Need some spring length and rate help.

[Macmatic]

I'm working on an '85 GS450L that is getting forks from an '82 GS550M along with cartridge emulators. The budget doesn't allow for new springs AND emulators right now so I need to cut down the springs and put in ye olde PVC.

I think I'm looking for around .8 kg/mm. The rider weighs around 170lbs.

Wire dia: 3.90mm
Coil dia: 25.5 (outside, so 21.6 center to center?)
Free coils: 50
OAL: 508mm
Emulator height: 14mm

Is there a calculator for this anyplace? I've looked around and all I can find are Racetech and similar site whi tell you which of their springs to use but nothing that will tell me what rate I've got or what shortening it will do.

/\/\ac

 

[GregT]

Shortening a spring raises the rate - makes it stiffer.

Do your OE 450 springs come close on length to the 550 ones ?

The 450 spring rate is very close to what you need - if you have to, make up the extra length with spacers.

 

[Macmatic]

Shortening a spring raises the rate - makes it stiffer.

Do your OE 450 springs come close on length to the 550 ones ?

The 450 spring rate is very close to what you need - if you have to, make up the extra length with spacers.

Do you say "very close to what you need" because of the number I came up with or because you know the 450 springs are about right for the rider weight on the bike? Because I pulled that number out of my, er.. ear, after looking at Sonic and Racetech numbers.

The 450 and 550 springs are almost the same length, I can get an exact number but I'd say within 1cm. The 450 springs are a larger coil diameter and smaller wire diameter. I have no idea what the spring rate is on an '85 GS450L but I can tell you that they feel VERY soft sitting on the bike. Both forks had a reasonable amount of fork oil but I didn't measure it on the way out so I don't know if they were low.

/\/\ac

 

[posplayr]

I'm working on an '85 GS450L that is getting forks from an '82 GS550M along with cartridge emulators. The budget doesn't allow for new springs AND emulators right now so I need to cut down the springs and put in ye olde PVC.

I think I'm looking for around .8 kg/mm. The rider weighs around 170lbs.

Wire dia: 3.90mm
Coil dia: 25.5 (outside, so 21.6 center to center?)
Free coils: 50
OAL: 508mm
Emulator height: 14mm

Is there a calculator for this anyplace? I've looked around and all I can find are Racetech and similar site will tell you which of their springs to use but nothing that will tell me what rate I've got or what shortening it will do.

/\/\ac

Someone over at gixxer.com posted a spring rate calculator once which used spring parameters as input. I tried to compare two springs by direct measurement and using the calculator. The problem is that the spring rate is so sensitive to the coil thickness i would think it hard to determine rate from the parameters. Just teh coil thockness variation along the coil made it hard to get an estimate that did not vary widely. (more than +/- 10%)

It would be better to use the race tech website to calculate what rate you should have as compared to stock and cut sufficient length off to reach the required increase. 10% stiffer comes from a 10% drop in length.

 

[Macmatic]

It would be better to use the race tech website to calculate what rate you should have as compared to stock and cut sufficient length off to reach the required increase. 10% stiffer comes from a 10% drop in length.

Thanks for the info but the first problem is finding out what the spring rate on these springs is. Maybe there is a resource for what they should be but I haven't found it yet. Anyway, can't figure out what percentage to cut off until I figure out what percentage I've got.

I'll do some more googling tomorrow and see what I can find.

Thanks
/\/\ac

 

[posplayr]

Racetech lists the stock rate.

Thanks for the info but the first problem is finding out what the spring rate on these springs is. Maybe there is a resource for what they should be but I haven't found it yet. Anyway, can't figure out what percentage to cut off until I figure out what percentage I've got.

I'll do some more googling tomorrow and see what I can find.

Thanks
/\/\ac

 

[GregT]

Despite the 450 springs feeling very soft, they're what I'd start with.
Only if you find that you're regularly bottoming the forks out would it be worth making major changes. adjusting preload should get you in the ballpark as far as ride height goes and you will be surprised how much difference the emulators make.
In general the OE springs are slightly too stiff - looking at the OE forks i have sitting here they're slightly stiffer than the Marzocchis on my bike which have Ducati Pantah springs and are just right for roadracing the 450.
Another 450 racing here is still using std forks and the owner & I agreed that they were only just too stiff - he's talking about looking at GSX250 springs.

 

[Macmatic]

Racetech lists the stock rate.

Do they for the GS450L? I've seen it for other models but not this one. Here is the link to the GS450L specs page, it only says "call" http://old.racetech.com/evalving/en...50L&year=85&TABLEINFO=street&langname=english

Despite the 450 springs feeling very soft, they're what I'd start with.
Only if you find that you're regularly bottoming the forks out would it be worth making major changes. adjusting preload should get you in the ballpark as far as ride height goes and you will be surprised how much difference the emulators make.
In general the OE springs are slightly too stiff - looking at the OE forks i have sitting here they're slightly stiffer than the Marzocchis on my bike which have Ducati Pantah springs and are just right for roadracing the 450.
Another 450 racing here is still using std forks and the owner & I agreed that they were only just too stiff - he's talking about looking at GSX250 springs.

Maybe the fork oil level was low or there was another problem but I could roll the bike a few steps and jam on the brake and get it to feel like it was about to bottom out. This bike is for a new rider and hopefully he'll never bottom out!
I'm surprised to hear that about the OE spring rates, I thought the general consensus was the opposite which was why "everyone" commonly changes to Progressive or other aftermarket springs and/or puts in a PVC spacer to add pre-load.

Any thoughts on the emulators and fork oil weight? I suppose I should just call Racetech and pick their brains but I feel kind of cheap doing that since my emulators are "replicas".

At this point I am 99% sure I'm using stock 1982 GS550M Katana forks:
Same outer dia but better (perfect) fit for the emulators over the 450 forks. They are both 35mm but the 450 has larger diameter springs and...riser thing you bolt down to the bottom of the lower leg. Those riser tubes (the part you drill out) also look like cheap crap in the 450 forks, seems like thinner wall thickness and a stamped dimple around the stock holes.

12mm shorter inner tube length shouldn't upset the apple cart.

Lower legs will be ready for a twin disk upgrade as soon as I master caliper carrier repair or the new owner wants to spend the money on a twinpot upgrade.

Thanks for the input guys,
/\/\ac

 

[posplayr]

When you run the custom spring rate calculator they list what is below. It usually includes the rate.

OK so you might be stuck with measuring your sag and trying to translate from there unless you can giev them a call and see what they might know. I woudl have my sag numbers handy though


<B>

FRONT FORK SPRINGS
Recommended Fork Spring Rate: 0.901 kg/mm (use closest available)
Stock Fork Spring Rate: Call kg/mm (stock)
No Fork Spring Series currently specified for this model.
Please call Tech Support 951.279.6655 for help.

REAR SHOCK SPRING
Please call for recommended rate.
Stock Shock Spring Rate: Call kg/mm (stock)
No Shock Spring Series currently specified for this model.
Please call Tech Support 951.279.6655 for help.
​

</B>

 

[GregT]

The twins are generally oversprung - the fours not so much.
The fours seem to have been set up for the US market in that they are a bit soft as stock and require better (progressive) springs to keep the weight off the deck in spirited use....At least at the front.

 

[Macmatic]

Pos, thanks for the info. I think I'm going to leave the springs alone for now and just set up sag for the new owner.

The twins are generally oversprung - the fours not so much.
The fours seem to have been set up for the US market in that they are a bit soft as stock and require better (progressive) springs to keep the weight off the deck in spirited use....At least at the front.

Ah, I get it now. This is my first time messing around with a Twin so I haven't read as more of the lore and history on them. Thanks!

/\/\ac

 

[old_chopper]

measure the free height of a spring (example 14.0")
apply a known weight (100 pounds)
measure the compressed height (13.5")
calculate that 100 pounds force = .5" compression
to find the rate per inch divide the weight by the compression
100/.5 = spring rate of 200 pounds per inch

I'm not sure if you needed this or not but it might help you compare your springs to the aftermarket ones they are trying to sell you. All this assumes the springs are not progressive. The coil spacing is not even on a progressive spring so as they compress you lose the number of active coils, therefore making the rate stiffer.

 

[Macmatic]

measure the free height of a spring (example 14.0")
apply a known weight (100 pounds)
measure the compressed height (13.5")
calculate that 100 pounds force = .5" compression
to find the rate per inch divide the weight by the compression
100/.5 = spring rate of 200 pounds per inch

I'm not sure if you needed this or not but it might help you compare your springs to the aftermarket ones they are trying to sell you. All this assumes the springs are not progressive. The coil spacing is not even on a progressive spring so as they compress you lose the number of active coils, therefore making the rate stiffer.

Great! I'd seen Pos's thread about rear spring rate checking but I hadn't really thought about how to apply it to the long, thin front springs. I bet I could make up a PVC and plywood test rig for under $15. All you'd need is a vertical tube a little bigger than the spring inside and a little taller, a known weight on a wooden stick & piston to compress the spring and maybe a rod going down the outside of the vertical tube from the weight showing the height of the spring top.

/\/\ac