Date: Wed, 1 Jul 1998 20:32:04 -0600
Reply-To: kenstich <kenstich@BEWELLNET.COM>
Sender: Vanagon Mailing List <Vanagon@vanagon.com>
From: kenstich <kenstich@BEWELLNET.COM>
Organization: Central Intelligence
Subject: Re: spacers and sproings
Content-Type: text/plain; charset=us-ascii
Fred,
Thanks for adding more clarity on an apparently murky subject - I'm adding
some comments below to splain why albeit with an engineering spin.
Fred Porter wrote:
>
> > If it can be done to a motorcycle, why not to each wheel on a car? Truth is
> > that it is in some applications...no reason why it can't be done here.
> > Pre-load is no different than inserting a stiffer spring...it is stiffer
> > throughout the range of travel.
>
> ***you obviously don't understand springs or are using 'stiffer' the
> wrong way.
Yep - all coil springs have a 'free length" that is the length in the unloaded
state. Any deflection from the free length is proportional to the force,
either tension or compression, up to the point of the elastic limit of the
spring material. The constant of proportionality is called the spring
constant. English units for a spring constant are frequently inches/pound.
Most spring geometries result in this proportional or linear force/deflection
relationship. Most folks equate the term 'stiffness' with the spring constant.
Preload only increases the force the spring pushes back by a
> constant amount through the range of travel. If the spring has no
> preload and the mechanism the spring is in is extended all the way--your
> motorcycle with no one sitting on it and no turns on the adjuster knob,
> or the rear of the van jacked up, wheels off the ground--the spring is
> neutral, exerting no push or is extended all the way. Inserting spacers
> or adding turns on the adjuster knob only means that when the mechanism
> the spring is a part of is extended all the way, the spring will still
> be compressed some and exerting 'push' forces. Conversely, if you
> stretched the spring (elastically) and extended the mechanism the spring
> would be exertind 'pull' force.
>
> The stiffness of a spring is a physical characteristic of the spring
> that you won't change by stretching or compressing the spring within its
> elastic range of use (all else being equal). Now if you stretch the
> spring to the point where you actually bend the coils, i think that
> would make the spring 'stiffer'. Take apart your ball point pen and
> play with that spring a bit.
When one exceeds the elastic limit of the the material, the material 'yields'
and this is called the plastic region - this is a no-no in spring design and
is why automobile spring applications have limit stops to limit compressive deflection.
>
> Don't confuse the amount of force you get from the spring at various
> points through its travel and stiffness...they are two TOTALLY different
> things. And don't do rediculous thought experiments like inserting 12
> inches of spacers cause that isn't a realistic application--you'd bend
> something.
> >
> > If Harold's spring's are not stiffer, why does it handle better in turns and
> > not roll so much anymore. Sag adjustment is supposed to do exactly what
> > Harry did.
>
> Because you're just preloading the spring and it will take more force
> to get the same initial compression and overcome the preload. youwill
> still get the same amount of spring compression for a given force
You know, preload just doesn't have any bearing on the above - the preload
force is still proportional to the preload length - the preload length is full
shock extension - but a vehicle at rest is loading the springs with its weight
- weight is a constant value for a vehicle at rest - the key parameters for
spring design are free length and the spring constant - and then the loading
characteristics - both static and dynamic. The dynamic characteristics of the
vehicle have as much or more to do with damping (shocks) anti-sway torsion
springs and tires.
Steven's concept of stiffness as it relates to preload is interesting:
- because preload increases the force required to produce additional spring
deflection (a motorcycle spring with 300# of preload requires a force greater
than 300# to produce any additional deflection) A bike could be set up so that
the preload is greater than the weight of the bike so that at rest, the bike
springs fully extended against the lower stop - When riding with a high
preload, it feels to Steven like the spring has additional "stiffness" -
Although the spring constant has not changed, the bike rides much harsher
becuase of any added preload and reduced deflection/travel - I can understand
the perception
> however.
> >
> > Anyway...seems like this will never be resolved through debate....I already
> > pissed derek off and probably others, too.
Hope he gets over it -> just boys having good clean fun.
>
> ***Right, cause i think some fundamental understanding of springs is not
> present.
> >
> > I know from 25 years of tuning suspension on bikes that I am right. That's
> > good enough for me. :~)
>
> That's like saying that my 13 years of driving with a clean record means
> i know the motor vehicle laws. Not really, it just means i haven't got
> caught. I think your 25 years of experience has given you a well tuned
> tush, but i don't think it necessarily means you understand springs in
> interms of the spring response, stiffness and preload
> (math/engineering). You know how to adjust things to get the ride you
> like. But isn't the tush factor what's important as long as you aren't
> compromising safety or reliability?
>
> i really think that from the content in the thread at least Larry and
> Steve would benefit by getting a book from the library about springs,
> not a web site from yamaha or whatever, but something about springs that
> explains forces, stiffness (spring constants), preload, and spring
> response. Just statics will do. You guys throw around preload,
> stiffness, travel and force interchangeably in your thot experiments and
> they aren't interchangeable--they are unique quantities with their own
> definitions.
>
> later,
> fred
Ken's Final Komments:
Its unfortunate that one cannot always take experiential data and translate
that into intuitive understanding - but if that were the case, I wouldn't have
had to spend 10+ years in college. Toilets flushed in the northern hemisphere
twirl one way - in the southern hemisphere the opposite way. I needed a course
in Dynamics to actually understand Coriolis acceleration and why this was so -
I couldn't tell from looking at the toilet - Some principles just don't jump
off the page - Spring design is not rocket science, but ultimately, to fully
understand mechanics, one needs to apply a little physics and math. I've
enjoyed this thread, but I think this is my final put - You folks take care.
Best Regards,
Ken Stich
80 Vanagon-L
85 Prelude Si
57 Beetle
