how to determine rear shock length ?

[84scrambler]

Ok, guys need some help I have a 1979 wr 250 and I need shocks for my restore. I need to know whats the proper way to measure for new shocks . So here is what I did (hope you can follow) the shocks on the bike now measure 14" eye to eye out of the bike. Those shocks are leaky and worn and to low for how I want the bike to end up. So I fabricated some shock extensions out of square tubing to add 1.5" to the top of the shock. The bike sits perfect for my height and weight . The shocks out of the bike measure 15.5" when installed in the bike they measure 15" and when I sit on it they are 14" but remember they are worn out . I guess what I am asking is how do I use this info and order my new shocks and end up with the same measurements as when I am on the bike? Thanks for the help...

 

[Kartwheel68]

I think the stock length for that bike is 15", that is what Works Performance lists for that bike.

http://www.worksperformance.com/pdf/app_guide/dirtbikes.pdf

You can order them as 15", or longer or shorter, ask for Sandy and tell him you want them 15.5" if that is what you want. They build each set of shocks to order and can build any length you want.

 

[Ron]

Rear shock length is going to effect the front fork angle and how the bike steers.

Raising the rear (longer shocks) steepens the fork angle, making the bike steer quicker at the expense of straight line stability. Lowering the rear (shorter shocks) increases the rake on the front forks, adding to straight line stability, but slowing the steering.

If you like the way the bike is with your extensions, what is your reasoning for that conclusion? Are you trying to establish a certain fork angle?

This is also a personal matter. Some people like the front end to ride high some don't. Some like the back end to ride high etc.

Pardon any assumptions I'm making, but you need to take into account is your sag numbers, which determine how the bike sits with the rider aboard.

Sag is the amount the bike compresses the suspension with the rider on board with full riding gear, from being fully extended.

Sag is also used to determine if your spring rates are correct. Having the correct spring rates will keep the bike balanced. Both front and rear should compress equally with the rider in the center of the bike.

The general rule is, you want to use about 25% of your available travel on the front forks and about 33% of the available travel on the rear for your "race sag" (rider aboard).

Preload is used to set your sag. Adding preload will raise the ride height (race sag) and decreasing preload lowers the ride height.

Using preload is where you can determine if your spring rates are correct. Always set your race sag first, then check your "static sag" (how much the bike compresses the suspension under its own weight.

After you set your race sag you should check your static sag. Your static sag should be about 10% of your available travel both front and rear.

If you have zero static sag, your using to much preload to set the rider sag, which indicates your springs are to soft. If you can not achieve the proper rider sag with zero preload your springs are to stiff.

Adding preload does not change your spring rate. It only increases the amount of force required to begin compressing the suspension.

Race Tech has a spring rate caculator on there web-site for determinig your front fork spring rate and Sandy at Works Performance will be able to determine the correct rear springs for you. Most likely he will be using dual rate springs on the rear.

No idea what formula either of them use for choosing spring rates, years of experience.

Hope this helps

 

[mci-owner]

Ron that explanation of the proper suspension setup was very good, you made the complex very simple to understand.

Thanks