How to ID the spring rate on a rear spring

[SULLY]

RM 2003 125. Rider has just gone up from 85cc and is light (approx 125 lb). I think the bike has had a stiffer rear spring fitted and I cannot set the race sag without the spring being too loose, so I assume it is too stiff for his weight, but how can I tell what spring rate it is, as it is not marked in any way. It is red.
I guess in theory to put a known weight on the spring and measure it's deflection is one way, but can this be relied upon in practice? I just need to know what it is, so I can work out what lighter rate one to get.

Also as a second Q, is there a danger of going too light on the rear spring?. IE If I get an appropraite spring for his small size, which allows me to set the correct race sag, is there a danger of it deflecting too much under the bike/rider weight when landing off jumps?

 

[elf]

Not sure about suzuki but yamaha has a small paint stripe on the end of the spring. different color for different spring rates. check your owners manual.

 

[terry hay]

Sully
The 03 RM125 came out with a 4.7kg/mm spring which will be too heavy for your rider. Racetech have a 4.4 and a 4.2 available. I would tend to go for the 4.4 myself as your rider would probably fall between these two rates. Unfortunately kids have a habit of growing and it wouldn't hurt to have a little room to accommodate this.
Terry

 

[SULLY]

Thanks guys, I have ordered a suitable spring.