotation of the strut. Besides having a unique anti rollbar AND not having all the above mentioned issues makes the Ford Advanced Revo-Knuckle the most sophisticated and most cost effective of all variants.

Cheers

Dynatune.

http://www.dynatune-xl.com

 

[GregLocock]

"Can this system replace the conventional McPherson struts in most cars (not only high power ones)?"

It could, but why would you bother? Why would you add weight, cost and complexity to a light and simple suspension if you didn't need the advantages that the more complex design provided? Torque steer is not an issue on your average econobox, so far as I am aware, and given the likely trends in the future will become less so.



Cheers

Greg Locock


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[sqrtminus1]

"In addition to reducing torque steer, HiPerStrut also is designed to isolate unwanted feedback from bumps and rough surfaces in the roadway, as well as improve steering precision and cornering performance in wet and dry conditions, by keeping more of the tire patch on the roadway."
Also: "... you’re not side loading as much.” (less stressed in comparison to ordinary McPhersons).

http://wardsauto.com/ar/gm_hiper_strut_100330

Yes, it is also more expensive, but many advantages are mentioned, other than reducing torque steer, and I will not be surprised if this system will find its way to many cars, not just the powerful ones.
Besides, I think that cost is also a function of volumes produced.

 

[GregLocock]

I guess we'll have to wait and see, since I suspect you have even less information than I do on that score. Big hint, I work in the same office and for the same manager as 2 of the names in this presentation.

http://www.autoline.tv/journal-wp-content/uploads/2009/02/revo-knuckle.pdf

Cheers

Greg Locock


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[cibachrome]

GM's Hyperstrut has been around for a long time and is still in use, mostly on the V8 ss models. But, there are several other ways to prevent torque steer (actually: Torque Induced Steer) in these FWD performance cars.

Tire brand is a player in this phenom because of the lack of a tire spec on MX. Toyota calls it 'pneumatic scrub'. Its a strong effect that produces a scrub radius offset much bigger that the static CAD recipe value and can be the wrong sign. Right sign: no torque steer, wrong sign: broken wrists.

 

[dynatune]

I know the exact costs of the system (which I unfortunately cannot share) and that by itself is a major roadblock for the system to become "popular" - if not for really high performance FWD cars. Furthermore with the arrival of electric power steering has also arrived an "anti torque steer" electronic tuning parameter that at least for relatively low powered FWD cars is sufficient to filter out any torque steer effects, eliminating the need for additional hardware/costs/weight.

On the topic HyperStrut/RevoKnuckle there are many claims to fame from many companies and many people. There are as always many fathers of success. Mind you please that the

http://www.autoline.tv/journal-wp-content/uploads/2009/02/revo-knuckle.pdf

presentation is the presentation of the first Revo-Knuckle that did not make it to production for various reasons. The "Advanced" Revo-Knuckle that is under the Focus RS MrkII is a different animal an does have some different inventors. Then not to forget, the Hyperstrut is violating that same patent of the link above using a revolute joint at the lower control arm.

Tire "Torque Steer" is indeed also a very important factor. On the development of the famous Focus RS Mrk I - having an ordinary McPherson Strut - the OEM Tire supplier was Michelin. In those days when FWD cars were developing into "unknown" territory with high power turbo charged engines the tire manufacturers learned a lot about "Tire Conicity" and it's effect on Torque Induced Steer. So thet started to work on tuning it. I assume that during the last decade some Tire Suppliers must have made great progress on this particular issue, allowing the standard McPherson strut to be used at even higher FWD power configurations.

Cheers,


Dynatune,

http://www.dynatune-xl.com

 

[sqrtminus1]

Well, at least Renault (with Ricardo) do not seem to violate some later patents, as can be seen in Ricardo's publication of 2004 about Renault Megane Sport (see the separate text box about the front suspension):

http://www.ricardo.com/Global/IA/Our-Markets-1/HPV_Motorsport/Renault X84 case study.pdf

As for the concept of all these complex struts, it looks as a clever "hardware" solution is preferable to some electrical steering buffering action to counter torque steering. I trust superior geometry more than battery power..., in particular, given that the secondary benefits of these complex struts, as mentioned before, will be lost if only electrical steering systems take care of torque steering, leaving the conventional Mac struts as they are.
I guess that solid rear axles are much cheaper than struts, wishbones, trailing arms, and twist-beams, and some sensible arguments had been made as to the cost/benefit ratio of all those independent, or semi-independent systems...

 

[dynatune]

I prefer the superior hardware myself too, as the Focus RS Mrk II has impressively demonstrated time after time again, but sometimes I fear that we are becoming ancient remainders of a long forgotten dynasty, with long forgotten rules and even longer forgotten principles of good engineering. And try to explain to an iPhone generation kid why he should buy a BMW M3 with al fancy top knotch multilink suspension geometry when a half of the price Mustang GT with sold rear axle blows it away on a track ..... good luck, we are in trouble ....

 

[sqrtminus1]

See your point on i-phone generation.
Been just wondering, why not use some sort of good old beam axle in the front instead of Mac struts to save a few more thousand dollars?
Your i-phone kid will be very happy with sub-5sec, or so, 0-60 mph acceleration, for even less money, shaming those very expensive Bavarian snobs. Can someone enlighten Mighty Ford with this deep insight
On smooth roads, and some great tires, even the lateral acceleration can be just fine.
No data on possible wheels hop at drive axle. That is not the kind of info shown in brochures. It is not a phenomenon as easily grasped and quantified as straight line acceleration.
However, there is the famous - "the test of the pudding is in eating it", principle. Even an i-phone kid will accept that.

 

[sqrtminus1]

dynatune,
A relatively new, suggested, version of the strut from Sweden, which is described quite clearly, even for those who are not suspension engineers...
See, in particular, parts 6.1 to 6.3 for the characterization of different flavors of the new strut.
As you can notice (in the last chapters), the guy prefers the Renault version for a separate link to prevent struts rotation, and the strut is supported, at its lower end, on a ball joint:

Development of a Hi-Per strut front suspension
Master’s Thesis in Automotive Engineering
ANDERS JOHANSSON
PETER ROUBERT
Department of Applied Mechanics
Division of Vehicle Engineering and Autonomous Systems
CHALMERS UNIVERSITY OF TECHNOLOGY
Göteborg, Sweden 2011

http://publications.lib.chalmers.se/records/fulltext/149741.pdf

 

[dynatune]

Hi Sqrtminus,

thanks for the link to the publication, good to see such a good essay on the various concepts to get a center point steering on a McPherson strut.
As far as I can read the writer mentions for the Ford Focus RS Mrk II Revo-Knuckle 2 "major" disadvantages, namely 1) possible problems with the Anti Roll Bar design and Loads and b) possible Ride Steer Problems due to the attachment of the rollbar on the Strut. If this article was written in 2001 I would probably have agreed but since it is written in 2011 the

Point 1) As you can see on the car Ford did basically make the Anti-Roll bar longer by adding material and creating a package routing for the bar. Besides that an Anti-Rollbar is probably one of the simplest parts in a Suspension (there is only so much you can design on a piece of tube steel) and since durability has not been a proven issue I do not support that statement.

Point 2) The writer is correct that the Anti-Rollbar Attachment joint on the strut is having a influence and it is also correct that the main reason for the "longer" ARB is the described effect of relative rotation of the strut on ride steer. However there are more parameters that influence ride steer and using their combined effects to the best this effect can be largely be compensated. Besides that being an ultra sportive car the wheel travel operating range was significantly reduced. And again here too, the Focus RS Mark II has not been criticized for having any bumpsteer problems.

At the end the main problem on these type of suspensions is how to avoid the rotation of the strut. This "how" is subject to many copyright patents and moreover comes at a certain cost. Considering all the factors mentioned in this report, I cannot see how adding twice 30 centimeter of 14mm steel tube (at an effective material price of 10 cents per car, not to forget the missing ARB drop-links with joints) is less preferable to adding an additional tie-rod with 2 expensive joints or complex revolute joints with earlier described limitations.

Cheers,

dynatune,

http://www.dynatune-xl.com

 

[GregLocock]

Using the steer effect from the a/r bar is commonly used to provide tuning between ride and roll steer. The a/r bar link can also be used to aid returnability.

I'm not wildly keen on either of those approaches but if it gets you out of trouble, that's what you do.

Cheers

Greg Locock


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