camber vs. ride height, what I do in class instead of learning Options

[Dave-O]

Hi guys/gals,
Now that the AX season is officially done up here in the north, it's time to start thinking about winter "improvements". At the beginning of this season I set my alignment/ride height/corner balance and just left it as is all year, focusing on driving. I'm contemplating trying to drop the front of the car down another inch in an attempt to transfer a little more of the weight up there. Right now I have about 2.5" of suspension travel in the front and don't want to sacrifice any of that.

In order to lower the car I would have to chop the struts and put shorten inserts in. To justify this, I wanted to verify that the camber curve would still be acceptable. I made a quick excell sheet graphing the camber gain as a function of A-arm deflection. Hopefully this will be useful to other people as well.

As of now, at rest the A-arms are at about +3 degrees. On the bumpstops they are at about -10. By lowering the car an inch they would start at about -3 degrees and assuming I cut the struts 1.5 inches on the bump stops would be at about -19.

I took some quick measurments on the car but they are by no means extremely precise.

Here is the graph...

[Dave-O]

Based on the graph I think I will still do the shortened struts as I could use some new struts anyway. I may not go down a whole inch but we will see when I get the car on the scales next spring.

here is the excel sheet...let me know if it works for you.

[Aaron Cox]

what about raising spindles?

A arms now point down farther starting out further positive at the same ride height, and giving you a bit more travel....

[Dave-O]

Step 0, read the rule book. Altered spindle location is not allowed in my class. (IMG:style_emoticons/default/sad.gif)

what about raising spindles?

A arms now point down farther starting out further positive at the same ride height, and giving you a bit more travel....

[Aaron Cox]

but you can shorten the housings? wow...

cool prject tho!

[groot]

Hey, Dave,

I love to see people using science to make cars faster!!! Good work.

For reference a certain vehicle (no specifics due to proprietary information restrictions) with a Macstrut front suspension that I have actual Kinematics and Compliance data on has a fairly linear camber curve of .168 degrees/cm around 0 (+/- 2.5 cm) on one side and .160 on the other. One of the limitations of the macstrut is the lack of ample camber gain.... ideally, you'd like it to be .4 degrees/cm or so... depending on many things.

More often than not most people measure camber gain by degrees per distance of wheel travel.... but I understand why you compared it to LCA angle to horizontal.

[brant]

but you can shorten the housings? wow...

cool prject tho!

Aaron,
alot of rules say exactly this.
this is the reason I don't have raised spindles either, but instead have chopped housings.

Moving a suspension point is not legal.. Technically the wheel spindle is a suspension point.

however the use of any shock choosen by the owner is legal.

brant

[Dave-O]

Hey, Dave,

I love to see people using science to make cars faster!!! Good work.

For reference a certain vehicle (no specifics due to proprietary information restrictions) with a Macstrut front suspension that I have actual Kinematics and Compliance data on has a fairly linear camber curve of .168 degrees/cm around 0 (+/- 2.5 cm) on one side and .160 on the other. One of the limitations of the macstrut is the lack of ample camber gain.... ideally, you'd like it to be .4 degrees/cm or so... depending on many things.

More often than not most people measure camber gain by degrees per distance of wheel travel.... but I understand why you compared it to LCA angle to horizontal.

I certainly have a "certain vehicle"'s suspension design in mind for a future project. If you take my excel sheet and add 10" to the "a" and "c" components the graph gets much much nicer!

Class rules aside...What do you think of using a later model porsche strut with a larger strut angle and moving the upper mount inboard? I haven't done any measurments on these struts but assuming you could move the top inboard by 2.5 inches. Here is what the graph would look like. With longer A-arms it would be more linear.

[groot]

I certainly have a "certain vehicle"'s suspension design in mind for a future project. If you take my excel sheet and add 10" to the "a" and "c" components the graph gets much much nicer!

Class rules aside...What do you think of using a later model porsche strut with a larger strut angle and moving the upper mount inboard? I haven't done any measurments on these struts but assuming you could move the top inboard by 2.5 inches. Here is what the graph would look like. With longer A-arms it would be more linear.

That's an interesting idea (using the later model strut), which I hadn't considered. I didn't even know that Porsche changed that angle. Seems like a reasonable way to get better camber gain.... but I have to think on it a bit.

The "certain vehicle" I was referring to is currently in production, but it's camber curves are fairly representative of the front macphearson strut suspension.

But, if you were thinking about my race car (thanks), please know that I optimized for roll center migration. I certainly weighed roll center migration higher than both camber gain (since I actually decreased it) and bump steer (I raised the rack, but my toe curve still got slightly worse than stock). But, I did that because I like to use suspension travel and think huge stablizer bars aren't the proper way to deal with roll. In my opinion this makes the car easier to drive.

[groot]

So, I've thought a bit more about the steeper angle on the strut.

If one makes this change without moving point 3 and point 4 on the LCA (where the lower control arm meets the body....maybe this is the "a" and "c" you referred to). There will be an increase in camber gain, increase in side load on the strut (resulting in more stiction), greater change in the toe curve (curve is less linear), slight (negligible) change to roll center... I think that's most of it. Well, not quite, there's kingin angle and scrub radius, too, but those are relatively minor contributors to this situation.

Now, if you move your steering rack you can center your toe curve to minimize the effect of the increased non-linearity. You can move points 3 and 4 inboard to settle your roll center migration down at the expense of some lost camber gain.

Seems like a reasonable thing to try.

[ottox914]

Cool stuff, Dave. Was this little exercise part of your application for the masters program for ME's? See you at the club meeting tomorrow, we can talk about that future "project" and see what taladaga mike has for us...

[Dave-O]

So, I've thought a bit more about the steeper angle on the strut.

If one makes this change without moving point 3 and point 4 on the LCA (where the lower control arm meets the body....maybe this is the "a" and "c" you referred to). There will be an increase in camber gain, increase in side load on the strut (resulting in more stiction), greater change in the toe curve (curve is less linear), slight (negligible) change to roll center... I think that's most of it. Well, not quite, there's kingin angle and scrub radius, too, but those are relatively minor contributors to this situation.

Now, if you move your steering rack you can center your toe curve to minimize the effect of the increased non-linearity. You can move points 3 and 4 inboard to settle your roll center migration down at the expense of some lost camber gain.

Seems like a reasonable thing to try.

the "a" value is the length of the LCA. the "c" value is the horizontal distance between the LCA mounting points and the upper strut mount. For simplicity, in my tables I just took the distances from the centerline of both mounting points of the LCA. In the graph above I simply decreased the c value by 2.5 inches to simulate moving the upper strut mount inboard by this amount.

I have not done any actual measurements on the late-model struts as unfortunately I don't have a 996 or a boxter sitting around to take measurements off of. I have heard that they have a larger strut angle but I really don't have any real data to back that up. The 2.5" difference is simply a guess.

It seems that with a larger strut angle and with something similar to your longarm suspension design, you could get a slightly more aggressive camber curve while keeping it relatively linear.

I'm entirely a newbie when it comes to suspension design. I had not accounted for toe-changes. I could potentially make another graph to look at this.

The other benefits of using a later model strut is that it is already set up for coil springs. Another concern we had was with the overall width of the vehicle (very important in autocross). Potentially with a larger strut angle we could keep the wheels further inboard. Finally, it also seems like a cool thing to do! (IMG:style_emoticons/default/biggrin.gif)

Later I will take some time to change the independent variable on the graphs to cm of suspension travel to make it more relative.

Here is a graph of the camber curve while moving the upper strut mount inboard by 2.5 inches and using 21 inch control arms.

[Dave-O]

Cool stuff, Dave. Was this little exercise part of your application for the masters program for ME's? See you at the club meeting tomorrow, we can talk about that future "project" and see what taladaga mike has for us...

I didn't mention any of this craziness on my application, which is probably why I got admitted! They still think I'm a mild mannered biochemist who is interested in mechanical systems.

[J P Stein]

Kevin:
Take a look at Bills Buckster Build pics.
There's a good pic of the front strut from which you can discern what they did to maintain the proper angle on the strut (to keep the side loads under control) while getting the front hub outboard.

I'm already tearing up Bilstein inserts with side loads....doan need to increase that angle.

[jhadler]

David,

Very interested in what you come up with, the Street Prepared rules regarding struts are pretty much the same as in Street Touring, so that could be of use to folks in other classes as well!! (IMG:style_emoticons/default/smile.gif)

-Josh2

[Brad Roberts]

But, I did that because I like to use suspension travel and think huge stablizer bars aren't the proper way to deal with roll. In my opinion this makes the car easier to drive.

For experienced drivers? Yes. For newbies? No. People who have ran go karts with no suspension travel can jump into anything with super high spring rates and small anti roll bars and drive the wheels off of it. Somebody who hasnt driven a stiffly sprung car and gets into one typically feels uneasy.

Everyone is different (IMG:style_emoticons/default/biggrin.gif)

Now back on topic!

How does this help me go faster? I already know not to turn the wheel a whole lot or the car slows down. I actually teach people this for AutoX. I make them push a car.. then push a car with the wheel turned.

The numbers are great. I understand everything being said, but bottom line, how does it help us? If a Boxster strut can be used (easily actually) why wouldnt I just run a Boxster?


B

[jhadler]

The numbers are great. I understand everything being said, but bottom line, how does it help us? If a Boxster strut can be used (easily actually) why wouldnt I just run a Boxster?
B

'cause a boxter costs more???? (IMG:style_emoticons/default/biggrin.gif) (IMG:style_emoticons/default/biggrin.gif)

But really, if a Boxter strut can significantly improve the camber curve of the front, that would be way cool. In my class I'm allowed _any_ strut, so long as I don't alter pick-up points...

-Josh2

[Dave-O]

The numbers are great. I understand everything being said, but bottom line, how does it help us? If a Boxster strut can be used (easily actually) why wouldnt I just run a Boxster?
B

'cause a boxter costs more???? (IMG:style_emoticons/default/biggrin.gif) (IMG:style_emoticons/default/biggrin.gif)

But really, if a Boxter strut can significantly improve the camber curve of the front, that would be way cool. In my class I'm allowed _any_ strut, so long as I don't alter pick-up points...

-Josh2

Josh, the talk of the boxter strut is for an XP 914 that ottox914 and I have been daydreaming about. The problem with the boxter strut on a stockish car is that the top mounting point will have to be moved inboard by a significant ammount. For our cars in Street Touring/Street Prepared rules we are stuck with the first graph that I posted. Since we can't change the A-arm and we can't change the upper strut mount, it doesn't matter what strut we run, that is the camber curve we will get based on those parameters.

Brad, as to why not just run a boxter, I had conisdered it. But we are thinking (dreaming) about building a car for a specific SCCA AX class. To reach the minimum weight in the class the boxter would require a lot of sawsall action. Also the initial cost is higher. Finally, we want to run a WRX STi drivetrain...sticking with the 914 chassis just makes more sense.

Why do these crazy mods to a 914? If we are going to spend a bunch of money building this car? Why go half-assed? Like you said, if the strut can be used easily. We also will get the boxter brakes, hubs, and coil-spring set-up. Seems like win/win/win.

This all just talk right now, I won't have any money to sink into such a project till I finish gradschool. But it seems like a much better idea to figure these things out before I fire up the credit card and the welder!

[Brad Roberts]

I have a 993RSR hub/carrier and a Boxster hub/carrier (both with struts) Tell me what to do and what to measure. I'll let you know what I find.

GT2 Porsches and 993RSR's use a "raised" wheel bearing. They lower these cars by raising the bearing in the carrier compared to a stock 986/996 carrier. The lower control arms are identical in a GT3 street/GT3 Cup/GT3RSR/GT2.

My friend Jason in San Jose has been buying complete suspension setup from wrecking yards for under $800.



B

[J P Stein]

I have a 993RSR hub/carrier and a Boxster hub/carrier (both with struts) Tell me what to do and what to measure. I'll let you know what I find.
B

OK, Brad, lay the Boxster hub/carrier up into the 914 strut area and tell me what needs to to altered (oher than the ball joint) to make it fit. I can't run my big wheels/tires in front due to scrub radius problems. The cure is a more room twixt the
hub & strut. I don't mind altering the upper strut mount within reason.