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PostPosted: June 30, 2021, 7:43 pm 
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Essentially, my suspension design is fixed in terms of the basic design. However, one of the few variables I have left is the motion ratio. In the graphic below that I swiped off the Internet, my B, C and D distances are fixed. I do have some latitude in determining distance A, the distance from the lower coilover mounting point center to the pivot center of the lower A-arms.

Using the approximation formula at the bottom of the graphic below, are there any ideal, or rule of thumb values, one should target?
Attachment:
Motion Ration Approximation Graphic.jpg


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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


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PostPosted: July 1, 2021, 7:38 am 
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As close to 1:1 as practical to minimize the LCA weight required to prevent bending at the coilover mount and maximize coilover piston travel (spring rate and dampening. This means the difference between A and B and the diff between C and D should be minimized. You can tilt the coilover if needed to compensate for insufficient piston travel or excess spring rate. When tilt isn’t enough, rockers can be used to make nearly any coilover work. If you have the coilovers already or have a specific one and rate in mind, you can design around that.

Place the coilover brkt as far outboard as practical, mock up the coilover angle to support the car at ride height, then check that the uca clears the coilover at full droop and clears the spindle at full steering lock. You can fit travel stop bushings to the rack inside the bellows or spindle rotation stops on the LCA.

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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12


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PostPosted: July 1, 2021, 7:42 am 
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In very simple terms, the further inboard your spring is on that diagram, the less resistance it will give you in roll, relative to the ride rate of the springs.

...which means the more anti-roll (sway) bar stiffness you will need to compensate.

Anti-roll bars reduce grip. They are the work of the Devil, and should always be kept to the minimum size possible, so the answer to your question taken in isolation is that dimension (a) should be as large as you can contrive it to be.

HOWEVER, if - due to the chassis location of the upper end of the coilover - this means that you have to lean the spring/damper unit over at a large angle from the vertical, you get into conflicting issues with potential geometric falling rate on the suspension, which is also a bad thing. Hence, to some degree, you pays your money and you takes your choice over the compromise between the two (...unless you adopt pushrod suspension, in which case you can have your cake and eat it, 'cos you can control the rising/falling rate by means of the bellcrank alignment).


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PostPosted: July 1, 2021, 9:47 am 
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I agree with MV8. I'll also add that coil over springs naturally put a side load on the shock shaft. This adds friction/stiction and negatively affects damping. The stiffer the spring, the worse it gets. In other words, the farther you get from 1:1, the more spring rate needed to maintain the same wheel rate, which in turn increases the side loading on the shock and decreases the quality of damping. Aim for as close to 1:1 as practical.

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PostPosted: July 1, 2021, 10:57 am 
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@MV8
@Sam_68
@RTz

Thank you, gentlemen. You each added something significant, and it's all pretty darn concise. Now, let's see how close to 1:1 I can get the A-arm side mount in practice.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


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PostPosted: July 1, 2021, 12:38 pm 
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To account for the coilover angle in your motion ratio, multiply by the cosine of the angle off perpendicular relative to the lower control arm. If bump travel brings the coilover closer to perpendicular with the lower control arm, it's rising rate. If bump travel brings the coilover further from perpendicular with the lower control arm, it's falling rate. My recollection is that most traditional Locosts have the coilover leaned over in a falling rate configuration. One option to deal with the angle would be something like this: https://www.locostusa.com/forums/viewtopic.php?f=5&t=1612

Sam_68 wrote:
In very simple terms, the further inboard your spring is on that diagram, the less resistance it will give you in roll, relative to the ride rate of the springs.
This is true of solid/beam axles, since the motion ratio inherently differs between ride and roll. However, this is not applicable here, as the motion ratio of a double wishbone is independent of whether ride or roll is causing the wheel displacement.

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Last edited by Driven5 on July 2, 2021, 11:30 am, edited 3 times in total.

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PostPosted: July 2, 2021, 6:26 am 
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Mid 0.6 is about the absolute minimum. Beyond needing heavier springs to achieve acceptable wheel rates, the shock's shaft speed is reduced as the motion ratio decreases. The piston needs velocity as it moves through the oil to create damping and low motion ratios result in this speed being too low.

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PostPosted: July 2, 2021, 10:01 am 
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Of course, if using an existing coilover such as a mono on a sportbike with very high rate and little travel, the piston passages are likely very restrictive and appropriate to the rate and travel.

_________________
Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12


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PostPosted: July 3, 2021, 10:19 pm 
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@Driven5
Thanks Justin. I don't see any practical alternative to inclining my coilovers for the falling rate scenario. The good news is that its been done for years on the 7/Caterham/Locost builds, and there isn't that much wheel travel on a 7-like car. The design Jack detailed is not practical for me at this point. I'm at 47 degree inclination now, just for the reccord.

@a.moore
Hmmmmm! I'm just at 0.6 with my latest simulation of the LCA/coilover. I've kept the upper end of the coilover mount as close to the upper chassis rail as practical. We've all seen the cautions about cantilevering out too far for those mounts. I could maybe get the upper mount out towards the wheel another inch (lessen the coilover angle), and I think my newest LCA design gets the lower coilover end out another 1/2" closer to the spindle, but I haven't run any numbers with those two changes yet.

Wouldn't shocks adjustable for bump and rebound give me some help dampening at a lower motion ratio?

Thanks, Andrew.

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


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PostPosted: July 8, 2021, 12:10 pm 
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To an extent yes. Regardless of how much damping your shock can theoretically provide, you still need shaft velocity for it to occur. With really low motion ratios (lets say below 0.5), wheel vertical displacement only translates to a small amount of piston movement and this movement can be entirely taken up by any "slack" within the damper or the installation like bushing play or just the amount needed to start opening and closing valving. There is a point where that small amount does not move the piston enough to do anything useful.

Speaking from past experience, it would have been easier to correct the geometry rather than trying to make a shock that works. Even after re-valving and using thicker oil, it was still not enough and they wore out quickly from the harsh environment.

I'd say you are safe at 0.6 but I wouldn't go any lower.

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PostPosted: July 8, 2021, 3:17 pm 
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@a.moore

Thanks, Andrew.

I managed to squeak out a little more, and I'm up tp 0.616 currently. It's maybe possible I can get a small amount more, but we're still talking ~ 0.625 or so. To get closer to the wheel, I'd have to go up and out with the A-arm mounting bracket, which would move the coilover towards the horizontal more, which has its own issues, so not worth doing me thinks.

I did find info about some race cars being in the range of 0.60 to 0.70, so it's not a fatal flaw, and may just be the compromise I just have to accept.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


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PostPosted: July 8, 2021, 3:58 pm 
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Will your UCA allow you to go up-and-out with the upper coilover mount?

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PostPosted: July 9, 2021, 8:03 pm 
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Driven5 wrote:
Will your UCA allow you to go up-and-out with the upper coilover mount?


This is something I did with my build. It can be challenging to get everything to clear through the full range of alignment possibilities, but it's effective.

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PostPosted: July 10, 2021, 2:54 pm 
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@Driven5

I'm not sure yet, Justin. We'll have to see what space is available when the UCA is finalized, and the coilovers are selected. The upper coilover mount is the last thing I'm going to design and fabricate.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


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PostPosted: July 10, 2021, 7:34 pm 
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Lonnie,
In order to get more clearance between UCA and coilover spring, I've switched from 2 1/2" ID springs to 2 1/4" ID springs. On the QA1 Promastar shocks, there's enough meat on the spring perches to turn them down to accept the smaller ID. Koni makes threaded collars/perches for both sizes, but the ID of the 2 1/4" version is smaller also. The shock/spring combos that Jack sells come with a spring that's just under 2" ID (IIRC). So, several ways to address the space limitations.

Ron


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