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Discussion Starter #1
Who ever you are.

See if you can solve this problem for me/ put a formula together.

If you have a 996 swingarm measuring 470mm and it takes a 7 kg spring, how much spring would you need to add if you extended the swingarm out to 495mm to compensate for the extra leverage. I am looking to use this math in differing situations, so I'm looking for some sort of formula.

I am sure that I am leaving something out, but some of you are pretty smart cookies.


thanks
 
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Re: Ok there Mr. Math guy (section8superbike)

jason, you forgot to mention the wingspan of the sparrow, if it is african or capistrano, and the weight of the coconut.

(python joke ... "wot's yer favorite color?")
 
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Re: Ok there Mr. Math guy (mh900e0974)

Red......oh no, I mean blue ahhhhhhahhhhhhhhaaahhhhhhh
 
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Re: Ok there Mr. Math guy (section8superbike)

Shazaam is the answer


You are only increasing is length but a little under 5%, are you altering the connection point of wear the spring sits down and connects to the swingarm or is it purely added to the back?

That will also make a difference to the calcution as you are altering the piviot and if I remember correctly the bending moments about the connections/joints.

You simply can't bag a 5% wigger wire dia spring in as it was be way over the top in stiffness.

If you can give us the points at which it is altered in can be worked out.
 
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Re: Ok there Mr. Math guy (mh900e0974)

Actually... the question was: "What is the air speed velocity of an un-laden swallow?"

King Aurthur said, "What do you mean... African or European swallow...?"

The 'Keeper of the bridge of death' responded: "I don't know." ....and off into the gorge of eternal peril he went.
 
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Re: Ok there Mr. Math guy (Mental998)

I left my slide rule at home. I'll find it tonight and post the answer tomorrow.
 
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Discussion Starter #7
None. Leave the spring as it is. Unless you want to change the total force required to compress the suspension, it should remain exactly the same.
If you want to retain the same ride hight, then that's a different matter, but I'd assume you would simply reset the ride height if you added a section that was not horizontal to the ground anyway?
The longer swingarm will provide a softer effective rate, but is compensated exactly by the additional travel - so unless you limit the suspension movement by some artificial means to not allow it to compress the shock as it did before the change, your overall suspension resistance to bottoming should be the same.

I think.
 
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Re: (Old Baldy)

From an engineering side that is not quite correct, the greater the leverage actuating the spring the the easier the wheel will travel.

Given that most wheels travel probably a good part of that distance horizontally over the chain adjustment life by way of manual adjusters or in this case the eccentric I would seriously doubt you would even need a bigger spring unless of coarse the spring is maxed out for you given rider weight.

Thus far at this point if you were to lengthen the swingarm are you are effectivly doing and creating a more supple ride and at an educated guess you mayhave to reset the spring prload a little to compensate.
 
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Re: (Monstaman)

"From an engineering side that is not quite correct, the greater the leverage actuating the spring the the easier the wheel will travel."

What part is not correct?

I don't dispute that the rate is softer, but remember that the softer rate is exactly offset by additional suspension travel - so the overall force required to compress the spring is the same - i.e it has exactly the same overall resistance to bottoming by added load from full extension to full compression (as long at the travel is not artificially restricted)
 
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Re: (Old Baldy)

I am not sure whether you would have to restrict the wheel travel or not, does the rear wheel on full compression get near the pipes?, I know with the MH it is quite critical as the wheel can touch the numberplate brackets cauing massive damage to the rear and can also touch the exhaust can.
 
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Re: Ok there Mr. Math guy (section8superbike)

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by section8superbike »</TD></TR><TR><TD CLASS="quote">Who ever you are.

See if you can solve this problem for me/ put a formula together.

If you have a 996 swingarm measuring 470mm and it takes a 7 kg spring, how much spring would you need to add if you extended the swingarm out to 495mm to compensate for the extra leverage. I am looking to use this math in differing situations, so I'm looking for some sort of formula.

I am sure that I am leaving something out, but some of you are pretty smart cookies.


thanks</TD></TR></TABLE>

Easy formula.


470/7 = 495/X Subtract 7 from X once you figure out what X is and there you have your answer. This is basic algebra.
 
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Discussion Starter #12
Re: (Old Baldy)

It's not that easy, the added length is additional moment arm and that will require a stiffer spring, but it doesn't end there. The shock works through a linkage that mechanically changes rate as it goes through it's travel. It's the very reason the send you a different rocker when you buy a 25mm longer magnesium swingarm. If you doubt the addition in spring rate, then think about changing a tire on your car using a 1/4" rachet or a two foot breaker bar. It just isn't a linear function, sorry.
 
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Discussion Starter #13
Phil - you're assuming that the rider wants to retain the same effective spring rate as before - that would, of course, require a different (stronger) spring to counteract the additional advantage of the longer lever (swingarm).

That may well be the requirement (keeping the effective spring rate the same as before) - in which case you've ALSO effectively increased the force required to reach full compression (bottoming resistance) of the suspension - which allows either additional weight on the rear, or ability to handle large bumps at a higher speed - BUT, if the requirement is simply to retain the SAME resistance to bottoming from load forces or bump absorption, then changing the length of the swingarm has no effect.

The longer swingarm will result in a softer rate for each inch of travel, but it will still take exactly the same amount of force to "bottom" the suspension as before - it just does it over a longer suspension travel. If it took 1000lbs of force/weight to bottom the rear with the stock swingarm, and you lengthened the swingarm, it will still take 1000 lbs to bottom the suspension afterwards, unless you change the spring itself, or limit the rear suspension movement.

The longer arm has no effect on the linkage ratio, other than a proportionate increase in travel, unless you change the relative sag value (preload), and ignoring the small effect of the swingarm angle difference relative to the horizontal axis.

I suggest that the reason a different linkage would be supplied with the racing arm, is to either change the linkage to a more racing orientated "straight-rate" vs the stock rising rate curve, or to adjust the location of the rear wheel for interference issues at full compression. I'd think it's the former....

All of the above is based on my very rusty high-school math, so I'd be happy to be proven wrong by some math whiz!
 
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Discussion Starter #14
Re: (Old Baldy)

there isnt going to be any real "easy" answer to this problem....

first of all its kinda like a torque issue (force at a displacement)....
you are increasing the length of the wing arm which is effectively
adding to the displacement.... however, Im making an assumption
that the force here will also change because of a change in the
geometry of the bike... weight will shift differently between the
front and the back with the longer swiingarm. (although by
how much... I cant wager a guess.. outside of slight)

And changing the length of the swingarm WILL have an effect
on the spring... I disagree with baldy. Its a complicated
statics/dynamics problem...
 
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Discussion Starter #15
Re: (ducati_atx)

Sorry Old baldy but I believe what you are saying is that the spring as it is will take the same load to compress it, however when you increase the leverage by way off a longer arm it will take less at the wheel end to compress the spring.

When all said and done you could recalculate the bending moment about the spring and work out the additional wire diameter need to remain at status quo, you would probably find a couple of turns on the spring reload would have damn near the same affect.
 
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Discussion Starter #16
Re: (Phil 998)

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Phil 998 »</TD></TR><TR><TD CLASS="quote">It's not that easy.</TD></TR></TABLE>

Yep. The effective rate at the rear wheel is what matters, not the amount of shock shaft travel used, so you'd want a stiffer spring. But the spring acts through a linkage that alters the effective rate depending on where the shock is in it's stroke, and if you're using less shock travel you'll change the whole curve, not just a single number. I'd just scale it for starters - (new spring rate) = (old spring rate)x(new swingarm length)/(old swingarm length) - and see how it feels. This would put you at 7.4 kg/mm, close enough. Or, since it's a small change, just leave it and see how it feels.
 
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Discussion Starter #17
Re: Ok there Mr. Math guy (section8superbike)

There's a real smart guy who's got an answer to your problem but it'll cost you. Check out the kinematic software on Tony Foale's website.
 
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Re: Ok there Mr. Math guy (Just Riding Along)

Jason, you did this just to spin the egg-heads up and watch them go, didn't you.

C'mon, admit it!!
 
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Discussion Starter #19
It is not a straight up formula based on total length of the arm. You are only adding length to the very end, thus only increasing the length from the mount point of the shock / spring linkage back. Granted the pivot point to the shock linkage can remain the same, or you could reposition. There are a lot of dynamics to this. Seems like to keep stock handling characteristics you would proportionately want to move the shock / linkage mount as well. Those are the measurements you need to know.
 
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Discussion Starter #20
When you correct the ride height with the longer W/B and then correct it back you are changing the geometry of the linkage, how much will that change the spring rate? A good question and the longer lever ratio has to cause more movement in the rear so more weight transfer and wheel excursion.

Is far as I can tell, you have to calculate the linkage ratios into the equation.

I know a guy who uses a longer swing arm on a turbo bike and he really cant find a strong enough spring.



Modified by NC Rick at 9:11 AM 6/30/2005
 
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