Spring length

Classic short-frame models

Spring length

Postby roadbum » Thu Feb 24, 2022 10:14 pm

What is the stock length of the springs in a standard 22.75 front tube? Thank you in advance, Tom.
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Re: Spring length

Postby EKHKHK56 » Fri Feb 25, 2022 5:04 am

Hi, I looked over my K and XL fork spring collection and 19” were shortest with most around 20” to 22” or longer depending on application or aftermarket etc. About 25 sets. XLCH were 2” longer than K.
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Re: Spring length

Postby roadbum » Fri Feb 25, 2022 4:21 pm

I am using pre 1968 shock absorbers and I measure about 19 3/8ths. from spacer to bottom of top retainer. Do these springs require any pre-load? Thank you, Tom.
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Re: Spring length

Postby Ferrous_Head » Fri Feb 25, 2022 6:17 pm

There are (to the best of my knowledge) two sizes of forks.
There are "high road" versions which are 1 inch longer than the "stock" ones.
You can't mix and match the springs. You really need the correct length.
Damper assemblies also cary depending on the version of forks you have.

I struggled greatly with this as I disassembled a set of mismatched forks. One side had the longer spring in it. Took me a while to understand what was going on. Can only presume the bike that it was on handled "funny".
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Re: Spring length

Postby EKHKHK56 » Fri Feb 25, 2022 6:22 pm

Sounds like 19-20” range is stock K with various sagging etc. I would preload just a hair past zero, just so spring is seated always.
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Re: Spring length

Postby Ferrous_Head » Fri Feb 25, 2022 8:12 pm

Spring preload will be set by installing the retainers to the correct depth.

Can't remember off the top of my head but it's in the FSM.
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Re: Spring length

Postby wz507 » Sat Feb 26, 2022 5:46 pm

As Erik noted, spring length for K/XL front forks vary a bit, with many such spring specimens sacked and thus, a bit shorter than the stock length.

LR K-XL Fork Springs.jpg
LR K-XL Fork Springs.jpg (400.01 KiB) Viewed 1200 times

A stock KHK fork tube had a length of 21-7/8” and spring length of 18-7/8”. In this case the spring was compressed about 5/8” when installed on the damper rod/valve body. The image below shows the spring to be slightly longer than the KHK valve body/damper rod assembly when fully extended.

LR KHK fork tube spring and valve body.jpg
LR KHK fork tube spring and valve body.jpg (129 KiB) Viewed 1200 times

Extrapolating to your XL situation with a tube length of 22-3/4”, it wouldn’t be unreasonable to expect the XL spring to be 7/8” longer and a damper rod/valve body combination with commensurate length increase?

Another common replacement spring used in XL front ends with sacked springs is the Progressive Suspension dual rate spring shown at the link below.

https://www.progressivesuspension.com/p ... spring-kit

Below is a comparative spring length image of the 11-1120 spring and a stock KHK spring, which shows the overall length would be workable in the longer XL fork tube.

Stock KHK and Progressive XL replacement spring.jpg
Stock KHK and Progressive XL replacement spring.jpg (69.69 KiB) Viewed 1200 times
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Re: Spring length

Postby Ferrous_Head » Sat Feb 26, 2022 6:46 pm

Any second hand spring you use will have some "sag".

Will that matter ? Probably not. The length of the spring and the preload as well as the spring rate will determine "Static ride height"

This is the height the bike sits at when seated rider is installed. You will notice the bike drops a small amount when you sit on it, in other words.
But if you weigh 350 lbs and you decide to put a rider on (for the next race) ho only weighs 97 lbs does that mean the static ride height will change ?

Yes it does.

If we were really concerned we would modify the spring length or the pre load to bring the bike back to where we want it to be.

The spring pre load on these forks is set by installing the retainer 9/16's down from the top of the fork tube. according to the factory that is. But that's for an "average" rider under "average" conditions.

With "sagging" springs the most likely impact is that they will "bottom out" with a 350 lb rider going over pot holes or big bumps in the road.
A 97 lb weakling may have no issue at all with these sagging springs.
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