Triangulated 4-link with lower links angled

[AMC Racer]

A couple questions on a triangulated 4-link:

1) A triangulated 4-link with the lower links mounted outboard on the axle and angled forward toward the driveshaft is a "Satchell linkage" ... but is there a term for the arrangement with the lower links mounted forward on the frame rails and angled inward and mounted low on the differential housing?

2) Due to clearance limitations, the arrangement with the lower links angled and meeting near the differential housing seems to give the best fit and, with adjustable link mounting points, can achieve near neutral roll steer or slight understeer, 100% anti-squat, etc. ... but it gives a RRCH of around 8" or less. Is this too low? How low can you go relative to FRCH?

Thanks for the help!

[exwestracer]

A couple questions on a triangulated 4-link:

1) A triangulated 4-link with the lower links mounted outboard on the axle and angled forward toward the driveshaft is a "Satchell linkage" ... but is there a term for the arrangement with the lower links mounted forward on the frame rails and angled inward and mounted low on the differential housing?

2) Due to clearance limitations, the arrangement with the lower links meeting near the differential housing seems to give the best fit and, with adjustable link mounting points, can achieve near neutral roll steer or slight understeer, 100% anti-squat, etc. ... but it gives a RRCH of around 8" or less. Is this too low? How low can you go relative to FRCH?

Thanks for the help!

If my humble learnings serve me, what you are trying to achieve is common roll moments front and rear (vertical distance from R/C to C/G axis). So, it depends on how high the weight is carried in the chassis at the rear. This can all be determined by angled weight checks on scales; but if you run an adjustable rear sway bar, you can artificially raise the rear RCH by stiffening the bar.

One note of caution on angled lower links: In ANY link type suspension, only the lower links carry all the forward thrust from the tires (the uppers are in tension under power) so having the links angled from the chassis centerline can (I didn't say "will") cause axle tramp under power. The tighter a diff you run, the more likely you are to encounter this issue. Also, since the tires are pushing the ends of the axle housing, and the lower links are mounted in the center (per your description), forward thrust force puts a bending load into the axle housing. This load gets worse the higher the upper arms are mounted above the housing.

[AMC Racer]

Thanks Ray! Lots of things to check and lots I need to learn! Wasn't really trying to achieve common roll moments ... but is this something that's good to try for? Basically just trying to fit something to improve all around handling capabilities for a mostly street car that will see some limited strip use and hopefully some autocross duty. The arrangement would look something like this:



If I went with a Satchell linkage instead, it gets similar characteristics, but a higher RRCH and the axle support is better since the lower links mount outboard near the wheels. However, the packaging isn't as good and requires a substantial cross member for mounting the front pivot of the lower links:



There is simply no room for a Watts behind the axle with the stock gas tank. A panhard bar may be possible ... but very tight and still may not work with the tailpipes. The traditional triangulated 4-link with angled uppers gets too high a RRCH and difficult to get good anti-squat with neutral roll steer, which is why the triangulated 4-link with angled lower links seems the best and simplest option ... but still learning this stuff, so if there are issues, please let me know what to look for.

Thanks!

[exwestracer]

With lower links level and parallel at ride height, you shouldn't have much of a roll steer issue... As the car rotates (rolls) about the RC, both links are gaining more or less the same angle, just one up and one down; so the position of the axle ends (relative to each other), won't change. With a panhard bar in compression, there is some jacking effect which will force some of the roll motion back on the inside tire and cause a small amount of roll steer.

Another thing to ponder about the Satchell type suspension is the tendency of the linkage to hold the car upright under power. The chassis ends of the upper links move sideways in roll (keep in mind the rear roll center is still at the projected intersection of the lower links - now some distance ahead of the rear axle); so in roll the upper links have some side angle as viewed from the top. When power is applied, the upper links are put in tension, and will naturally try to straighten out their load path. This has the effect of putting some amount of reverse-roll load on the chassis (kind of like an anti-roll bar that only works under power). This isn't necessarily BAD... if the car has a normal power understeer condition, it could actually help; just thought you should be aware of it.

Regarding your question about common roll moments F and R... I does help reduce diagonal jacking as the car rolls. If one end of the car has a much longer roll moment (easier to roll), lateral force will put some torsion load in the chassis structure. If the structure is as stiff as it should be, this force gets distributed diagonally to the opposite outside tire. With a common roll moment at both ends, weight transfer tends to stay more at each end of the car. Seems like that would be a good idea in most handling applications...right?

[AMC Racer]

The upper links seem to control the roll steer with the Satchell-type arrangement, so have them essentially level and parallel to each other at normal ride height to give neutral roll steer. Angling them down, in side view, gives roll understeer. The bottom links are angled up slightly. With spherical ends in the links, there is no bind.

So the question of whether 8" RRCH is too low is largely dependent on the specifics of the weight distribution, CoG and relative front/rear RC locations (roll axis)? By memory, the current weight distribution is approx. 57% front and 43% rear, so the lower RRCH may be a positive step in helping the F/R roll moment balance? Need to see what can be done to improve weight balance and chassis stiffness then try to get it on a scale to check.

Interesting on the effect of the Satchell-type arrangement under power. Stock certainly tends to understeer under power, so it may be a good thing.

So is there a negative to having the lower links angled rearward toward the differential, putting the intersection behind the axle, compared to the Satchell with the intersection considerably forward of the rear axle? The existing unibody structure is strongest where the links would mount to the chassis if angled inward to the differential (essentially mounts to the boxed section that joins the rear frame rails to the rockers i.e. close to the existing front leaf spring mount) so should be a better load path relative to the stock construction.

Thanks!

[exwestracer]

The upper links seem to control the roll steer with the Satchell-type arrangement, so have them essentially level and parallel to each other at normal ride height to give neutral roll steer. Angling them down, in side view, gives roll understeer. The bottom links are angled up slightly. With spherical ends in the links, there is no bind.

So the question of whether 8" RRCH is too low is largely dependent on the specifics of the weight distribution, CoG and relative front/rear RC locations (roll axis)? By memory, the current weight distribution is approx. 57% front and 43% rear, so the lower RRCH may be a positive step in helping the F/R roll moment balance? Need to see what can be done to improve weight balance and chassis stiffness then try to get it on a scale to check.

Interesting on the effect of the Satchell-type arrangement under power. Stock certainly tends to understeer under power, so it may be a good thing.

So is there a negative to having the lower links angled rearward toward the differential, putting the intersection behind the axle, compared to the Satchell with the intersection considerably forward of the rear axle? The existing unibody structure is strongest where the links would mount to the chassis if angled inward to the differential (essentially mounts to the boxed section that joins the rear frame rails to the rockers i.e. close to the existing front leaf spring mount) so should be a better load path relative to the stock construction.

Thanks!

You're right about the upper links...we do SO much triangulated 4 link stuff here, I automatically flipped that over in my head...! Yes, with upper links angled down toward the chassis, you would get roll understeer. I would try to keep the lower links as close to level as possible. Keep in mind those links are in compression under power, and you will get thrust anti-squat (bad) with more than a couple of degrees of up angle.

As far as angling the lower links out from the center of the housing, the only down I side I can see is the bending force in the axle housing I mentioned earlier. It actually controls axle movement better as the fixed point in the suspension system (roll center) is closer to the housing.

[AMC Racer]

The axle bending can take care of with a brace as it will need extra reinforcing for the mounts anyway.

Another interesting thing is if the upper links are horizontal but angled in slightly toward the front, it gives roll oversteer (out gives understeer). If you angle the upper links in and down you can basically get back to neutral roll steer. With some adjustability built in, should have reasonably good ability to fine-tune the handling.

Thanks!

[BillyShope]

...100% anti-squat, etc. ...
Thanks for the help!

Happened to notice this. 100% antisquat is what you want for a dragrace-only car, but, if it's also a track car, you'd best back off a bit to avoid wheel hop during braking.
http://www.racetec.cc/shope

[AMC Racer]

Thanks Billy! Since planning to do some strip runs, the 100% anti-squat was a goal for occasional strip use, with adjustability for less for track use.

On your site, there's good detail on the asymmetric 3-link to cancel driveshaft torque and get equal tire loading. Is this primarily for drag or also helps for track? Does it cause unbalanced braking? With the triangulated 4-link, would it be good to offset the angled links central pivots, similar to the asymmetric 3-link, or stagger the mounting points vertically to give a higher IC and greater anti-squat % for the right link pair vs. left or best to keep symmetrical for simplicity?

Thanks!

[BillyShope]

On your site, there's good detail on the asymmetric 3-link to cancel driveshaft torque and get equal tire loading. Is this primarily for drag or also helps for track?

This was first used (to my knowledge) on the Jaguar C-Type.

Does it cause unbalanced braking?

Only with total wheel lockup. Again, note its use on the C-Type.

With the triangulated 4-link, would it be good to offset the angled links central pivots, similar to the asymmetric 3-link, or stagger the mounting points vertically to give a higher IC and greater anti-squat % for the right link pair vs. left or best to keep symmetrical for simplicity?

Thanks!

Only you can answer this one. It would appear the simplest solution...if you want to cancel driveshaft torque...would be the staggered vertical mounting points.
http://www.racetec.cc/shope

[exwestracer]

With the triangulated 4-link, would it be good to offset the angled links central pivots, similar to the asymmetric 3-link, or stagger the mounting points vertically to give a higher IC and greater anti-squat % for the right link pair vs. left or best to keep symmetrical for simplicity?

Thanks!

One thing to keep in mind with an assymetrical T4L... If your C/G is on the chassis centerline (where it should be for balanced handling), the offset R/C location will give the C/G different leverage on the suspension in left vs. right hand corners and cause inconsistent handling. Honestly, unless your primary focus is drag racing, I'd recommend keeping the links centered and symmetrical. This isn't as critical with the 3-link, as you will need a lateral locater and that will set the R/C location, not the top link.

If you are thinking about offsetting the lower triangulated links in the Satchell design, you would be giving one tire increased leverage on the chassis in thrust, and I can't see any good coming of that...

[AMC Racer]

Sounds like symmetrical is the best way to go for balanced handling, then have some adjustability in the link mounts to get the anti-squat and driveshaft torque canceling effects if wanted for the occasional run at the strip.

Thanks!