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"Link-X" suspension

"Link-X" suspension

"Link-X" suspension

(OP)
I just heard about, and started looking into the "Link-X" suspension system.  The concept is interesting, but I haven't had a chance to lay one out in the comptuter yet, to see how things work.

The setup requires crossing the A-arms, ie attaching the "upper" A-arm's inboard pickup points below the inboard pickup points of the "lower" A-arm.  The concept is to work the cars natural roll against itself, to resist the roll, eliminating the need for an anti-roll bar.

I'm curious if anyone has looked into this type of setup, and any pro's, con's, or thoughts regarding it.

Some links:

The Link-X inventors webpage:
http://www.wageng.com/

Articles on it:
http://www.autofieldguide.com/articles/040203.html
http://www.autofieldguide.com/driven/0602dri05.html

http://www.caranddriver.com/article.asp?section_id=29&a...

-Dave
http://www.moslerauto.com

RE: "Link-X" suspension

We did discuss this a while back but the thread has vanished. That's a shame since the inventor himself did participate.

He thinks he has solved the obvious problem with camber control, I don't know how and he wouldn't say.

Cheers

Greg Locock

RE: "Link-X" suspension

(OP)
I had heard it was discussed, via a google search, but searching here didn't turn up the old thread, so I figured I'd ask again.  I should be meeting the inventor tomorrow, so I've been trying to find out all I can on it.  Hopefully I can get a demo of it too, since I can't find any good pictures online.

I'll post back, if I learn anything from him regarding it.

-Dave
http://www.moslerauto.com

RE: "Link-X" suspension

I would anticipate sever camber change problems.

The only method I can think of to reduce this would be to have the top arm very long (like past the centreline of the chassis) and the bottom arm quite short and inclined in the same direction as the top arm, but not so steeply.

Say the top arm is 800 mm long, at 10 deg to horizontal, and the bottom arm is 400mm long at 5 deg to horizontal for example. I have no idea as to how these numbers would work out, and what the horizontal distance between the ball joints has to be, and if the inboard top arm pivot would end up below the bottom one, but with the concept in mind, and a drawing board, or better still a CAD program, it could be pretty easily roughed out to see the camber changes.

It might be a bit more difficult to work out the roll resistance and the effect on ride and bump steer, but I won't stick my neck out any further as I am no where near being a suspension guy.

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: "Link-X" suspension

Sorry I was sloppy and replied to the wrong thread.

The inventor is quite paranoid. He pulled posts from his own forum that mentioned his patent number. I bet he asked to have the old thread removed. If someone can find the text in a google cache or somewhere else we should repost it.

The original thread was Thread108-21170. There is another thread with the patent number here: Thread800-54517

The idea of having the roll center near the C.G. to reduce roll is very old. I doubt that the patent would hold up in court. There are probably huge issues with jacking because the C.G. of an SUV is very high. His videos look like that is a problem.

I also would like to know what happens when truck is full and the C.G. moves below the roll center. This will create roll moments in different directions at each end of the vehicle.

RE: "Link-X" suspension

(OP)
Anyone able to get the images to show up for the patent?

I haven't seen an actual "link-x" yet, but I have seen what he refers to as being 75% as effective...which basically looks like a traditional SLA suspension, with some radical anti-dive goemetry, and a vary high rollcenter (near the CG), and very soft springs.

The drivers impression wasn't very good, but it was just assembled, and had no time to tweak it yet, so I'm not making any judgement yet.

-Dave
http://www.moslerauto.com

RE: "Link-X" suspension

Sorry for the off topic commentary, but the following information might be useful:

To view the tiff images on the USPTO website, it is recommended that you use the one of the viewers referenced on this page:

http://www.uspto.gov/patft/help/images.htm

I copy the individual images to a Word document and save the entire patent electronically.

Best regards,

Matthew Ian Loew
"Luck is the residue of design."
Branch Rickey


Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: "Link-X" suspension

I'm not a suspension man, but from my observation of the drawings, there are some extreme angles re anti dive, anti squat, scrub radius and camber change, The camber change must be horrific, and I imagine would lead to problems re bump steer.

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: "Link-X" suspension

I think the toe curve could be controlled by the longitudinal angle of the upper and lower control arms.  The biggest problem I see is the short and severely angled instantaneous center causing severe camber change and track width change through suspension travel.  While the body roll is minimized by the high roll center, the wheels will camber out due to a larger roll moment on the unsprung mass and tire deflections.  The large track width changes will wear tires out rapidly.  For trucks without ride height control, the camber change with varying payload will wear tires out unevenly, similar to the twin i-beam suspension.

RE: "Link-X" suspension

The swing axle is simpler, probably safer and not under patent. Simply move the points of articulation outwards until the roll center is above the CG and you get the same effect. Porsche did the same thing when he stole Ledwinka ideas (Ledwinka’s designs placed the point of articulation at the center of the chassis), and produced the worst handling cars ever. There might be some geometric refinement to the Link-X that I am not taking into account, but it isn’t enough to make it viable.

RE: "Link-X" suspension

Hi All,

I'm the inventor of Link-X.  I'm a mechanical engineer and have invented several manufacturing processes.  When I invented this suspension for my solo car, I didn't know that much about suspensions in general.  If I had, I probably wouldn't have pursued it!

You are right about the fact that known theory suggests that the camber change (there isn't any CL track change, but the camber change moves the contact patch) will make a vehicle with Link-X shift laterally over bumps and wear out the tires.

But...with some help from a generous venture capital firm (over 5 long years) we have solved all of the 'issues' via clever engineering and in some cases via other bad suspension characteristics that act in the opposite direction!  For example, we use a lot of scrub radius to offset lateral disturbances.  As a result, we have less steering wheel shake over bumps and driveway lips than most OEM vehicles.

We did a tire wear test with our '99 Expediton prototype (converted to all indep. suspension) against a '03 independent suspension Expedition.  Over the 8500 mile test the Link-X machine got 16-31% LESS wear.

Camber gain is part of the solution, not the problem anymore.  We have both tires leaning into the corner so we get optimal grip on both inner and outer tires.  The tires wear more evenly as well.

The biggest remaining problem which someone pointed out is using Link-X on the rear of a truck.  Load variations lead to high static camber.  In moderation, this effect offsets load-induced transition to oversteer.  SUVs seldom are loaded heavily, so the extra wear isn't that meaningful.  Regardless, we are focusing on using Link-X only on the front.  

We've found that this setup yields about 70% of the value of Link-X and has zero drawbacks.

Cheers!
Todd

RE: "Link-X" suspension

Thanks Todd for your comments.  If this system is used in a front suspension with a more conventional rear suspension, do you see adverse affects to having such a high front roll center with a lower rear roll center, opposite of conventional implementation.  Does this create stability issues due to the phasing of the front and rear suspensions as lateral load is building?  Usually the rear builds faster than the front for driver comfort/predictability.  With your setup, the turn-in response must be much quicker, but wonder how the axle phasing is handled.  With that much camber gain, is there straight ahead stability issues/wandering due to camber thrust?  Or are toe curves used to compensate for this.  Just curious, you've obviously been refining this.

RE: "Link-X" suspension

Good question.  The only vehicle that we have applied Link-X to in this way was an ATV.  It completely transformed it and actually made it easier to handle, you could just drive it like a car instead of leaning all over the place.  It took most of the 'fear of death' factor out of riding an ATV.

We had Prodrive do an extensive (expensive) Adams analysis on a Link-X front and a stock live axle rear.  It did show 31% faster yaw response - it got to the intended yaw 31% faster.  This is good, but what is better is that there was negligible overshoot!  The Prodrive guys rated the Link-X setup as 3 times more stable than the stock setup.  Of course ours also had better ride quality and less roll.

A lot of people talked to us about camber thrust, but it doesn't really amount to much in the real world.  In order to get a meaningful amount of camber you have to hit a 30mm or larger bump and the bump is only instantaneous...so it's 'easy' to use load-based toe changes to offset any camber thrust, tire-patch movement, etc.

I'm working on grip-phasing now with a vehicle, because the rear has significantly taller sidewalls than the front.  We compensate for grip phasing on the Link-X setup by running a much lower-than-normal bump:rebound damping ratio.  We don't have any 'jacking down' problems with it.

As I understand it, having a higher rear roll center improves driver comfort by pitching the car forward slightly in corners.  We get this same effect by installing rebound springs in the front shocks that contact the shock piston immediately below ride height.  We also have another method that is still trade secret.  

People ask us about lift, but we actually have slight negative lift!  Having this allows us to make both (inside and outside) tires camber into the corner, but with the outside cambering slightly more...no other suspension I've ever encountered can do that.

Todd

RE: "Link-X" suspension

I must admit I am sceptical about the use of four bar mechanisms to push up the roll centre on off road vehicles from where a panhard rod would put it. The problem I usually face is that the majority of inputs are one wheeled. The lateral acceleration greatly reduces the driver comfort (I work with 60 tonne trucks).

I have looked at similar systems to lower roll centre to ground height, but decided in the end that independant is the way to go (ie vertical movement). Designing an active steering system into the axle does help, by allowing the wheels to move laterally without vehicle input, but it does begin to get complex (viscous couplings reacting against springs etc). In the end the thing always ends up active, so you may as well fit active roll control - an electric motor is so much cheaper! Seems to have helped Land Rover...

Mart

RE: "Link-X" suspension

The Prodrive guys are actually working with a firm that builds Dakar rally-type vehicles.  The fact that Link-X makes sway bars obsolete is very appealing for the very reason you mention...most inputs are single-wheel.  

If I remember correctly, they said the 'cross axis' stiffness is very low with Link-X.  Thus it is perfect for off-road vehicles.  One problem is that our suspension gets too much camber at extreme amounts of droop.  I like to limit droop to about 3".  

Bump doesn't seem to pose a problem.  Our ATV prototype actually has more overall wheel travel than it's stock counterpart.

RE: "Link-X" suspension

My experience with off road racing is that things like camber change and track change really don't matter so much, as the surface is loose and uneven, and traction low.

Maximum travel and rebound control are paramount.

I can see a great advantage with completely independent action on each wheel, so long as there was another method to control roll.

Regards
pat

eng-tips, by professional engineers for professional engineers

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: "Link-X" suspension

Interesting. Must admit I'm a fan of the Lotus floating subframe technique. The subrame held independant suspension to the wheels, but was then attached to the vehicle by a four bar link. The original purpose was to put subframe roll centre below ground height, so that wheel camber was always optimised - particularly on corners.

Modern design techniques (ADAMS) have rendered the technique obsolete, but I have wondered about a variation on the idea. The subframe is allowed to slide sideways in the vehicle, but two angled links connect to the stub axles, with top ends constrained to the vehicle by a DeDeon arrangement. Like X-link, the roll centre goes up (so you still need roll rate controlled passive steering), but track remains constant. Roll centre could even be put above mass centre. Dynamically it would be excellent, package wise it could prove a nightmare.

All sounds very complicated, when hydraulics and an electric motor can force the vehicle to lean into the corner. I was a great fan of Lotus's work on active suspension. It was a very sad day for motor sport when Williams were banned from using the technique...

Mart

RE: "Link-X" suspension

wageng
    Have presented any SAE papers or similar technical presentations with the info from the ADAMS analsis?

  Thanks for continueing the join the conversation. It's too bad the orginal thread on your design is gone.

ProEpro
www.whitelightdesign.com

Pro/E FAQ www.whitelightdesign.com/servicestips.htm

RE: "Link-X" suspension

I presented a SAE 'small engine' paper on our ATV work, but haven't presented anything to the big SAE Congress.  I didn't remove the original thread.  I guess it just went unused for too long.  Mostly the same questions came up, though.

About a year ago we looked at Graviman's idea of a floating subframe to give us more rebound travel.  It seemed too complicated and the need for further droop wasn't compelling enough for us to try it.  

We're working with the Military on the next-gen Hummer type machine (all indep susp).  The Hummer rolls over a lot more than people know.  Also, the 50mm machine gun is hard to control because of body roll...even when the vehicle is sitting still!  Our moment-inversion principle can keep the platform stable.

The ADAMS model predicts a higher rollover threshold as well because of lower cg displacement & lower rolling momentum (less stored energy because the sway bars are gone).

RE: "Link-X" suspension

Been thinking about this a while now. More I think about it the more it makes sense. Basically it allows the vehicle mass centre to flow along the same axis over rough terrain. The only down side is the need to allow lateral movement of the wheels. The only ideal method is to have the steering linked to a mass constrained to the axle by a torsional compliance (along x in car line) - has to be the axle, not the vehicle.

Presumably this is what you mean by a load based toe changes? The system ends up looking like McPhearson strut, with an axle between, using the axle rotational inertia to control the wheel toe angle. The struts take the vehicle weight. Not sure how the steering input goes in yet, but I'll get there! Packaging wise this looks good too, since the wheel tops stay about the same position (in plane of mass center).

How big can these axles be made? I take it 20 tonne axle weight is out of the question!

Mart

RE: "Link-X" suspension

Here are links to two SAE Technical Papers on the "link-X":

2004-01-1545 Link-X Suspension for Roll and Pitch Elimination
2003-32-0071 Link-X Stability System

Best regards,

Matthew Ian Loew
"Luck is the residue of design."
Branch Rickey


Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: "Link-X" suspension

Thanks for this - these are now ordered.

After further thought I am having doubts as to how well such a system might respond to the step input of say a curb strike. Steering would not allow the wheels to move sideways quickly enough. OK this is fairly extreme, but I know that active suspension and ground height roll centre will cope with this, and still eliminate roll. We are talkng about an off-road suspension system...

BTW is there any merit in designing a conventional beam axle to have bump steer? I'm thinking steering towards bump, to reduce centre of mass lateral movement. The trucks I'm involved with have very high cabs that would benefit greatly from reduced lateral movement (Articulated 6x6 tractor trailor combination), not to mention higher ground speed from reduced mass center disturbance.

Mart

RE: "Link-X" suspension

GraviMan,

The lateral motion that you're talking about is not near as large as you're envisioning because the instant center of rotation moves up with the tire.  Load-based toe change in the case of my suspension means that upon an impact on the front of the tire, the bushings are designed to generate a tiny bit of toe-out so the tire steers its way over the face of the bump.  It works beautifully!

I would think that a lot of the lateral movement you're getting in the tall cab is from the roll of the vehicle coupled with the large distance between the roll axis and the position of the driver.

Link-X will reduce that distance and generate less overall roll, so lateral movement of the driver himself should be lower...I think.

RE: "Link-X" suspension

"The lateral motion that you're talking about is not near as large as you're envisioning because the instant center of rotation moves up with the tire..."

But roll centre is still above ground height!?!
I certainly agree with the idea of load based toe change. I'm trying something similar (albeit less sophisticated) with the truck system. This helps the truck to rotate about it's mass centre, rather than roll centre.

Trouble with bump steer avoidance is that it really needs to be linked to rate of change of wheel height. Load based toe change would approximate this due to the damper load, and I can certainly see how this would work with smaller bumps. When the bump contacts nearer tyre front, I can't see how steering would help. An example of this is curb strike - steering just ain't gonna help reduce the contact patch scrub.

"I would think that a lot of the lateral movement ... large distance between the roll axis and the position of the driver."

Yup!

"Link-X will reduce that distance and generate less overall roll, so lateral movement of the driver himself should be lower...I think."

Well, yes and no. Truck will need lower roll stiffness, true. Unless bump steer works perfectly, large lateral movement with high roll centre will cause high lateral accelerations. I can see how this sytem would cope very well with less rough ground, but in the extremes that out trucks have to deal with...

The final concern is cost. Don't forget the hidden costs of reliability, something I have to think about every day. There are hydraulic systems for active spring adjustment, which become more practical every day. I have read the paper sent to me, but it dealt more with the theory than application. Engineers love pictures of oily bits...

Mart

RE: "Link-X" suspension

One last thought I forgot to mention: Jacking. Examples of this include: Triumph Herald; Volkswagen Beetle (earlier design, i think); Chevy Impala; and indeed any swing axle type of system. Basically car continues to roll, after a critical angle is reached (dynamically unstable). The net result is the the vehicle eventually stops rolling, once the roof has intervened.

I can understand the need to keep the driveline length constant, but having tyre roll centre outboard of centreline is asking for trouble. As a fan of the "handbrake technique", I would be worried about sudden and unforeseen roll above a certain cornering g. This is why I prefer, at least, the idea of a floating subframe (keeps contact patch distances across car the same).

It's good to think outside the box, as long as you see the big picture! Believe me I'm the first to get overenthusiastic.

Mart

RE: "Link-X" suspension

GraviMan

I think the Chevy you mean is the Corvair.

You forgot early Porsche and some old Mercedes Benz and Auto Union

Regards
pat   pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: "Link-X" suspension

Ahhh the dreaded swing arm!  Link-X has a high roll center like a swing arm, but the similarities end there.  

We have less camber gain, ability to build-in a

The most important difference is that the Link-X roll center doesn't migrate.  A swing arm's roll center moves up as the wheels droop - that is unstable.

RE: "Link-X" suspension

OK. Good points well made. I have then, only two concerns:

1. Ride comfort (lateral acceleration) in extreme off-road conditions - bearing in mind my point about bumps contacting near tyre front.

2. Structural stability. The need to have overlapping links reduces the ability to reinforce against lateral loads - bearing in mind we see at least 20 tonnes per axle.
This usually ends up requiring, at least, expensive (in small volume) ADI castings.

Don't get me wrong, I really like your concept. I just remain to be convinced that it can handle extreme loads and terrain without becoming prohibitively expensive. Admitedly the size of the tyres we use will help to keep contact patch at centre. We have, at this stage, even had to reject hydraulic suspension on cost grounds - although the view is still that this offers the best dynamics...

Mart

RE: "Link-X" suspension

(OP)
Excuse me if this is obvious, but...

I'm curious on locating the rollcenter of the Link-X setup.  In a standard SLA setup the instant center is on the opposite side of the car centerline, so the 'instant center' to 'tire contact patch center' line crosses the car centerline.  Thus diagraming the instant rollcenter is easy.
http://www.miracerros.com/mustang/t_roll_center.jpg

On the Link-X, the instant center looks to be between the wheel and the frame, where the link bars cross.  Putting it on the same side of the car centerline as the wheel, thus the 'instant center' to 'tire contact patch center' line never crosses the car centerline...so how is the rollcenter height determined?

-Dave
http://www.moslerauto.com
"Everything should be designed as simple as possible, but not simplier"

RE: "Link-X" suspension

Dave,

The geometric roll center determined by the intersection of lines through the each suspension's contact patch center and IC. This suspension, just as a swing arm suspension, indeed has the IC on the same side of the centerline as the CP as you noted, but this does not change the method to determine the roll center. Lines are infinite, line segments have a finite length.

Best regards,

Matthew Ian Loew
"Luck is the residue of design."
Branch Rickey


Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: "Link-X" suspension

(OP)
Ah, gotcha...it should have clicked to extend the line back the other way.

Thanks!

-Dave
http://www.moslerauto.com
"Everything should be designed as simple as possible, but not simplier"

RE: "Link-X" suspension

Our third patent issued a month ago.  It applies to the concern GraviMan has about arm length.  Note that there are some 'trade secrets' that work in conjunction with the new method of Link-X.

We use arms that are similar in length to a traditional SLA, but still exist on the same planes as would be the case with a 'long arm' Link-X setup.  The IC is still in between the wheel and vehicle centerline so the rollcenter is still very high.  The knuckles are very short, so unsprung weight is reduced.

The roll center can be designed to be absolutely motionless and as an auxilliary benefit total camber gain is reduced and droop camber gain is considerably less.  High-droop, off-road setups are more reasonable in this configuration.

We have found that you still get about 85% of the roll reduction just from the high roll center without extending the arms all the way to get 'moment inversion' benefits.  Crossing the arms is still better, but considering the cost/weight/footprint savings...

This is the setup we have on the supercar, and it works spectacularly well.  The turn-in is almost magical.

t

RE: "Link-X" suspension

As you raise the front RCH, aren't you impacting the effectiveness of shock/spring/ARB tuning to the overall roll moment distribution.  Or are you pretty much accepting that change and using the rear only to balance the desired understeer.  How effective is body roll being damped?  Seems to me the front is now a go-kart (except over bumps) which is why the turn in is more responsive.

RE: "Link-X" suspension

After thinking a bit more, raising the front RC only reduces the sprung mass roll moment, which reduces total roll angle.  The turn in response may result from the front high roll axis causing the body to yaw into the corner when rolling, vs. conventionally it would yaw out of the turn (which I believe has been mentioned by someone else).

RE: "Link-X" suspension

The high front RC does contribute to the immediate turn-in.  The tires leaning into the corner slightly also helps (camber thrust into the corner vs out of the corner on other setups).

A high front RC will typically make the car feel like it's rolling onto it's rear.  We've compensated for that and achieve slight forward pitch with roll.  This has been proven to inspire the most confidence in drivers.

RE: "Link-X" suspension

maybe I am completely wrong, but isn't it also true that a swaybar controls wheelloads in corners?
As one wheel travels in more than the other, the wheelload is compensated by the anti swaybar.
I can imagine the x-link setup has no compensation for wheelloads when cornering.
Offcourse the vehicle stays horizontal with the x-link layout, but what happens to the wheelloads?
Don't you need a swaybar for this?

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