Spring help

[nathanm]

Bought jayne a DH bike and trying to get it set up for her weight. It's got a vivid shock with a 550x2.75 spring.
I bought a 53x67.5-75mm 300lb spring to replace it, but it's much physically longer and I would need to compress it just to get it onto the shock and the thread collar started on the threads

Did I get the wrong spring or do I indeed need to somehow compress it onto the shock?

 

[T-Rex]

Compressing it will preload it and make it stiffer. Also you need to make sure it doesn’t coil bind at full compression this could result in something breaking. Eg frame, rider, your love life.

 

[Staunch]

Wrong spring, shouldn't need to compress just to get the collar onto the threads.

What are the dimensions of the shock?

 

[teK--]

Bought jayne a DH bike and trying to get it set up for her weight. It's got a vivid shock with a 550x2.75 spring.
I bought a 53x67.5-75mm 300lb spring to replace it, but it's much physically longer and I would need to compress it just to get it onto the shock and the thread collar started on the threads

Did I get the wrong spring or do I indeed need to somehow compress it onto the shock?

you need a shorter spring

 

[nathanm]

Wrong spring, shouldn't need to compress just to get the collar onto the threads.

What are the dimensions of the shock?

8.75x2.75 I believe. 2007 demo8

 

[Staunch]

Given the dimensions you posted, I'm assume you purchased a 'metric' spring.

Now that newer shocks are going to metric measurements, 67.5-75 is a common stroke size for a 250mm eye-to-eye shock, in which case a 174mm spring (which is the length of a Rockshox 67.5-75mm spring) isn't too bad. In the case of an older 8.75-2.75, which is only 222mm eye-to-eye, that 174mm becomes a little too long.

Easiest solution would be to buy a 2.75x300lb spring specifically (which should be shorter in overall length), instead of a new metric 67.5-75mm (even though logically 67.5mm = 2.75")

TL;DR
'Metric' and 'imperial shocks' have different spring lengths, even for the same stroke

 

[nathanm]

Given the dimensions you posted, I'm assume you purchased a 'metric' spring.

Now that newer shocks are going to metric measurements, 67.5-75 is a common stroke size for a 250mm eye-to-eye shock, in which case a 174mm spring (which is the length of a Rockshox 67.5-75mm spring) isn't too bad. In the case of an older 8.75-2.75, which is only 222mm eye-to-eye, that 174mm becomes a little too long.

Easiest solution would be to buy a 2.75x300lb spring specifically (which should be shorter in overall length), instead of a new metric 67.5-75mm (even though logically 67.5mm = 2.75")

TL;DR
'Metric' and 'imperial shocks' have different spring lengths, even for the same stroke

Cheers mate, perfect explanation, appreciate it.