73 replies to this topic

[TOAD]

Hi Chaps, can I ask a question please.

I wonder if any one would have any thoughts on a problem I have. Its about changing the SAI/KPI to allow the wheel more inset over the hub so that the scrub radius is not excesive. The car will have a lower wishbone instead of the normal track control arm normally found on this car under a semi strut, full adjustment and design is open other wise.

Many Thanks in advance

Regards

Roger...................

[Greg Locock]

I wonder if any one would have any thoughts on a problem I have. Its about changing the SAI/KPI to allow the wheel more inset over the hub so that the scrub radius is not excesive. The car will have a lower wishbone instead of the normal track control arm normally found on this car under a semi strut, full adjustment and design is open other wise.

The only concret thought I have is that it sounds possible. What is your actual question? For that matter why do you want to reduce scrub? what do you use the car for? does it have PAS? what is a semi strut? Are you ging to modify the spndle?

[TOAD]

The only concret thought I have is that it sounds possible. What is your actual question? For that matter why do you want to reduce scrub? what do you use the car for? does it have PAS? what is a semi strut? Are you ging to modify the spndle?

Hi, first thanks for replying, the car is for circuit racing and the struts unbolt from the hubs. The car will have pas and and present I was not thinking of changing the stub axle/spindle other than for bearing size. The car would normally have 5 x 13 rims, because the new set up will be around 11 x 18 this may push the rims to far out thereby increasing steering kick back up to the steering wheel and so on. The car will be a cross between something called the Fiat SE031 (There was only One) and a Zakspeed Capri.

With the wheels pushed so far out does the question work now ? excessive scrub radius

Many Thanks

Roger..................

[Greg Locock]

I don't know whether you've got positive scrub or negative. Changing the wheel offset is the easiest way of adjusting the scrub, obviously, but you may run into problems with wheel strength, and the wheelbearings may complain as well.

So, what is the actual suspension architecture? It sounds like macpherson with a lower wishbone. If so I can't really see what you are asking about, yes, as you move the shock wrt the spindle the KPI and camber and scrub will all move in obvious ways.

[TOAD]

I don't know whether you've got positive scrub or negative. Changing the wheel offset is the easiest way of adjusting the scrub, obviously, but you may run into problems with wheel strength, and the wheelbearings may complain as well.

So, what is the actual suspension architecture? It sounds like macpherson with a lower wishbone. If so I can't really see what you are asking about, yes, as you move the shock wrt the spindle the KPI and camber and scrub will all move in obvious ways.

Hi Greg, this car will be a Grp5 type build for the track, with as you said a Macpherson strut and a lower wishbone. I think the car may have to much Pos, Scrub with an eleven or twelve inch wheel. The entire suspension and wheel set up is being made for this car as a one off and as such I am trying to get the angles just right so as not to wast any of the expense that's going into building it. I have just come to the conclusion that the changes will be a reduction in rid frequency because with the strut leaning over more the leverage will reduce the effectiveness of the spring/damper units that would be there, and all the joints and mounting will need to be stronger as well because of the extra leverage.

The wheel bearing is a separate issue but that can only get better as Pos Scrub is reduced closer to Neutral. The wheels are being made to fit so that shouldn't be a problem.

Many Thanks for inspiring the old brain here

Regards
Roger.........................

[cheapracer]

Don't go too close to neutral or strange things will start to happen especially under braking. If you have neutral and one side goes positive and the other side goes negative under braking forces then the car is then steering as both wheels are pointing the same direction .

Group 5 - you mean like in the 70's? What are you using it for classic Targa real road type events or just smooth race tracks - obviously the meaning is that you will notice a lot of scrub on bumpy stuff compared to smooth stuff.

[TOAD]

Don't go too close to neutral or strange things will start to happen especially under braking. If you have neutral and one side goes positive and the other side goes negative under braking forces then the car is then steering as both wheels are pointing the same direction .

Group 5 - you mean like in the 70's? What are you using it for classic Targa real road type events or just smooth race tracks - obviously the meaning is that you will notice a lot of scrub on bumpy stuff compared to smooth stuff.

Hi smooth tracks, I have to recreate the front end of a Zakspeed Capri on a Fiat 131, its all space framed, well it is as I work along it, and I'm just staring the front suspension top mounts, if you look at one of these Capri's you can see I have to work in a lower wishbone as the TCA is too short, and the Air jack tubes that hold the top mount up, and all this must miss the strut.

Do you have any recommendations on how near to Neutral might be considered getting close ? There will not be much roll, I'm looking at around 40mm bump and 75mm drop I don't expect the suspension to move around to much.

Thanks

Roger..............

P.s the car will look like a modern Fiat SE031

[britishtrident]

ISTR the Zakspeed Capris were double whisbone at the front --- I had some (minimal) contact with the one fitted with a Boss Mustang engine Douglas Niven used in special/supper saloon racing in the UK

[cheapracer]

Hi smooth tracks, I have to recreate the front end of a Zakspeed Capri on a Fiat 131, its all space framed, well it is as I work along it, and I'm just staring the front suspension top mounts, if you look at one of these Capri's you can see I have to work in a lower wishbone as the TCA is too short, and the Air jack tubes that hold the top mount up, and all this must miss the strut.

Do you have any recommendations on how near to Neutral might be considered getting close ? There will not be much roll, I'm looking at around 40mm bump and 75mm drop I don't expect the suspension to move around to much.

(At full bump)

No less than 10mm heim and ball joints or 20mm urethene bushed should be ok.

I wouldnt lose much sleep over the spring shocker leverage thing, the wheel is connected directly to the strut remember - I would be worrying about dramatic camber changes first - your going to have to get as long as possible control arm to control the camber and then probably the arm will need to be starting on its upwards/inwards arc (from center of car the control arm will be angling up at ride height). I hope you have a bump steer and ackerman answer then because you have created a pain in the ass.

and dont call me smooth tracks!

[meb58]

If his scrub is a hair negative couldn't scrub become very close to zero or zero as the tire deflects say in a turn? Not sure this is part of the concern, however.

Edited by meb58, 05 June 2009 - 19:03.

[TOAD]

ISTR the Zakspeed Capris were double whisbone at the front --- I had some (minimal) contact with the one fitted with a Boss Mustang engine Douglas Niven used in special/supper saloon racing in the UK

As all the pictures I can find show as below a strut and single wishbone, I must hope your friends car was ether the later wide wing car or, that it had been modified after being sold on him.
Thanks for giving me the heads up though, I will be careful on which one I'm looking at for now. I just thought, do you remember if the front airjacks only went part the way up inside the tubes ?

With thanks
Roger..................


This is from the web site
Chassis:
Space frame chassis from welded alloy piping. Quick lift jacks. Kevlar 49 plastic coachwork with skeletal Capri steel cockpit area. MacPherson/live axle suspension, alloy hubs, brakes as late works. Side rads and coolers, Garrett twin intercoolers at front. Deep front spoiler and separate rear wing.

[Greg Locock]

Scrub radius is not a helpful concept when cornering.

The reason I say that is that the sr is the moment arm of the effective point at which longitudinal forces are reacted at the contact patch, in front view. But, that location is highly unstable under combined slip conditions, and anyway the tierod forces will be dominated by the lateral force creating a torque around the steering axis via the mechanical and pneumatic trail.

Scrub is important, in my opinion, for two things, on a RWD.

(1) parking effort when stationary, brakes off
(2) large scrub radius increases kickback through teh steering when under braking due to unequal forces on the front wheels. I would have thought this might be a good thing, not a bad thing.
(3) it used to be that neg scrub was good for split mu braking, we don't care about that no mo'.

Edited by Greg Locock, 06 June 2009 - 00:39.

[Bill Sherwood]

FWIW on my old Corolla KE-55 rally car I used to run Toyota wheels on the front for road use (only car I had at the time) and for club work I'd fit the Nissan/Datsun wheels. The Datto wheels had far less offset and pretty close to zero or even negative scrub, and they made the car a far better thing to punt around - It'd stop far better and also felt like it turned better as well.
I'm a fan of zero or negative scrub.

[Greg Locock]

Citroen were big fans of centrepoint steering, so I am prepared to believe that there are advantages, as your experience suggests. Guessing that the difference between the two might have been 50 mm, that's almost within the tuning range we'd use.

But could it not have also been the narrower tires, etc?

[cheapracer]

No doubt it was Mc Struts....

http://www.geocities...bo/history.html

[Bill Sherwood]

Citroen were big fans of centrepoint steering, so I am prepared to believe that there are advantages, as your experience suggests. Guessing that the difference between the two might have been 50 mm, that's almost within the tuning range we'd use.

But could it not have also been the narrower tires, etc?

I don't think there was as much as 50mm between the two wheels - I can check if you really want me to go digging through the shed though. The funny thing is that the struts that both sets of wheels were used on were Toyota, and so you'd think they'd work best with Toyota offset. Turned out to be not the case.
From memory the rally tyres were 185/70-13's and the road tyres were 205/60-13's and the roadies were a little smaller diameter than the rally tyres. And as you mention above, this would accentuate the difference in scrub radii as well.

Psst .... this is Neil's car, driven by me at the Cootha Classic ->


Car needs a bit of work but it's sorta okay to punt around. Only had 10mm of droop on the rear end, I had to be super careful for it to not swap ends .... it's one of only two in the country.

Edited by Pascal, 06 January 2011 - 14:39.

[TOAD]

Scrub radius is not a helpful concept when cornering.

The reason I say that is that the sr is the moment arm of the effective point at which longitudinal forces are reacted at the contact patch, in front view. But, that location is highly unstable under combined slip conditions, and anyway the tierod forces will be dominated by the lateral force creating a torque around the steering axis via the mechanical and pneumatic trail.

Scrub is important, in my opinion, for two things, on a RWD.

(1) parking effort when stationary, brakes off
(2) large scrub radius increases kickback through teh steering when under braking due to unequal forces on the front wheels. I would have thought this might be a good thing, not a bad thing.
(3) it used to be that neg scrub was good for split mu braking, we don't care about that no mo'.

Hi Chaps, thanks for posting on this one,

Cheapracer; HaHa (smooth Tracks) The winsbones will be as Zakspeed and mounted inboard over the top of the chassis rail, so drop is critical so as not to bend the wishbones. Bump and camber yep, that will have to be a look see as I go, but with little suspension treavil I'm hopping that will not be to much of a problem. Ackerman, some people go ether way with that, so I'll have to see how it handles as built to see if there's a problem. Oh, and thanks for the web link.

Meb58; Thanks I'll have to give that some thought

Greg ; #2 is the one I was thinking of, steering snatch approaching 100% braking with too much Scrub.

Bill, Ya got that, I think it shows very clearly that my concerns with Scrub on

I get the impression that a small amount of scrub may be disirerable, I fact I just thought, the wheel manufacturers should have and idea about this because of setting wheel offset.
My car is not what you'd call light weight, but then the minimum allowed is 945Kg so there was no point, but it's Stiff and Strong for sure. Those touring cars seem very Delicate when getting there wings rubbed, Toad (My car) will not have the frailty.

Thanks chaps for your support

Roger...............

[Greg Locock]

Bill- we can't do anything much with the information anyway, so leave your old wheels undisturbed among the redbacks. Still, it does offer up an avenue that I might get around to exploring with some wheel spacers, we're always looking for improved precision in the steering feel.

[Bill Sherwood]

Bill- we can't do anything much with the information anyway, so leave your old wheels undisturbed among the redbacks. Still, it does offer up an avenue that I might get around to exploring with some wheel spacers, we're always looking for improved precision in the steering feel.

If it works, can I have a cheap F6, pleeeeeeeeeeeeeeeeeease?

[robroy]

we're always looking for improved precision in the steering feel.

Hi Greg, sorry if this disturbs you, but may I just take this moment to say God bless you and the other Ford suspension engineers out there. Thank you so much for making my cheap runabouts so much fun to drive! I really do rate Ford high in this department with their early Focii, early Mondeos, early Kas and Pumas.

Edited by robroy, 07 June 2009 - 11:28.

[cheapracer]

Hi Greg, sorry if this disturbs you, but may I just take this moment to say God bless you and the other Ford suspension engineers out there. Thank you so much for making my cheap runabouts so much fun to drive! I really do rate Ford high in this department with their early Focii, early Mondeos, early Kas and Pumas.

Things really are bad when Henry 111 has to resort to this.

[gordmac]

Ground offset seems to affect more than steering feel from the brake force moment. I modelled a generic wishbone system on wingeo and varied, as nearly as I could, one thing at a time then added steer angle to see what happened (for education purposes). It was a while ago but from what I remember according to the results changing the ground offset would affect pretty much everything else including dynamic toe and dynamic corner weights.
Many years ago I tried wheels with "normal" offset and another set with a lot of offset on a race saloon, despite the narrower track the car felt better and was quicker with the smaller offset.

[cheapracer]

Ground offset

Huh? You mean the distance from the center of the wheel on the ground to where the steering axis intersects the ground?

If so thats scrub radius.

[meb58]

I had a discussion a year or so ago with a fellow racing a gravity racer - down hill no power. He was absolutely shocked by the differences betwee pos. neg and zero scrub...zero allowed a faster car. I am assuming that all else was equal... This was a casual discussion so I have no idea what the actual performance differences were. But this struck me from one power driven perspective...fuel mileage.

[gordmac]

I do mean centre of the wheel to the steering axis intersect, if you wish to call it scrub radius that is fine with me! I have seen both terms used in various places. Technically the important distance is actually the moment arm which is at right angles to the steering axis rather than parallel with the ground. And the centre of force of the contact patch is more significant than the wheel centre although I don't know how to accurately find that.

Interesting comment about the downhill racer, any idea if the difference in drag was all the time or when steered? How rigid are the uprights, steering etc?

[meb58]

I'll see if I can dig up the discussion...from memory it was in stright line but let me confirm.

EDI: stright line, but the car had to be steered for road camber...and it wasn't zero scrub but very close. Perhaps the benefit is lost a wee bit on powered cars. I found this interesting but don't know what to do with it.

Edited by meb58, 09 June 2009 - 17:04.

[TOAD]

Hi Chaps thanks for every's ones input this is what I'm thinking. ( THESE ARE ONLY MY THOUGHTS )don't know if it correct so don't kill me.

The conclusion I am coming to concerning SAI and Scrub amount to this, as far as scrub goes a little bit of Negative will add some steering feel, with out getting excessive kick back. The steering kick back was my starting concern because of braking on the limit, if one wheel brakes loose, the sudden loss of grip on that side, would cause the car to snatch over in the opposite direction. This side way’s yaw would destabilise the car when you need stability the most.
By increasing SAI you would load up the suspension more, changing the Suspension rates in the process, but I feel this is a lesser evil compared with running to much Scrub, this being caused be the increase in rim width. The suspension rates can then be re-adjusted with testing to compensate for the revised suspension loadings.
Ackerman will be increased with the re-positioning of the front top mounts forward of there original position. This being done to give more wheel space and some sort of reasonable steering lock. My understanding of Ackerman is that more is better for tight corners, because with increased slip angles on the inner tyre due to weight transfer, the tyre needs as much help as it can get. Oppositely, for long fast corners, that use less steering angle, Ackerman is not needed as much because the wheels are not turned through such an acute angle that induce large slip angles. Further more, with chassis sitting more balanced on both front wheels, more of the steering load is shared with the inside tyre on these fast turns. I think the reason for this is that on a long bend, body roll builds with increased G, but the dampers are still able to keep control of the suspension movement. On the other hand, in a tight hairpin bend, suspension movement is faster and more dynamic, allowing the inside tyres to momentarily unload some of there down force, and therefore grip on the track. To compensate for this the inside tyre is turned in further to get the last available grip. (Toe out on turn)

• Static Toe out, may give to much Toe out when turning with the Ackerman addition if set to much in the first place. Caution needed here.

• Set the least static Camber for maximum grip on roll in long corners, and keep this to a minimum for best braking grip.

• Adjust Caster to increase Camber in the tight corners to compensate for a lower static Camber angle as set above.


If any one has a thought or correction for me to think about please let me know (I expect I have it all wrong anyway but there you go got to start some where ).

Many Thanks to you all

Roger...............................

[cheapracer]

• Static Toe out, may give to much Toe out when turning with the Ackerman addition if set to much in the first place. Caution needed here.

• Set the least static Camber for maximum grip on roll in long corners, and keep this to a minimum for best braking grip.

• Adjust Caster to increase Camber in the tight corners to compensate for a lower static Camber angle as set above.

Well thats a KISS plan right there.

I tell you one of my secrets (ok the rest of you stop reading right now - well go on, move on nothing to see here - ok, your not taking the hint so - nick off! - now that ought to do it).

You can seperate the top pivot from the KPI point so as to allow camber gain but not be punished with extra KPI. Keeps the steering a bit lighter and the inside tyre a bit flatter on lock because you don't need excessive caster to compensate to put it simply.

[TOAD]

Well thats a KISS plan right there.

I tell you one of my secrets (ok the rest of you stop reading right now - well go on, move on nothing to see here - ok, your not taking the hint so - nick off! - now that ought to do it).

You can seperate the top pivot from the KPI point so as to allow camber gain but not be punished with extra KPI. Keeps the steering a bit lighter and the inside tyre a bit flatter on lock because you don't need excessive caster to compensate to put it simply.

well Ok, could you put that in even simpler please. ( Seperate top pivot from the KPI ) ? Sorry

and ( don't need excessive caster to compensate )


( to put it simply ) Ok so I'm thick

Thanks for helping though

Roger...................

[cheapracer]

well Ok, could you put that in even simpler please. ( Seperate top pivot from the KPI ) ? Sorry

and ( don't need excessive caster to compensate )


( to put it simply ) Ok so I'm thick

Thanks for helping though

Roger...................

Well every Mc strut you see combines the camber pivot and steering pivot together - commonly a big rubber bush actually or spherical bush on some race cars.

You can make an offset bush that seperates the camber pivot from the steering axis pivot.

Some warnings though, if you use this system you must be aware that the setup puts a bending stress on the strut shaft the further you seperate the points (see the arrow) however it does gain a slight amount extra travel also depending on the separation distance.

Here is my 3D CAD modelled drawing to help..

Edited by cheapracer, 05 July 2009 - 09:15.

[TOAD]

Well every Mc strut you see combines the camber pivot and steering pivot together - commonly a big rubber bush actually or spherical bush on some race cars.

You can make an offset bush that seperates the camber pivot from the steering axis pivot.

Some warnings though, if you use this system you must be aware that the setup puts a bending stress on the strut shaft the further you seperate the points (see the arrow) however it does gain a slight amount extra travel also depending on the separation distance.

Here is my 3D CAD modelled drawing to help..

Thanks for that, its an interesting set up to consider, If I have this correctly, and just as a check on my understanding. Would this mean the Caster would change with steering? Because the top mount is moving in an ark forwards and backwards as its turned. ?

One other thing, I was going to transer a picture I have of the Zakspeed Capri (straight on the front), over to my CAD program. I thought I could get it to put an angle on the strut to see how much differant it is from standard.


Roger...............

[cheapracer]

Would this mean the Caster would change with steering? Because the top mount is moving in an ark forwards and backwards as its turned. ?

standard.


Roger...............

You would be wrong, no caster change with steering, the steering pivot doesn't change position.

The camber pivot inner bush is of single plane rotation only but it must be a neoprene etc bush with give because as the strut goes up and down there is some twist if you have any caster or anti dive etc. and the caster gains or loses through the travel.

You can not use a heim or bearing there because of the twist - a heim will allow back and forward uncontrolled movement and a bearing will not allow the small amount unavoidable of twist.

Your quite lucky, I found an old picture that has the setup.

[TOAD]

You would be wrong, no caster change with steering, the steering pivot doesn't change position.

The camber pivot inner bush is of single plane rotation only but it must be a neoprene etc bush with give because as the strut goes up and down there is some twist if you have any caster or anti dive etc. and the caster gains or loses through the travel.

You can not use a heim or bearing there because of the twist - a heim will allow back and forward uncontrolled movement and a bearing will not allow the small amount unavoidable of twist.

Your quite lucky, I found an old picture that has the setup.

I see what your getting at now, and the No Caster bit. It's definitely interesting though.

Thanks

Roger.....................

[cheapracer]

I see what your getting at now, and the No Caster bit. It's definitely interesting though.

Thanks

Roger.....................

Oh and you can have a slightly lower bonnet height or more suspension travel as the mount is too the side not on top.

That one in the picture has 30mm offset, not a lot but the strut shaft wasn't all that hefty either, 18mm from memory.

[TOAD]

Thanks, I haven't been able to stop for Toad just yet, Our Thema has developed and air leak that I've been trying to trace.
God help us when Toads engine go's back in, that was tight with all its plumbing.

Roger............

[Greg Locock]

Cheapy's just reinvented the double wishbone, quick, patent it!

[gruntguru]

Cheapy's just reinvented the double wishbone, quick, patent it!

Look closer - the top wisbone connects to the upright via a sliding joint (the shock rod) not a spherical or knucle joint.

[Greg Locock]

Look closer - the top wisbone connects to the upright via a sliding joint (the shock rod) not a spherical or knucle joint.

Now draw the velocity diagram.

[gruntguru]

Now draw the velocity diagram.

Sure, at any a given instant it behaves as a double wishbone but so does a Mac strut. You can't build a double wishbone that has the same geometry through the entire travel. (I think??)

Edited by gruntguru, 06 July 2009 - 06:42.

[Greg Locock]

Sure, at any a given instant it behaves as a double wishbone but so does a Mac strut. You can't build a double wishbone that has the same geometry through the entire travel. (I think??)

and how much travel will it have, given the constraints?

[gruntguru]

and how much travel will it have, given the constraints?

Slightly more than the original Mac strut if the pivot is on the engine side of the rod - slightly less if its on the wheel side.

Hey is this some kind of test?

Edited by gruntguru, 06 July 2009 - 11:13.

[Greg Locock]

Slightly more than the original Mac strut if the pivot is on the engine side of the rod - slightly less if its on the wheel side.

Hey is this some kind of test?

No, it's a learning.

Now combine your two most recent observations - the velocity diagram is the same as a double wishbone, for the drawn configuration, and the suspension travel is virtually the same as the original MacP. From the latter it is obvious that the upper arm doesn't move much (it can't, it is overconstrained).

Therefore, for the range of movement that is available, the velocity diagram is the same as for a double wishbone, hence my original comment that you decided to disagree with.

Edited by Greg Locock, 06 July 2009 - 23:29.

[gruntguru]

Fail

Whoops.
You'd better explain that one to me.

Edited by gruntguru, 06 July 2009 - 23:28.

[gruntguru]

No, it's a learning.

Now combine your two most recent observations - the velocity diagram is the same as a double wishbone, for the drawn configuration, and the suspension travel is virtually the same as the original MacP. From the latter it is obvious that the upper arm doesn't move much (it can't, it is overconstrained).

Therefore, for the range of movement that is available, the velocity diagram is the same as for a double wishbone, hence my original comment that you decided to disagree with.

Shouldn't you say "for the range of movement that is available, the velocity diagram is the same as for a Mac Strut"? To replicate a Mac strut or Cheapies design in double wishbone would require a top wishbone of infinite (or almost infinite) length.

Edited by gruntguru, 06 July 2009 - 23:57.

[cheapracer]

Cheapy's just reinvented the double wishbone, quick, patent it!

Ummm? to be double wishbone I think you need errrr, 2 wishbones?

Where the strut goes into that block of steel it does so via a thread (standard top of the shaft thread) with a locknut on top, there is no joint there - the block is a solid part of/extension of the shaft.

It just sidesteps the pivot point a bit and maintains the KPI while increasing camber gain.

In this excellent and time consuming Professional CAD drawing where the strut shaft goes through (I should say threads into) the red thingy just below the arrow, it is a 100% solid connection.....



By the way, this little trinket has nothing to do with a previously mentioned suspension invention. I'll speak to you about that before the years out Greg.

Edited by cheapracer, 07 July 2009 - 13:45.

[Greg Locock]

Ummm? to be double wishbone I think you need errrr, 2 wishbones?

Where the strut goes into that block of steel it does so via a thread (standard top of the shaft thread) with a locknut on top, there is no joint there - the block is a solid part of/extension of the shaft.

It just sidesteps the pivot point a bit and maintains the KPI while increasing camber gain.

In this excellent and time consuming Professional CAD drawing where the strut shaft goes through (I should say threads into) the red thingy just below the arrow, it is a 100% solid connection.....



By the way, this little trinket has nothing to do with a previously mentioned suspension invention. I'll speak to you about that before the years out Greg.

gruntguru is correct, the kinematiics are more like a MacP than an SLA, but for the very limited range around the as-drawn position the kinematics is also very similar to a double wishbone. As a mechanism the top link is redundant, it is over constrained and can only move at all because the joints etc are compliant. In practice it'll put lots of sideforce into the strut.

[gruntguru]

gruntguru is correct, the kinematiics are more like a MacP than an SLA, but for the very limited range around the as-drawn position the kinematics is also very similar to a double wishbone. As a mechanism the top link is redundant, it is over constrained and can only move at all because the joints etc are compliant. In practice it'll put lots of sideforce into the strut.

The overconstraint wouldn't be a problem as long as the axis of the knucle joint is parallel to the lower "wishbone" axis of rotation. Only a tiny bit of compliance required. It won't add much sideforce to the strut (and they have quite a lot already) but does apply a fair bit of bending moment to the top of the rod during damped motion.

Edited by gruntguru, 08 July 2009 - 01:28.

[cheapracer]

but does apply... bending moment to the top of the rod during damped motion.

..relative to the distance the shaft is from the pivot, I've never used more than about 30mm through fear of breaking a shaft at that point, unlikely, just paranoid.

I actually originally did it many years ago to get a little extra travel with a longer strut in a rally car and the benefit of a little more camber gain was just a bonus.

[meb58]

In carting we sometimes change front axle material/diameter as a way of helping introduce some neg camber thru compliance. The red component in your diagram could be made to be flexible enough to help remove some fear of breaking the shaft? Or am I seeing this all wrong?

I think most Mac Struts have flexible upper mounts that more or less deflect to the forces acting upon them...

..relative to the distance the shaft is from the pivot, I've never used more than about 30mm through fear of breaking a shaft at that point, unlikely, just paranoid.

I actually originally did it many years ago to get a little extra travel with a longer strut in a rally car and the benefit of a little more camber gain was just a bonus.

Edited by meb58, 08 July 2009 - 13:30.

[cheapracer]

In carting we sometimes change front axle material/diameter as a way of helping introduce some neg camber thru compliance. The red component in your diagram could be made to be flexible enough to help remove some fear of breaking the shaft? Or am I seeing this all wrong?

The Fulcrum Uberous Camber Kinetic Energy Device could be flexible but then you lose the purpose because then the pivot length is shortened to the flex center point.

Edited by cheapracer, 08 July 2009 - 15:34.