Rear spring question...
 

Hello hello. Right, just got a second hand shock for mi bike that came with 2 springs. The original and another one which has the same info printed on (ie same diameter/flexibility rating etc) but is approx 1 in shorter when fitted. How will this effect the shock? I like the idea of a shorter spring as it allows me to fit the hydraulic preload jobby off another shock. Cheers as always. K.

Well you've lost an inch of preload or even travel before you start with your "hydraulic jobbie"...If as you imply it's an inch shorter than the standard spring.....strange, I would ask why ??

A shorter spring even with the same wire/spring rating will have shorter travel range. Think about it becoming 'coil bound' (where the wire touches the wire above and below it), you loose the travel of the longer springs 'space between the wire' of the bit that is missing in the shorter spring.

If the spring is 1" shorter and the wire is 1/4" thick and two turns for the missing 1" you will loose 1/2" of travel (= 1" - 1/4" * 2).

But why would someone fit a shorter (but otherwise the same) spring instead of the standard one? That's what i want to know. There must be some benefit..?? Will it act like a stiffer spring, for carrying luggage etc?? (Surely springs don't compress to the extent that they become 'coil bound' on a motorbike shock?)

Lowering the bike would be the only good answer I think - less spring, and it sits lower for a short person. Are the forks pushed up in the yokes? If so you'll want to find out what the correct position is.

And yes, springs can and do become coil bound if not the right one. Do some measuring and you'll be surprised how close to coil bound they are at max compression. Shock (not suspension) travel minus the total of all the coils = clearance between springs at max compression.

Any number of reasons;
Lower seat height.
Fitting spring preload thingy ...

If it is a 'good idea' is another matter. Might be 'good' for them and bad for you. Depends on the use and abuse that it is subjected too.

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If the shock limits travel before the spring becomes coil bound depends on the shock and coil. You can determine that by measurement.
You can also see if the shorter coil 'tops out' with the shock fully extended - is the coil still applying some pressure to the seating surfaces?

You should be able to think these things through for yourself by simply looking at the coils and shock.

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A spring with the same rating (amount of force it takes a compression or extension spring to travel a proportionate amount of distance) will not act like a stronger or weaker spring simply by shorten it .. I assume you mean by cutting a bit off it, not cold forming it. If it is cold formed then the spring rating may well change.

btw, yes a shorter spring identical in all other respects to another will be stiffer. Think of a long piece of wood, like a 2'x4", at 12 feet long it's got a fair bit of flex. Just try and bend a one foot piece.

This works well if you're shortening a spring for a shorter rider, as in order to prevent bottoming out you WANT a stiffer spring. The negative is that it is stiffer all the time, meaning not as plush a ride. As always, trade-offs.

Humm your right
I used https://www.acxesspring.com/spring-calculator.html to runs some numbers.

A 10% reduction in height would be ~10% increase in spring rate.