Fork spring questions

[balky1]

Hi!
The time has come to replace old springs in my 85 front fork. I have a shop where I can have them tailor-made, but they need the old ones as a size reference. Since they are compressed, their size isn't relevant. The questions are:

• does any one know what was the length of OEM springs when new and their rate (I think they were progressive so if the rate question is silly, disregard it)?
• a recommendation is needed. Straight rate or progressive ones for forks without any valves/emulators? For straight rate springs Race Tech recommends 0.95 kg/mm for a rider of my size (without equipment and passenger), comments? 

I can not afford any valves/emulators and/or factory-made springs so this is the reason for it. A set here would cost me around 35 USD. 

[balky1]

I have found on some site progressive ones with rating of 35/50 lbs/in. If I remember correctly, I should divide it with 56 to get kg/mm? If so, those are too soft and might be the stock rate.

[FJ1100mjk]

According to the Factory Service Manual, the OEM spring's free length is 383mm (15.08 inch). It shows it is dual rate stiffness, 0.42 kg/mm for 0 - 100mm of initial stroke, then 0.6 kg/mm for 100 - 150mm remaining stroke.

If someone here has a set of Race Tech springs out of their bike, perhaps they can provide their free length value. They may be longer. According to the Factory Service Manual, the spring spacer "collar" is a 100mm long. I put Race Tech springs in my FJ1100, and I thought (not sure though), that the spacer I cut for them, was less than 100mm, so I'm thinking that the Race Tech springs are longer than OEM ones. I put the same spring rate ones in that you have, and I weigh right around what you do. New, stiffer, straight rate springs, and good fresh oil in the forks are a noticeable improvement in fork action.

35 USD for the springs are a real bargain.

[balky1]

According to the Factory Service Manual, the OEM spring's free length is 383mm (15.08 inch). It shows it is dual rate stiffness, 0.42 kg/mm for 0 - 100mm of initial stroke, then 0.6 kg/mm for 100 - 150mm remaining stroke.

If someone here has a set of Race Tech springs out of their bike, perhaps they can provide their free length value. They may be longer. According to the Factory Service Manual, the spring spacer "collar" is a 100mm long. I put Race Tech springs in my FJ1100, and I thought (not sure though), that the spacer I cut for them, was less than 100mm, so I'm thinking that the Race Tech springs are longer than OEM ones. I put the same spring rate ones in that you have, and I weigh right around what you do. New, stiffer, straight rate springs, and good fresh oil in the forks are a noticeable improvement in fork action.

35 USD for the springs are a real bargain.

Thanks for the info!
Yes, it is a bargain. It is cheap enough that I am willing to risk 35 bucks to see the quality. I will report how they behave once I install them. I am looking to order the OEM length springs to be able to retain the spacers because I don't have a lot of tools nor space at my disposal to complicate things for myself.
One more question now pops for me. How do you determine the length of spacer needed if you install springs different in length than the OEMs?

[FJ1100mjk]

One more question now pops for me. How do you determine the length of spacer needed if you install springs different in length than the OEMs?

Do a Google search with this line: race tech fork spring installation instructions

From the results, there are documents (PDFs) from Race Tech that you can download, and they'll answer your question(s).

[balky1]

 

[Pat Conlon]

a recommendation is needed. Straight rate or progressive ones for forks without any valves/emulators? For straight rate springs Race Tech recommends 0.95 kg/mm for a rider of my size (without equipment and passenger), comments?

Replacing fork springs was one of my first mods I did to my '84. Race Tech Cartridge Emulators were not yet on the market.
I was warned not to use straight rate springs with the stock damper rod forks. The ride will be too harsh.
The damper rod can not control the heavier straight rate springs. There is a reason the oem Yamaha springs are progressive and not straight rate.

Later in the '80's when Race Tech came out with their Cartridge Emulators yes, straight rate springs were specified. The Emulators had better control vs the oem damper rod setup.

So, back in the old days, before I had Cartridge Emulators, I installed progressive springs made by Progressive Suspension.
http://www.progressivesuspension.com/product/1465/fork-spring-kit


Sorry, its been so long ago, I don't recall the spring rate. I do recall that it was an improvement over the soft Yamaha oem springs.

Using Marty's information on the oem Yamaha progressive springs I would stay with the progressive nature and just increase the values.
For example 0-100mm use .5kg/mm and 100-150mm use .7kg/mm

I can almost guarantee that you will not be happy with .95 straight rate springs in the oem damper rod forks.

Hope this helps

[edit] I just got off the phone with Frank from Progressive Suspension and he informed me that the progressive springs they sell for the Yamaha FJ100 start 1-100mm .63kg/mm and 100-150 .89 kg/mm... so... I would use these values on a progressive spring.
Make sure you are getting good high quality spring steel.

[Alf]

The valve cartridge emulators are designed for straight rate springs, I see. Randy, Progressive and Race Tech design that way.

But most Yamaha bikes like the YZF 600 with cartridge forks use progressive rate springs

Does anyone have tested progressive springs with cartridge emulators?

[balky1]

a recommendation is needed. Straight rate or progressive ones for forks without any valves/emulators? For straight rate springs Race Tech recommends 0.95 kg/mm for a rider of my size (without equipment and passenger), comments?

Replacing fork springs was one of my first mods I did to my '84. Race Tech Cartridge Emulators were not yet on the market.
I was warned not to use straight rate springs with the stock damper rod forks. The ride will be too harsh.
The damper rod can not control the heavier straight rate springs. There is a reason the oem Yamaha springs are progressive and not straight rate.

Later in the '80's when Race Tech came out with their Cartridge Emulators yes, straight rate springs were specified. The Emulators had better control vs the oem damper rod setup.

So, back in the old days, before I had Cartridge Emulators, I installed progressive springs made by Progressive Suspension.
http://www.progressivesuspension.com/product/1465/fork-spring-kit


Sorry, its been so long ago, I don't recall the spring rate. I do recall that it was an improvement over the soft Yamaha oem springs.

Using Marty's information on the oem Yamaha progressive springs I would stay with the progressive nature and just increase the values.
For example 0-100mm use .5kg/mm and 100-150mm use .7kg/mm

I can almost guarantee that you will not be happy with .95 straight rate springs in the oem damper rod forks.

Hope this helps

[edit] I just got off the phone with Frank from Progressive Suspension and he informed me that the progressive springs they sell for the Yamaha FJ100 start 1-100mm .63kg/mm and 100-150 .89 kg/mm... so... I would use these values on a progressive spring.
Make sure you are getting good high quality spring steel.

Thanks for the values and recommendation! I thought by reading some older posts here that straight rate will be too harsh.
Regarding quality. I've read some good comments about that manufacturer on the local motorcycle forum. It's a firm that makes all kinds of springs for industrial and personal use so I will give it a shot. Furthermore, if the springs show up as a good quality, I'm thinking of replacing a rear shock spring, too (if it is possible to disassemble it).

[Pat Conlon]

..... anyone have tested progressive springs with cartridge emulators?

Hola Alf, not me personally but I recall back in the mid '90's some of the Net Path FJ folks did....(pre-Yahoo)
They already had the aftermarket progressive springs in their FJ and they did not want to (or see the need to) spend another $100 for new straight rate springs when they installed the Race Tech Emulators.

With aftermarket progressive springs you still get *some* fork dive with hard braking, although much less than the soft oem springs.
With the Emulators, they found that they still had the progressive spring fork dive. Just the nature of the springs.

When they went to straight rate springs, the fork dive was much less.

[simi_ed]

The valve cartridge emulators are designed for straight rate springs, I see. Randy, Progressive and Race Tech design that way.

But most Yamaha bikes like the YZF 600 with cartridge forks use progressive rate springs

Does anyone have tested progressive springs with cartridge emulators?

I have the YZF 600 front end on my FJ, with RaceTech revalving & straight rate springs.  This is the way to go with RaceTech goodies, otherwise YMMV.  No data on progressive springs ...

Ed

[racerrad8]

The valve cartridge emulators are designed for straight rate springs, I see. Randy, Progressive and Race Tech design that way.

But most Yamaha bikes like the YZF 600 with cartridge forks use progressive rate springs

Does anyone have tested progressive springs with cartridge emulators?

Alf,

Progressive springs will not work with the RPM fork valves. The RPM valve uses the deflection of the shim stack and the inertia bypass technology to control the fork travel.

The progressive springs are designed to start soft and get stiffer as they compress. The RPM valves require a minimum rate to keep the valve seated on the damper rod. Progressive springs are too soft. Then if the spring becomes to stiff and slows the travel the shim stack is not longer controlling the travel and makes the ride harsh.

Straight rates springs only for RPM fork valves. And unlike conventional theory, the softer the better which allows better valve control.

Randy - RPM

[Alf]

I know Randy

But please, any explanation why Yamaha fit progressive springs with cartridges?

[camo]

Alf Im gonna have a wild shot at this one...... most cartridge vavles are not shim stack design (theyhave a hole for the oil to flush through). The RPM valves are a shim stack. A shim stack will need a spring that will keep it in place from the outset (likely to be stiffer than the initial rate of a progressive spring) and hence the liner spring rate reccommended for randys valves. Other brands that have a ......shall i say glory hole.....design dont need to have the same initial spring rate to keep them working as the fluid will flow through the hole with much less initial resistance to a shim stack design.....

Its a stab in the dark so happy to be corrected......

[Alf]

Thank you very much, Camo!  Question solved!