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PostPosted: September 29, 2008, 10:04 am 
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Joined: February 10, 2008, 12:36 pm
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Location: SAN DIEGO
REALLY need to do your homework here.............

I have a Stalker and it has 2 inches of shock travel in compression at the most! It also has 180 springs, to make a long story short I am bottoming out ALL the time on bumps, to the point that some have been painful.

The problem, the rear end is naturally loose on a Stalker (straight axle). If you make the springs stiffer in the rear to keep it from bottoming out this will make it even more loose.


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PostPosted: September 29, 2008, 11:23 am 
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This was done to accommodate a LONGER shock actually. They have about 5.5" of travel and currently have 200lbs springs, so they wouldn't fit in the 'book' location. I guess moving the upper mount further inboard would solve most of the issues. And no, I haven't done any calculations yet.

What is the calculation anyways? Lets assume that the mount on the axle is where I drew it, and the upper mount is moved inwards (so they are in line). Lets also assume that they are at 10º from the side. So a total of 10º of angle... 200lbs springs (although thats easily changed). Anything else needed to make the calculation?


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PostPosted: September 29, 2008, 12:23 pm 
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Joined: August 6, 2008, 9:40 am
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Location: Greenville/Charlotte NC
Just out of curiosity, don't the shocks need to be mounted the other way? All bike shocks are mounted so that the reservoir is on top and the sliding portion is actually on the bottom. I'm not sure if this is done to keep the oil in a specific orientation or something like that. Also, I believe unsprung weight would be decreased by mounting it this way.

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PostPosted: September 29, 2008, 12:36 pm 
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killernoodle wrote:
Just out of curiosity, don't the shocks need to be mounted the other way? All bike shocks are mounted so that the reservoir is on top and the sliding portion is actually on the bottom. I'm not sure if this is done to keep the oil in a specific orientation or something like that. Also, I believe unsprung weight would be decreased by mounting it this way.


THEY AREN'T BIKE SHOCKS! (sorry, seems like that needs clarification here).


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PostPosted: September 29, 2008, 12:56 pm 
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gtivr4 wrote:
This was done to accommodate a LONGER shock actually. They have about 5.5" of travel and currently have 200lbs springs, so they wouldn't fit in the 'book' location. I guess moving the upper mount further inboard would solve most of the issues. And no, I haven't done any calculations yet.

What is the calculation anyways? Lets assume that the mount on the axle is where I drew it, and the upper mount is moved inwards (so they are in line). Lets also assume that they are at 10º from the side. So a total of 10º of angle... 200lbs springs (although thats easily changed). Anything else needed to make the calculation?

Sorry, saw the remote reservoir and made an assumption.

The 10 deg in itself doesn't hurt too much, degrading installation ratio by cos(10), about a 2.5% decrease which is fine.

The problem is that the shocks are mounted roughly half way between the tire contact patch and axle centerline. Half way means that's your installation ratio, so you need four times the spring rate to give a specific wheel rate.

Interestingly, this is only true in a one-wheel bump. When hitting a dip in the road that compresses both shocks at the same time, the installation ratio is 1:1. It's going to cause handling issues because it'll be stiff when hitting bumps common to both shocks, but four times softer when hitting one-wheel bumps or cornering.

Think of the extreme case, moving the shocks so far inboard that they end up at the center of the axle. When hitting common bumps everything's fine, but when hitting a one-wheel bump or cornering, the car has zero roll resistance amd will fall over!

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PostPosted: September 29, 2008, 1:33 pm 
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KB58 wrote:
gtivr4 wrote:
This was done to accommodate a LONGER shock actually. They have about 5.5" of travel and currently have 200lbs springs, so they wouldn't fit in the 'book' location. I guess moving the upper mount further inboard would solve most of the issues. And no, I haven't done any calculations yet.

What is the calculation anyways? Lets assume that the mount on the axle is where I drew it, and the upper mount is moved inwards (so they are in line). Lets also assume that they are at 10º from the side. So a total of 10º of angle... 200lbs springs (although thats easily changed). Anything else needed to make the calculation?

Sorry, saw the remote reservoir and made an assumption.

The 10 deg in itself doesn't hurt too much, degrading installation ratio by cos(10), about a 2.5% decrease which is fine.

The problem is that the shocks are mounted roughly half way between the tire contact patch and axle centerline. Half way means that's your installation ratio, so you need four times the spring rate to give a specific wheel rate.

Interestingly, this is only true in a one-wheel bump. When hitting a dip in the road that compresses both shocks at the same time, the installation ratio is 1:1. It's going to cause handling issues because it'll be stiff when hitting bumps common to both shocks, but four times softer when hitting one-wheel bumps or cornering.

Think of the extreme case, moving the shocks so far inboard that they end up at the center of the axle. When hitting common bumps everything's fine, but when hitting a one-wheel bump or cornering, the car has zero roll resistance amd will fall over!


Fair enough. So I should do everything I can to get the out as wide as possible? How do I calculate the installation ration in this respect?


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PostPosted: September 29, 2008, 2:18 pm 
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gtivr4 wrote:
Fair enough. So I should do everything I can to get the out as wide as possible? How do I calculate the installation ration in this respect?

It's two different numbers depending if it's a whole-car bump you're hitting, or one wheel/cornering.

The installation ratio is the geometric factor by which the spring moves versus the wheel. If the spring is located half way between the diff and tire, it means the spring compresses .75 as much as the tire (because it's 25% of the way from one wheel to the other.) If the spring is leaned over any amount, there's another factor of cos(angle) to apply.

The catch is that the installation ratio must be squared in order to calculate wanted spring rate. For example, let's say 150 lb/in wheel rate is wanted. The spring rate is 150 / (.75^2) = 266 lbs.

The above is true for cornering or a one-wheel bump. In the case of hitting a bump with both tires simultaineously, it's just the spring rate with none of the above, 150 lb/in. So there's quite a difference in ride rate versus roll rate.

To answer your question, yes, move the springs as far outboard as possible. Of course at some point it becomes an issue of dimenishing returns; where that point is depends on what you're happy with.

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PostPosted: September 29, 2008, 3:28 pm 
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OK, makes sense. THANKS!


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PostPosted: October 5, 2008, 5:28 pm 
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OK, after a bit more thinking, I might be able to just mount the shocks off of the back of the trailing arm bracket.

I made a drawing and animation to check maximum droop, bump and ride height (4.5")

More of the full size drawing. I have been using Adobe Illustrator for this. It's by no means perfect (and only 2D), but I know how to use it well, so it becomes a quick tool for simpler things like this.


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PostPosted: September 16, 2009, 12:21 pm 
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Joined: December 17, 2007, 1:17 pm
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WOW! It's been WAY too long since I have updated my build log!

I've got the rear four link hooked up and working well. Next I need to rig up a panhard bar, and figure out rear shocks. I think the shocks I have are going to be too big both front and rear, so I might just sell them and buy some QA1s that will fit!

I've also been working on the front suspension geometry, and have some questions, so onto the images and labels:

Attachment:
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So is it more, or are the upper mounts too low? They are in the right spot according to wishbone, but my (unedumicated) instincts say they are off.


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PostPosted: September 17, 2009, 8:01 am 
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As a starting point, you want to set your inboard lower control arm mounting points at the same height as the rotational center of the lower balljoint. (measured at desired alignment bolted to desired tire/wheel combo) So yes, just by looking at the Wishbone numbers I can see they are too low.

Another thing of note is the upper balljoint center is only .75" behind the lower which isn't going to give you enough caster for the steering to self center. I designed mine 1" behind the lower and still had to adjust in more caster. It seems extreme but if I did it again I'd put it about 1 1/4" behind the lower balljoint.

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PostPosted: October 5, 2009, 12:22 pm 
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OK, did some revisions, looks much better.

Attachment:
Mobile Photo Oct 5, 2009 12 16 47 PM.jpg

Attachment:
Mobile Photo Oct 5, 2009 12 16 12 PM.jpg


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PostPosted: October 5, 2009, 1:29 pm 
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I'm curious about your front LCA's. It seems like many folks have gone the route of using a larger rectangular member for the front leg of the "A." It seems like that is backwards. The front wheels ONLY see significant loads that point rearward right? (unless of course you're running front wheel drive.) It seems like the real enemy we're fighting off is buckling in compression, and those loads would be the greatest in the rear leg of the A under hard
braking. Why do you not have the stiffer leg on the higher loaded rear?

- I forget who's car it was but one of the Locosts in the Car & Driver article bent his LCA in buckling on the rear arm from exuberant braking...

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PostPosted: October 5, 2009, 1:41 pm 
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Well this is the Kinetic front suspension, so not really my design. I think the theory is that the rectangular LCA is more aerodynamic than just a square tube. So less of a structural issue.


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PostPosted: October 5, 2009, 2:49 pm 
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Joined: April 23, 2006, 8:26 pm
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What's with that positive camber? Is that due to it being a quick mockup, camera angle, or is that for real? You don't want positive camber except in droop when it doesn't matter as much. If that's for real, the upper arms need to be about 1" shorter, but then you'll have to recheck everything with the suspension program.

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Midlana book: Build this mid-engine Locost!, http://midlana.com/stuff/book/
Kimini book: Designing mid-engine cars using FWD drivetrains
Both available from https://www.lulu.com/


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