wheelsmithy
wheelsmithy UltraDork
3/21/20 8:12 a.m.

Short version of my question:

What is the "ideal" set up for a strut front suspension? I of course, know ideal doesn't exist, but would love any insight the hive may have.

The longer question:

I am working on a Fiat X1/9 Challenge car. There is already surgery that needs to happen in the suspension arm location, so this would be the ideal time to build it right. Variables that are nailed down so far are Aw11 struts, cut down for rear inserts and coil overs, 1/2 inch heim joints, and I'd like to keep the locating point of the stock front strut arm.

So, I have a pretty decent grip on A-arm suspension geometry.

The rack width and steering rod length should fall within the quadrilateral described by the upper and lower A-arms:

The thing is, struts just describe long arcs. I believe job #1 is to make the control arm as close to horizontal at ride height, also, have some negative camber dialed in. Then I get a bit lost. Watching Binky, I know raising/lowering rack height will change the situation. There is a limited amount of space where the rack can fit, but I have control arm length and rack width to play with.

I have heard it said that, for instance, I used a rack like this:

and lower control arms that almost meet in the center of the car, I could get "zero bump steer". Note the tie rod attachment points for the rack above are very close together. 

I do have books on this somewhere, but am having difficulty laying hand to them.

Here's another rack option:

, but having those mounting points so close together looks suspect. Lots of force going through a front suspension.

 

dps214
dps214 Reader
3/21/20 10:02 a.m.

Most of my knowledge is for double wishbone as well so this could be wrong. But I would think the tie rod sizing is the same concept, but instead of a quadrilateral of the four control arm joints, it's the triangle defined by the two control arm joints and the strut top pivot point.

jimbbski
jimbbski SuperDork
3/21/20 10:03 a.m.

For a strut suspension I like  the lower arm to angle downward towards the ball joint if at all possible. Ride height requirements may require that to not happen.  Then the alternative is to either raise the inner mounting point of the lower arm up so the arm still has the desired slope. You can also move the mounting point outward to build in neg. camber.  Alternatively you can lower the outer end of the arm by getting a longer ball joint stud. These are not easy to find on many not so popular vehicles. They can be made but it requires access to machine tools which not everybody has.

A spherical bearing, a housing, and a stud are the major parts needed. The first two can be bought, the last one needs to be machined.

Stefan
Stefan MegaDork
3/21/20 10:19 a.m.

There really isn't one.  Due to the design, the "upper link" aka strut is constantly changing in length so dialing it out at one length changes when under compression or extension.

There is a narrow range where it's ok or manageable and there are setups that are really not.

Your best bet is to replicate the locations and angles of the donor pieces as much as possible as they've done most of the engineering for you.  Keep the suspension travel within reason and keep the desired amount of static camber in mind when configuring it in the car.

spandak
spandak HalfDork
3/21/20 10:28 a.m.


 

Stole this from google. But you can see how as you lower the vehicle and the lower control arm levels out the roll center drops down closer to the pavement. This increases the distance between the RC and CG (roll couple) and essentially the car is going to roll more in the corners. You want some downward angle on the control arm if possible. 
 

Afaik there is no way to totally eliminate bump steer in a macstrut but you can control its window of motion to limit it as much as possible. I can see how the first rack would be beneficial but I would need to dig into this all again to say more than that. However, I've never seen an OEM rack like that and I suspect there is a reason. 

wheelsmithy
wheelsmithy UltraDork
3/21/20 12:10 p.m.

Nice conversation fellas, responses appreciated.

The conversation mirrors what I largely had in mind. To be clear, this will be a low vehicle, with very little suspension travel, and very stiff springs.  I am realistic about not having Zero bump steer. I just wondered about the theory.

I have a book that details moving the inner control arm pivots up a corresponding amount to what the car was lowered. Makes sense, and other people correct the same problem by putting spacers between the bottom of the strut and ball joint to re-establish the factory arc described by the control arm.

What I'm wondering is does a longer control arm help? Clearly, it would need a shorter rack to go with it.  Spandak- excellent point about never seeing a OEM rack like the one I pictured. 

dps214
dps214 Reader
3/21/20 12:28 p.m.

All else equal for the situation you're describing, yes a longer control arm would be an improvement because the same amount of vertical travel of the outer joint results in less of a change in the angle of the control arm, which results in less horizontal movement of the joint. However, if you draw it out for the situation you describe of a stiff suspension with minimal movement and a roughly horizontal control arm, I would expect you'll find that the change is pretty small. I'd guess it would be worth a lot more to put the same amount of effort into moving the inner joints up than in, unless there's some reason moving up is substantially more difficult.

TurnerX19
TurnerX19 Dork
3/21/20 4:12 p.m.

The minimum bump steer on a strut system is accomplished by making the tie rod the exact length as the control arm as well as keeping it in the same plane in both axis. Look at Volvo 240 to see the best production setup I have seen. The AW11 is probably very close to the same, but I have never been under the front of one.

wheelsmithy
wheelsmithy UltraDork
3/21/20 6:14 p.m.
TurnerX19 said:

The minimum bump steer on a strut system is accomplished by making the tie rod the exact length as the control arm as well as keeping it in the same plane in both axis. Look at Volvo 240 to see the best production setup I have seen. The AW11 is probably very close to the same, but I have never been under the front of one.

So, to work backwards, the distance between inner control arm mounts should equal inner ball joint to inner ball joint length, including rack? That would make things easy.

Picture of Volvo 240 suspension:

It appears the rack is nearly in the same plane as the lower control arm. This would not be an option in my situation. The rack has to live 4-6" above the LCA mount,

Feedback greatly appreciated, as always, people.

Pete Gossett
Pete Gossett MegaDork
3/21/20 6:54 p.m.

In reply to wheelsmithy :

Draw a line between the inboard control arm pivot point, and outboard ball joint pivot point. The distance between those to points should match the distance between the pivot points of the inner & outer tie rod ends.

Then you need to setup the steering so the control arm & tie rods stay parallel to each other both horizontally & vertically...or as close as you can get.
 

 

 

TurnerX19
TurnerX19 Dork
3/21/20 8:46 p.m.

In reply to wheelsmithy :

Why does the rack have to live above the LCA mounts? There is plenty of room down there in the X1/9 chassis.

Vigo
Vigo MegaDork
3/21/20 9:01 p.m.

If you're trying to get the inner tie rod pivots as close to the center as possible, look up 'center take off rack and pinion' . 

wheelsmithy
wheelsmithy UltraDork
3/22/20 7:49 a.m.
TurnerX19 said:

In reply to wheelsmithy :

Why does the rack have to live above the LCA mounts? There is plenty of room down there in the X1/9 chassis.

Well, that may be true. Beard scratching to ensue...

Thanks Vigo, I'm not sure what I'm trying to do. I don't understand the benefits of the center take off rack. As of right now, nothing is convincing me it is worth the hassle.

 

TurnerX19
TurnerX19 Dork
3/22/20 12:06 p.m.

In reply to wheelsmithy :

Center rake off rack is for cars with control arms that meet in the middle of the car. The earliest example I know of was the Rootes Imp. which had a swing axle front end. I worked surprisingly well when de-cambered, but oh how crude!

Vigo
Vigo MegaDork
3/23/20 10:10 p.m.

As of right now, nothing is convincing me it is worth the hassle.

That's my overall impression of overthinking suspensions on cars that will never have their changes objectively measured. If there's no data before AND after you just have to do what will give you the right mix of 'feeling better afterwards for no objective reason' and actual seat of the pants result.  My .02..

As far as center take off racks, they ARE used in otherwise normal cars. Hondas, old mopar LH cars, etc. It may be more for packaging than for anything related to bumpsteer, but suffice to say that having the inner tie rod and lower control arm pivots in wildly different places didn't make anyone jump out of one of those cars and yell "there is an obvious geometry issue here!". Meh 

wheelsmithy
wheelsmithy UltraDork
3/24/20 7:41 a.m.

In reply to Vigo :

Thank You, I thought I remembered some of them (center steer racks), but wasn't sure if I was thinking golf carts, or what. I finally found the passage in my Herb Adams Chassis Engineering book. His take on strut suspension: Don't Bother.

Still, something good gas come from this conversation- It made me realize that the closer the rack is to the LCA, the more favorable the geometry- Thanks for that TunerX1/9. 

This all gets me in a good place to start designing the suspension. I'll still whack the MR2 struts apart-it's got to be done anyway, and start trying to see what I can see.

SkinnyG
SkinnyG UltraDork
3/24/20 10:31 a.m.

The rack height is largely dictated by the ratio of distances between the upper and lower balljoints and the outer tie rod location.

The rack width is largely dictated by the inner pivots of the upper and lower control arms, and the rack height.

I briefly touch on this with some graphic figuring out in this video queued at the right spot HERE.

Try to pick a rack width that will work at the place you need it, and then move the rack up or down to fine-tune any toe changes in (mostly) bump, and (some) droop.  Try to focus at and around ride height.

On various projects, I've winged it, I've done rudimentary estimates, I've done careful measurements, and I've run numbers through SusProg3D.  Get close, it'll work.

One car was totally winged, and it drove fine, though there was something about the steering you just couldn't quite put your finger on. Then I re-did it using SusProg3D and shortend the rack considerably, and the steering was a whole lot better.

I also did some work on a business customer's X1/9, and I found the toe change to be absolutely horrific.  Like: I would have wanted to cut it all out and do it all again.  MASSIVE toe changes.

TurnerX19
TurnerX19 Dork
3/24/20 1:56 p.m.

I drive an X1/9 every day. With the original suspension the only way to prevent bump steer is to prevent bump travel! Fortunately the motion is in the benign direction, I.E. it toes out with compression. Wheelsmithy has the opportunity here to improve the situation massively.

wheelsmithy
wheelsmithy UltraDork
3/24/20 6:06 p.m.

Thanks, Skinny, I had watched before, but wasn't in suspension design mode at the moment. Very helpful.

This conversation has helped my analysis paralysis, and I appreciate you all.  I knew about keeping the rack as close to inline as possible with the LCA on a double wishbone, but now recognize its benefit on as strut suspension. You guys are the best.

 

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