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PostPosted: April 10, 2020, 1:19 am 
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Hey guys, just looking for some input from people actually running inboard suspension with motorcycle shocks.
How much actual wheel travel do you have?
What spring weight do yours use?
What disadvantages?
Would you build it the same again?

I’m considering using an inexpensive option for my pushrod shocks, but looking for opinions. The ones I’m considering have 3” of travel, 4 way adjustable but only a 450# spring.

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PostPosted: April 10, 2020, 12:31 pm 
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Finding someone that did that, knows all that, and is still on the forum regularly after completing their build is probably asking too much. Even if you try to pm or email, they still may not get it.

I'd be looking at running it with a 2:1 rocker to act like a 225lb rate with 6 inches of travel, keeping the coilover as close as practical to the rocker so the rocker doesn't end up very long. Rocker arm 90 to the 1/3 to 1/2 compressed coilover at ride height with minimum preload. Make the double shear plates bolt-on so you can just make new plates instead of the whole rocker in the future. You can put the lca pushrod bush further out and the pushrod more vertical without hitting the uca in droop, which will help with a long arm rocker.

You can mock it up with some spare tubing and bolts to figure out travel and if you have the coilovers, check everything for the concept before making anything.

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PostPosted: April 13, 2020, 5:28 pm 
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6” of travel sounds like an awful lot to me. Is that what most people aim for? Does that 225lbs rate sound normal as well? Not sure what my busa Willy’s will weigh yet, I don’t think much over 1000 if at all.

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PostPosted: April 13, 2020, 5:53 pm 
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Get the basics down so you can better understand how to adapt coilovers that are not the best choice.
viewtopic.php?f=5&t=11815&p=112797

It is more about the reduction in damping and rate than using all the travel available in the coilover.
Four inches of compression and two inches of droop from ride height is reasonable. You don’t want to run out of suspension travel.

Early miatas are twice as heavy and come with springs around 200# but have a highly effective rate due to the mounting angle. Tilting a high rate low travel coilover for a poor effective rate is another way to cope instead of rockers.

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PostPosted: April 13, 2020, 6:27 pm 
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I went thru a lot of this in the Bellcranks post. I based it on a 5" shock travel and a 1:1 at-rest shock to wheel motion travel ratio. I figgered this with vectors and can try to recreate my notes if interested. Vectors are cool for this.


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PostPosted: April 13, 2020, 6:38 pm 
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When you're designing the length of any arm of the bellcrank, I suggest it be long enough so the maximum angle of rotation be 60-degrees or less to accomplish the required stroke, to prevent going over-center.


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PostPosted: April 13, 2020, 6:39 pm 
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When you're designing the length of any arm of the bellcrank, I suggest it be long enough so the maximum angle of rotation be 60-degrees or less to accomplish the required stroke, to prevent going over-center.


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PostPosted: April 13, 2020, 7:33 pm 
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A 450# spring will be reduced to a 112# wheel rate with a 2:1 motion ratio.
I machined new perches for the Yamaha shocks and ran 1000+# springs on the Wycked7.
I am running 500# springs on the SkaytR6 which is an under 1000# car. Setup for less than 3" wheel travel.

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PostPosted: April 13, 2020, 8:15 pm 
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Kartracer47 wrote:
A 450# spring will be reduced to a 112# wheel rate with a 2:1 motion ratio.
I machined new perches for the Yamaha shocks and ran 1000+# springs on the Wycked7.
I am running 500# springs on the SkaytR6 which is an under 1000# car. Setup for less than 3" wheel travel.


That’s what I was afraid of. Though I’ve read through most of the suspension stuff on here, I still don’t understand the 4:1 ratio you’re mentioning and others mention too.

If I setup my wheels to travel 4” total, and my pushrod at the end only moves 2” in the plain of travel intended, I should be able to build my rockers to be a 1:1.5 rate at the coil. (My wheels would hit fenders at current ideal ride height with 2” bunp based on minimum OD) This would mean I have 337.5# at the wheel right?? I’m not ordering shocks until I have this completely figured out as it’s one of the most expensive portions.

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PostPosted: April 14, 2020, 6:42 am 
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Kartracer47 wrote:
A 450# spring will be reduced to a 112# wheel rate with a 2:1 motion ratio.


I don't think his motion ratio will be that bad. I was referring to the rocker ratio.

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PostPosted: April 14, 2020, 11:28 am 
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Wait a minute. Don't we have to square the motion ratio to get the spring ratio? A 2:1 motion ratio gets you a 4:1 spring rate ratio? I have to go back to school for this one.


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PostPosted: April 14, 2020, 1:15 pm 
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A guy who is ~#2 at the AutoX Nationals removed the pushrod/rocker activated shocks and installed conventional shocks and went faster.

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PostPosted: April 14, 2020, 1:39 pm 
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One can safely assume then that by doing so, he improved the suspension geometry; it does not mean that rocker suspension is "bad", it just means that it was designed wrong.

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Last edited by KB58 on April 14, 2020, 7:05 pm, edited 1 time in total.

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PostPosted: April 14, 2020, 4:20 pm 
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You can make the spring rates progressive or regressive with bellcranks. A regressive rate would screw up the handling. I would like to see the angles in his bellcranks.


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PostPosted: April 14, 2020, 8:02 pm 
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Velix wrote:
I’m considering using an inexpensive option for my pushrod shocks, but looking for opinions. The ones I’m considering have 3” of travel, 4 way adjustable but only a 450# spring.
I've done the math, and even bought a Hayabusa coilover to work with, but not pursued it beyond that. It can work, but has some pitfalls to be careful of, and is very application dependent. First issue of concern being is your 3" of travel total shock travel without a bump stop, or free travel before contacting the bump stop?

As far as bike shocks go, the research I did at the time, the Hayabusa shocks seemed to have the best combination of spring rate and travel. The 1st gen were 732 lb/in with 2.2 inches of travel before contacting the bumpstop, and I believe the 2nd gen upped it to 810 lb/in with the same travel. So that's 1610 lb total force for the early, and 1782 lb total force for the late. A 450 lb/in spring with 3 inches travel maxes out at 1350 lb total force, and considerably less if there is a bumpstop cutting into that too.


Velix wrote:
6” of travel sounds like an awful lot to me. Is that what most people aim for? Does that 225lbs rate sound normal as well? Not sure what my busa Willy’s will weigh yet, I don’t think much over 1000 if at all.
The honest answer is, it depends. Remember that's total travel, accounting for all available droop, bump, and maybe even bump stop. So it doesn't go as far as you might think, especially on a street biased suspension. So while a 3" bump/droop (6" to bumpstop) suspension is not unreasonable, neither is a 2" bump/droop (4" to bumpstop), depending how stiff you want the ride to be.


Velix wrote:
That’s what I was afraid of. Though I’ve read through most of the suspension stuff on here, I still don’t understand the 4:1 ratio you’re mentioning and others mention too.

If I setup my wheels to travel 4” total, and my pushrod at the end only moves 2” in the plain of travel intended, I should be able to build my rockers to be a 1:1.5 rate at the coil. (My wheels would hit fenders at current ideal ride height with 2” bunp based on minimum OD) This would mean I have 337.5# at the wheel right?? I’m not ordering shocks until I have this completely figured out as it’s one of the most expensive portions.
A 4:1 spring force ratio only occurs if you have a 2:1 motion ratio. The spring force ratio is the square of the motion ratio. This is because spring force is displacement dependent. So you're applying the motion ratio once for the force leverage and again for the displacement leverage.

So you're looking at an overall ratio that compresses the shock 3 inches for 4 at the wheel. That's a 1.333 motion ratio, netting a 1.778 spring rate ratio that reduces your 450 lb/in springs to 253 lb/in at the wheel...Which may or may not be in


Miatav8,MstrASE,A&P,F wrote:
Kartracer47 wrote:
A 450# spring will be reduced to a 112# wheel rate with a 2:1 motion ratio.


I don't think his motion ratio will be that bad. I was referring to the rocker ratio.
I don't follow. A rocker ratio is by definition a motion ratio, and mathematically works out the same. So a 2:1 rocker ratio that gets you 2x the travel, does so in exchange for 1/4 the rate. Of course, this now gets compounded with the remaining control arm ratio and pushrod angle effects, to an even more severe overall ratio.

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