[71roadster]

I built a five bar rear suspension similar to the C4 model. I used the longest toe control rods I could fit and set them for the minimal amount of toe-in on compression as possible. By measurement I get about 0.8 degrees of toe in from a normal resting position to full compression. which on 25 in diameter tire gives about 0.175" of toe change from ride height to full compression.

Unfortunately this seems to be far too much. When I hit a bump, the resultant toe steer is so strong it really pushes the rear of the car around. A bump on the right side pushes the rear left and vice versa. It is scary actually and I can't leave it like this. There is limited room to decrease the toe change a little more, but everything I have read, (and that is a lot - so much for self study!) says toe-in on compression is a desirable thing. And I think if I set the toe control rod closer to parallel at rest, I risk going to toe-in at partial compression to toe-out at full compression. So I really don't know where to go at this point and I don't really like driving it the way it is.

Here is a shot of mine.

Here are shots of a couple of others that really take the toe change to opposite ends of the extremes.

This one would have a LOT of toe-in on compression

And this one would have toe-out on compression

Maybe somebody here knows how much toe change a stock C4 suspension produces? I checked the C4 forums and there does not seem to be too much hard technical discussion over there. Any ideas?

 

[TwinTurbo]

See this is why I had the toe control rods set up to be perfect in relation to the halfshaft length and pivots, making sure it's all in a single plane. That way you can achieve 0 toe change. An easier way to do it is to do what riley&scott and guldstrand did, use 2 lower stuts and set the toe by changing the length of the front vs. the rear. Both rods will be in perfect geometry giving you 0 toe change also.

To see how to do it w/ the rear rods check out how greenwood did it, they had it perfect (I had mine very similar)

You can also minimize your problem significantly by moving the outboard mount up. It does matter if it's further back (from top view pointing to the rear) because you want to retain a parallelogramme here also. It's important that your rod's outer pivot is inline with the stub axle and the virtual swing arm center that the 2 forward rods make (think of it as a long virtual swing arm), you can not get this perfect for the entire travel is the 2 forward links are not symmetrical placed around the center horizontal plane of the stub axis (similar to an intersecting plane though the length of the stoick trailing arm) as the swing arm center will move avout a bit. In that case the best compromise would most likely be the center of the trailing arm (assuming you started w/ chopped off arms) or if you fabricated from scratch in the center plane of the bearing. Yours is mounted under that so the rod is not parallel with the halfshaft, this way the geometry relation between the 2 is dynamic over the entire susp. travel.

That German car has it wrong also (they even have trailing arms there visible by the u channels welded on them where the coil overs mount), they most likely have a floating front bushing. That last pic has it dead on! The pic is a little deceptive as it's taken from a low angle but if you would look at that one from the rear the toe control rods would be in the same plane as the halfshafts at the same angle.

Mounting the rods in the center is a big compromise as the center molunt is not directly inline w/ the stub axle u joint during the entire susp. travel. The C4 uses this as a compromise and it's cheaper (less exact placement needed).

If you want I can post pics later to explain it a little better.

This is the 5 bar I built

It's missing the toe plate and rods in those pics, I don't even have pics of it (and I scrapped the whole thing after I had it finished)

Here are some pics of the C4 susp (I dropped that also, figured I could improve it w/ my own design)

 

[71roadster]

You can also minimize your problem significantly by moving the outboard mount up.

If I did that the toe control rod would go past the horizontal and start toe-ing out the wheel before it hit full compression, just as I think the suspension in that last photo I posted will do. Ideally wouldnt you want the longest toe control rod you can have (to create the least amount of toe change) and you want it to come to the horizontal position at full compression? Toe-in on compression is actually preferable to no toe change at all. Any formula race car or the C5/6 suspensions are designed for it. If I take that as a truth, all I could do is raise the inner and outer points and there is only about 1" of space to do that before the toe rod would start hitting the crossmember on full compression.

A stock C4 at ride height (or maybe even lowered) probably has half shafts pointing down and the toe control rods are parallel. (wish I could find good pictures...anyone???) The problem for me is that my half shafts point up somewhat and if the toe control rods were parallel I would be back in the toe-out on compression configuration. In order to get them parallel without jacking the car way up, I would have to raise the pumpkin into the crossmember by at least 2 inches. Modding the crossmember is easy enough, but the front mount would also need to be modded, and that is getting more complicated and changes this from being able to go back to stock easily.

That last pic has it dead on! The pic is a little deceptive as it's taken from a low angle but if you would look at that one from the reqar the toe control rods would be in the same plane as the halfshafts at the same angle.

Why would this configuration not cause the tires to toe-out on compression?

 

[TwinTurbo]

the angle of the rod vs. horizontal has nothing to do with the toe control, the angle in relationship to the halfshaft is what controls your toe now. You have 2 forward links like the c4 right? They form a virual swing arm that controls the wheel position in the wheel well and the virtual swing arm/anti dive/anti squat characteristics. Not the camber and not the toe. The lower strut in relationship to the halfshaft controls your camber and the toe control rod in relationship to the halfhsaft controls your toe. If your halfshaft and toe control rod remain parallel all the time, meaning that the 3d positioning and the distance between the halfshaft and toe control rod remains the same then you have a parallelogramme and NO toe change. With your setup this can't be done, as the system w/ the longer rods relies on the longest possible rod for reduced angular effect on total effective rod length (horizontal length for toe control), it is a comprimise but a workable one (since the C4 also has it) you CAN get the outboard ones spot on though. You need to position that pivot exactly behind and in line with the u joint. This means moving it inboard and up. Up so it's halfway up the rear of the trailing arm or whatever you're using for a hub, this centers it in the y joint/spindle plane. And inboard to move it directly behind the u joint, meaning it's as far inboard in relationship to the bearing hub as the u joint is. This is the only way to assure you have a proper geometry.

The stock C4 system is marginal at best, it's a half assed design and the old versions have a too high roll center inducing jacking during cornering. You can do a much better job by carefully positioning the components.

The easiest way is still the guldstrand/riley&scott method since the 2 camber struts are always in perfect geometry and you can adjust the toe by adjusting one relative to the other. You have built a C4/greenwood type setup (I had that also but I didn't use the long rods/angular effect system I used rods that were exactly the same length as the halfshafts and the geometry was in relation to the halfshaft craeting a perfect parallelogramme that retained perfect toe control, this must be done because from a side view the wheel doesn't only move up and down, it makes an arch over the virtual swing arm pivot, or if you have a non stereoscopic forward rod setup, the virtual arm pivot travel path)

Check out this picture by greenwood and note where he placed the pivots, especially on the cut off trailing arm. The rear mounts are much wider than yours, just like I had it, this is to be in compliance w/ the inner u joints.

See that the toe control links and halfshafts are parallel and same length AND pivoting in the same planes. This last thing you can't do unless you change the center mount but you CAN adjust for parallel rods/halfshafts and get the outer mount right. Forget what the C4 looks like, the general copied what these bright racers already did with their older rear ends, they modified them to get rid of the horrible toe change over suspension travel and resulting torque steer.

This may give a good idea of the center plate, this was how I had it set up, the holes are for lightening, the outer most holes are where the toe control rods attached. The spacing is the same as the u joints in the diff, they are horizontally direct in line w/ the u joints.

 

[TwinTurbo]

Why would this configuration not cause the tires to toe-out on compression?

Because the whole suspension movement has no influence on the toe control, toe is now determined by the halfhsaft and toe control rods, those 2 angle the bearing block in or out for toe and if these are in perfect parallelism they will be a parallellogramme (sp?? damn what a word) and this means that effective toe does not change, sure the angles of these components will change but not relative to each other. THAT is the whole idea behind the 5 bar system where you use a set of sectioned forward rods to create a virtual swing arm but NOT to rigidly mount the bearing block to teh front with a fixed toe setting that changes (like a trailing arm) due to effective horizontal length of 2 components (the halfshaft and camber rod) moving the wheel in and outboard resulting in a toe change. Anotehr benefit of the 5 bar is that you have a virtual swing arm, not a true one like a trailing arm. This allows you to tinker with the anti dive/squat settings and virtual arm pivot height to tune the suspension. THIS is the reason why a 4 link setup for solid axles is so popular, it offers that same ability.

 

[yellow73sb]

Can I achieve the same results with a johnny joint in the front instead of the two rods and heims?

We have a C4 so I can get some pics for you. Not very good though as the exhaust covers a lot up.

 

[71roadster]

Because the whole suspension movement has no influence on the toe control, toe is now determined by the halfhsaft and toe control rods, those 2 angle the bearing block in or out for toe and if these are in perfect parallelism they will be a parallellogramme (sp?? damn what a word) and this means that effective toe does not change, sure the angles of these components will change but not relative to each other. THAT is the whole idea behind the 5 bar system where you use a set of sectioned forward rods to create a virtual swing arm but NOT to rigidly mount the bearing block to teh front with a fixed toe setting that changes (like a trailing arm) due to effective horizontal length of 2 components (the halfshaft and camber rod) moving the wheel in and outboard resulting in a toe change.

I think I had a bit of an epiphany while considering my response. I was assigning ALL of the toe control exclusively to the toe-control rod. I agree completely that when the rod is a perfect parallelogram with the halfshaft and the same length there would be zero toe change, but since toe-in is desirable, the toe control rod should be designed to force the toe-in. I think the problem in my setup is that while the toe-control rod swings up moving the back of the carrier outward, the halfshaft is pulling the front of the carrier inward, causing more toe than is desirable. Still not sure how much is perfect, but clearly more than I am getting. And I was stuck thinking that if the toe control rod were pointing upward, it would be pulling the rear of the carrier in as it swung even higher on compression. What I hadn't considered is that because it is longer than the halfshaft, the halfshaft would be pulling the front of the carrier in faster than the toe control rod, thereby creating toe-in on compression.

Then the trick is to get them closer to parallel. The problem is this...

That is the trailing arm at full compression. There is only a fingers width until it is hitting the crossmember mount. I cannot move it up any higher for fear of interference in a hard bump (haven't seen any to date, so I think the spacing is good) and taking the solid mount out would only buy about .25" - not enough since I need to relocate about 2.5". So that means I am forced to move the inner mounts lower. I think this might work and I can build with a piece of angle iron slipped between the rubber pad of the leaf spring (but would that cause my pumpkin cover to snap somehow - it is the heavy duty model...).

I can test the toe control pretty easy prior to full mod too...

 

[redvetracr]

not passing judgement on what anyone here has done but (I believe) this is the guy that designed the 5-link Guldstrand sells...Bob Riley a very smart cookie!
...redvetracr

http://www.rileytech.com/

 

[TwinTurbo]

but since toe-in is desirable

Static toe in is a setting, during suspension movement you want the toe to remain at the setting that it's at and NOT change, this gives you rear oversteering issues in case the suspension toes out. The stock suspension does that, if you set the susp. w/ level hafshafts the toe is maxed out, any pos higher or lower in the travel and the toe will turn more positibe.

You do NOT want the toe to change period, this is the same deal as eliminating bump steer for the front. Maintaining proper toe settings is what it's about.

With the suspensions et up perfectly using the parallellogramme method you can set an initial toe to what you want and it will stay that throughout susp. travel

As for your pics, 1st of why is that suspension so low? If the rod is interfering why not move the whole deal backwards a bit. You will gain a much preciser toe adjsutment also by moving the rod away from the spindle center. There's room jsut after the crossmember, just be sure to make sure it's all aligned parallel w/ the halfshafts from the top (or bottom) also, meaning you have to move the centermount backwards also.

That last pic w/ the lines drawn in won't work either, it's grossly misaligned w/ the halfshafts.

How did you come up with your design? I did quite a bit of thinking and crunching on this stuff, I modelled it all in solidworks and observed the results throughout the whole movement. It's all simple geometry and you can't bend the rules there.

Check out the greenwood pics, especially the one w/ the alu batwing. You can see behind the openings in the batwing where the inner mounts of the tie rods go, then check out my CAD drawing. Same deal there, getting this stuff in the proper spot is key. If you stick with the center mount rods, using as long as possible rods/reduced angular effect on horizontal component then you will always have some toe change but for a limited susp. travel it's not a bad compromise.

Yellow, with just a johhn y joint all you do is allow the trailing arm to move more freely over the arch that it makes. You CAN achieve a similar result though by doing what I have proposed many times. Float the front bushing and use a rod like that to control toe. It won't be as perfect as you can do w/ a corectly built 5 bar but it'll be damned close and much better than stock. (I modeled this also), you can also do it w/ dual lower struts like guldstrand did. The only drawback to the dual struts is that as the trailing arm angles the toe control rods and the halfshaft go out of perfect geometry, the 2 won't be perfectly aligned but it's a lot better than the crappy stock deal and with a reduced susp. travel it's almost a non existent problem/

I only have a pic of the floater setup w/ the 2 lower rods. I cat find my other model. You can see the slight geometry issue here. Notice the 2 struts not being perfectly parallel. The rear toe rod won't have this problem because it's outer pivot will always be in alignment w/ the trailing arm pivot. For the 5 bar w/ dual sectioned forward links the 2 lower rods is an easier way to build it with exactly the same results in toe control. For the floating front the rear control rod is a better deal.

another view to see parallelism of rods (and johnny joint spherical end in railing arm)

I'll see if I can find my old models for the 5 link.

Like Howard says, Riley is a smart cookie indeed. He did what greenwood did but significantly simplified the build. Instead of perfectly positioning the rear rods (and not hacking it up and taking the easy way out like GM did by using a center mount and long rods to keep is almost accurate throughout the susp travel) he made the design simpler w/ 2 rods that set toe w/ their relative settings and he KNEW that those 2 struts would be easy to do, just use a longer bolt on both ends to give them the same axis and you're golden. Making a system that works as good if not better, is simpler to do and also fits better in a tight space (like you see from your crossmember issue), me thinks the guy was a genius! Too bad GM had to take the cheap way out and relied on a hacked up version of the greenwood system but were not preparedd to properly do the toe control setup (I'm sure the engineers would have... must have been those damned bean counters again) They did learn from their msitakes though when they gave the C5 (and later 6) double a arms. Double a arms offer a FIXED toe setting over the entire range. This is what you are aiming for.

 

[71roadster]

Static toe in is a setting, during suspension movement you want the toe to remain at the setting that it's at and NOT change, this gives you rear oversteering issues in case the suspension toes out. The stock suspension does that, if you set the susp. w/ level hafshafts the toe is maxed out, any pos higher or lower in the travel and the toe will turn more positibe.

I get that the stock suspension toes out. old news. But I have to disagree with your contention that zero toe steer is most desirable. Marginal toe-in helps correct for oversteer, especially power induced. My goal would be zero or marginal toe in. Look at almost any purpose designed racing car. They use the longest toe control rods they can design in so as to achieve some toe-in on compression. This is also one of the advantages of a solid axle design, the compressed side moves forward creating toe-in. Additionally, when you brake in a corner there is going to be toe out due to slop in the suspension from bushings compressing, bearing play, tire scrub, etc. There is no practical way to absolutely eliminate this. A little toe-in will help compensate for the slop.

As for your pics, 1st of why is that suspension so low? If the rod is interfering why not move the whole deal backwards a bit. You will gain a much preciser toe adjsutment also by moving the rod away from the spindle center. There's room jsut after the crossmember, just be sure to make sure it's all aligned parallel w/ the halfshafts from the top (or bottom) also, meaning you have to move the centermount backwards also.

I am running 17 inch wheels with 255/45 tires with an approximately 25 inch radius. The suspension is lowered so that the fender lip is at 26.75 inches. Also, lowering the center of gravity is the single most effective of improving handling. Moving the toe rod back means adding about 2 inches to the trailing arm. Haven't seen that on any arrangement. Not saying it is not something I would not consider, but at this point I do not yet see a pressing need. The hiem joint is at the same position rearward as the greenwood design.

That last pic w/ the lines drawn in won't work either, it's grossly misaligned w/ the halfshafts.

This sketch is intended to show the toe control rods in alignment with the half shafts, just lower. I could also use the same concept to move the inner mounts outward if need be.

Check out the greenwood pics, especially the one w/ the alu batwing. You can see behind the openings in the batwing where the inner mounts of the tie rods go, then check out my CAD drawing.

yes, but his outer mounts are several inches outward from being in alignment with the outer U joints. Hard to tell from the photo, and on your cad drawing, would it be the long upper tabs or the short lower tabs to which the rod goes to?

Thanks for your help by the way. Just the open discussion helps build the pictures in my head to gel into actions.

 

[TwinTurbo]

marginal toe in does help w/ oversteer, but you can just set a marginal static toe in which will then be maintained throughout suspension travel. There's no need for a toe curve like there is for a camber curve because of body roll. You are comparing this to a solid axle, which is a much less rigid mounting option than a proper designed 5 bar or double a arm suspension (assuming you're using solid bushings and all) The flexing in a solid axle is what causes the brake toe out and the torque to in condition. This does not mean you can apply that to an independant suspension as something that needs to occur. Having said that, not even all live axles give toe in under power, some even toe out, especially those w/ weaker cases. Porsches had a similar problem w/ their semi trailing arms. They had terrible torque steer, something not very common in rwd cars but the semi trailing arm setup is notorious for this because of the arms being mounted on a diagonal axis towards the car centerline. This causes toe in under acceleration and toe out under deceleration 9letting off the gas or braking) because of the loading in the outer bushing and thus allowing deflection. Their solution was the weissach axle, basically splitting the trailing arm w/ a bushing in the front so that under deceleration the rear wheel toes in (especially important on the loaded outside wheel). These are all attempts to create a toe in situation to give understeer as understeer is much easier to control than oversteer.

What i want to illustrate is that you can just set a static toe in and have a suspension that does n ot change the toe so you wil never run into any toe control issues, especially toe out oversteer. However too much toe in is also not desireable. If you have a semi trailing arm suspension that does not use a sectioned arm like the weissach (and a lot of cars use the semi trailing arm) you will see a lot of static toe in being set just to counteract any toe change towards 0 or even out. This is not because more toe in is better but because they want to improve upon the shortcomings of the IRS design.

I agree on lowering the CG. I moved a lot of stuff aroudn on my car and think I lowered the cg quite a bit, my gas tank is about as low as it gets. By lowering the car you lower the CG but also the roll center, a higher roll center will reduce (weight shifting induced) body roll and this will let you run softer springs for a more responsive suspension and smaller sway bars (and thicker bars take away from the independence of the suspension), the only big drawback is that it puts more load into the tires however w/ modern tires in a wider size tat won't be a big problem. Given the same tire it would reduce the scrub angle significantly.

Your outer rod end is most definitely not in the same position as the greenwood one. Take a closer look at the sketched drawing and the picture. They use a chopped off trailing arm. The rod is about mid plane in relation to the spindle. It looks like it's a bit lower

Yours is significantly lower. This is the main reason for the geometry misalighment w/ the halfshaft. I'm willing to bet that the stock eaton type diff has a u joint center in the stub axle that's 8.25" off center or very close to that.

Have you even seen that the drawing depics WHERE to weld the center bracket? 8.25" from the centerline.

Here's a little sketch I made of that later type susp. See where the u joints and pivots line up and are all parallel w/ the driveline axis?
This was the latest version of that greenwood 5 bar system.

The other picture is just an installation drawing, I feel the picture of the real thing is a lot more valuable source of info.

 

[TwinTurbo]

A long time ago I had this same or a similar discussion w/ flexusmark regarding camber control and stub axle float/movement in a 6 link design. Here's a couple of different designs (I copied the text from his posts)

These models represent several variations, parallel and non parallel rods & halfshafts, lined up and non lined up pivots/u joints and a combination of these. The last ones are the shared instantaneous center for camber control. This all works in a similar way as the toe control issue you're dealing with.

____________________________________

Ok guys, here it is. I just spent 4 hrs in the cad software and this is what I have come up with. All models start with .5 deg camber at ride height and are shown travelling thru a total of 4" of syspension travel (plus 2 and neg 2 from ride height)

Twin Turbo. Here is your model. I completed This model first and came up with .9 deg of camber at 2" of suspension travel. With this design you have yoke travel of .40-.32 of .08" (2mm) with the suspension traveling 2" in each direction of ride height.

Norval. If I understand correctly, this is the system you are decribing. Set up for the same amount of camber gain the total yoke travel is .09"

I have had to split this over more that one post because of the number of pictures. Please continue with the next post.

 

[TwinTurbo]

continued....

This is a system using the origional strut rod bracket for the lower link. I has been set up the same .9 deg camber angle. No I see that I have a lot of yoke travel, .31"

The reason I was not concerened with the amount of yoke travel is that I had my system configured with a greater camber curve. This is what happens to the system if you drop the inside top link position 7/8". The camber at 2" is now 1.7deg and the total yoke travel is .13" One interesting thing about this setup is that at neg 2" you get negative camber like the stock setup does.

I am still having trouble trying to decide what the amount of camber gain should be over 2 in of suspension travel. For reference here is a stock corvette

And a Smart strut setup. I have droped the strut mount 1-1/2" down.

One of the reasons I was trying to stay with the origional strut rod bracket was for exhaust clearance. In the first few pictures I have a blue circle which is my exhaust.

It now 1:00 o'clock in the morning, and I don't know what to think of all this. Better sleep on it.

Mark

 

[TwinTurbo]

...last

Twin Turbo,

Ok here is a new design that works as you said it would. The yoke travel is almost zero. I just modified my other design, so this one used the stock strut rod bracket. The camber curve for this design is the same as stock. Stock strut rod bracket and no travel at the yoke. It only makes sense that the camber curve would be the same as stock.

TT, what sort of curve should I be looking to get? Can you give me a value that you would like to see at 2" of travel and what you would like to start with at ride height.

One other thing I am also looking at with these 6 link systems is the amount of toe change. I know TT is not concerened with that because he is not using a single bolt conection for the front of the trailing arm.

Mark.

 

[lsejlowe]

wow, there's so much to learn from this thread it's rediculous. Thanks for all the info guys :thumbsup:

 

[SMYDA]

wow, there's so much to learn from this thread it's rediculous. Thanks for all the info guys :thumbsup:

:agree: :agree: :agree:

It's all Greek to me at the moment but I'm quickly learning the language.

 

[DaveL82]

Thanks for the PM's guys; now I know where the suspension discussions went to.

I'll be working on my C3 suspension in a few weeks (adding rear toe links and front trailing arms).

I have a motorcycle track day next week and put a roll bar in the wifes Miata last weekend so working on the vette has been bit limited.

 

[TwinTurbo]

Hey Dave, did you get my PM? Are you converting to dual forward rods like C4/Greenwood/Guldstrand or modifying the stock trailing arm (johnny joint, floating front)????

How is your rear coil over setup working?

 

[DaveL82]

Marck - got yours and one from Chris (yellow73sb).

I'm thinking about two trailing links tied to a modified T-arm assy. I feel more comfortable with this arrangement. Just need the time to do it.

The coil overs are working well. I drove the vette to work today and that was fun.

 

[lsejlowe]

Check out this picture by greenwood and note where he placed the pivots, especially on the cut off trailing arm. The rear mounts are much wider than yours, just like I had it, this is to be in compliance w/ the inner u joints.

See that the toe control links and halfshafts are parallel and same length AND pivoting in the same planes. This last thing you can't do unless you change the center mount but you CAN adjust for parallel rods/halfshafts and get the outer mount right. Forget what the C4 looks like, the general copied what these bright racers already did with their older rear ends, they modified them to get rid of the horrible toe change over suspension travel and resulting torque steer.

is it desireable or undesireable to use the halfshaft as one of the links? It seems like it would cause accelerated wear in the diff and the resulting yoke endplay would cause your toe and camber settings to change. Obviously Greenwood didn't mind relying on the halfshaft (or GM for that matter), but if I were designing from scratch I'd rather avoid it.

What if you used the parallel toe control rods down low and added another link up above the halfshaft for camber control (similar to many "6-link" systems)

Essentially you'd have this set up:

but with the parallel rods down low for toe control and use two links in place of the trailing arm.