914 Suspension motion ratios, Comparing notes Options

[groot]

I measured my front and rear motion ratios yesterday and wanted to compare notes with any that have done the same.

For the rear suspension I took ~20 data points and did a best fit analysis to come up with 1.00 to 1.00 damper travel to wheel travel with strong statistical correlation.

My front geometry is a bit different than a standard 914, so the motion ratio should be different, too. I measure ~10 points and found 0.83 damper travel to wheel travel, again with strong statistical correlation.

I'm about to order my rear shocks and this is one of the bits of data they need to build the shocks.

[groot]

And....

If anyone had determined the height of their Cg, that would be useful, too.

I can't do mine now until I reinstall the powertrain......

[Brad Roberts]

Your making me think.. (IMG:style_emoticons/default/biggrin.gif)

I have all of this info on a laptop that we use for data acq on a FP 914.

I'm at home this weekend doing NOTHING.. but the laptop is at the shop (not in the trailer) so... I might have some info for you by Monday evening if you still need it.



B

[slivel]

Kevin,
I realize that I sent the wrong numbers to you in a PM. Sorry. My measured front MR is .909 and rear is 1.296. But this has to be squared to get Wheel Rate. Apparently I can't attach an Excel file. Let me know if you want me to mail you the file.

Steve

[Brett W]

Kevin
Who are you having do your shocks?

Steve
What is your setup? Stock suspension points and components? Could you email me the file as well?

Brad
Please email me that info as well. I would really be interested in comparing it to my own notes and theories. On that FP car what was the setup? Relocated mounting points, custom trailing arms and struts, etc?

[slivel]

Kevin, Brett,

I would love to compare notes with you guys about setup and will gladly send you the Excel file via email but can someone tell me how to do that. I spent hours studying formulas and measuring suspension components and creating spreadsheet with imbedded formulas. I think Kevin sent me his address but some filtering software seems to have removed it on my end or the Board's end. There is no way via reply to attach the file. Am I missing something?

Really want to maximize my car's handling and give more of those 911 drivers fits. Sharing information makes sense.

My car uses stock suspension mounting points except that front shock towers have modified camber caster plates. I am running RSR front struts and no torsion bars. Alll suspension mounting points have been strengthened through plating and tubing to the cage. Bilstein shocks front and rear.

Steve

[Eric_Shea]

Nice to have a Brother in the shock business... (IMG:style_emoticons/default/biggrin.gif)

[Thorshammer]

Kevin,


What do you plan on using for shocks????
I like the Ohlins

Slivel,
Y
ou may find the RC will be high, especially in the rear causing the instant axis from front to rear to be much higher in the rear. This causes a number of issues, Brad may feel different, but I have found 2.5-3.0 inches of rear suspension pick up point movement (up) to fix this and restore the rear trailing arm angularity. This will make the rear of the car much more compliant while enabling you to lower the rear RC to an acceptable point. This will make the IA similar front to rear althought the MA will change due to the difference between Struts and trailing arms.


Erik

If you really want something good, add 4.5 inches to the front a-arms and relocate those points 2.0-2.5 inches up. Then add 2.5 degrees to the strut knuckle angle and reposition the front camber plate. This will give you almost 0 scrub and a better camber curve, and allow for the proper amount of front track, well at least in my class. This is my plan for next year when the car gets a rebuild. Kevin is on the right track with his front suspension. But, he may find the rear pick up point change will give him immense benefits.

[Jeroen]

I just uploaded Steve's excell sheets.
You can find em here:

Suspension worksheet

Spring rates

[Brett W]

I too spent hours measuring the points for all the suspension mounts and marked them on the floor and measure to each point but none of my point measurements were relative to the program I was using so I get to do it all over again. (for the third time) To bad I don't have any of my old suspension components. As soon as I figure out this program I will be runnning numbers on the rear. I feel that lengthing the trailing arm and moving the rear pickup points are steps in the right direction but I will not argue why until I have the numbers to back up my theories. Same goes for the front. Simply moving the pickup points only fixes part of the problem.

Steve, I'll send you an email with my address in it.

[groot]

I'm going with Koni 30s in the rear. They have an incredible range of adjustment.

I know we've beat the RC discussion to death a few times, but let me add a few statements from what I've learned.

The front roll center on a lowered 914 front stock suspension is below the ground. Bad, very bad. There are several approaches to fix this. Eric mentioned one method.

The rear roll center is higher than the front...that's okay, it's supposed to be that way. The rear roll center, from what I've read, is determined by the angle of the mounting points to the chassis in the horizontal plane, in our case 12 degrees. Another line is drawn from the intersection of the 12 degree line with the wheel centerline to the center of the tire patch. The roll center is where this line intersects the center line of the car. So, the rear roll center is between the ground and the center of the wheels and it's height is determined by the angle of the trailing arm points relative to the wheel. BTW.... thanks to Brett W for helping me figure this stuff out. We traded many e-mails working through this discussion.

So, raising the points in the chassis does nothing for the rear roll center. But, raising the points does change the angle of the trailing arm relative to the body, which is where there can be a benefit.

Gillespie says that if a semi-trailing arm is angled up front the rear wheel, it contributes to roll oversteer, if it's level, neutral steer, and angled down from the rear wheel, roll understeer. And this is due to the track change at the outside wheel. However, Gillespie does not quantify this.

Now, we disagree about the severity of this effect.

[ottox914]

So how low is TOO low on the front of a stock 914? I have heard a rule of thumb for this is not to let the front lower A arms go below parallel to the ground. Comments for us stock guys?

Thanks-

[groot]

Angled down towards the wheel (better than parallel) to the ground is a decent rule of thumb, but .... see my signature.....

You can always just drive it and have fun (IMG:style_emoticons/default/wink.gif) and not stress about it. If the roll center is too low, you just need more roll stiffness to counter it. I think that's why there are so many big ARBs for the 914.

Since I know I'm not Montoya I need to build a car that is easier to drive and I like the engineering exercise behind it all.

[Bigbohr]

If the roll center is too low, you just need more roll stiffness to counter it. I think that's why there are so many big ARBs for the 914.

That would reduce actual body roll but you'd still get weight transfer and loss of front traction. Or did I understand that wrong?

Also, how do you increase the spindle to strut angle by 2.5 degrees? Customized knuckle I assume. Or do some strut assy's, like RSR, have a larger strut to spindle angle?

[groot]

That's how I see it.

I didn't increase the angle between the strut and knuckle, but I do have custom (read home modified) knuckle/strut assemblies. Eric would have to enlighten us on how he's going to do that.

I worried less about the scrub radius and more about roll center, right or wrong. I did raise the steering rack to reduce bump steer.

[groot]

Well, I re-measured and still get 1:1 on the rear, but.... since I'm going to use spherical bearing mounts, I think I'll move the upper mount to get the loads straight into the cage. So, I'll lean the damper more forward in-car than factory, which results in a .92:1 motion ratio (damper travel:wheel travel).

BTW...... Koni has expanded their 30 series line to include a lot longer dampers than before. So, now they make 30 series dampers up to 24.5" long extended (the longest 30 series they used to make was only 17.4" long extended).


Does anybody have any Cg height information? Preferably for an open 914.

[Brett W]

I was told by someone I trust to figure about 16 inches.

[groot]

That sounds like it's in the ballpark.

For reference a Ford GT has a Cg of 17.7"

[Brett W]

I guess the DOHC V8 moves some of the weigh a little higher.

[TimT]

I like this thread