OEM front fork spring rate

oldandgrey

oldandgrey

Joined
Jan 21, 2008
Messages
6
Location
New Brunswick, Canada
Bike
ST1300 CBR1100X
Hi all. I am thinking about buying new front fork springs for my 04 ST. Does anyone know what the original equipment fork spring rate is? Thanks for the help!
 
oldandgrey said:
Hi all. I am thinking about buying new front fork springs for my 04 ST. Does anyone know what the original equipment fork spring rate is? Thanks for the help!
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The OEM rate is reportedly appx. 86 mm/kg. That is found on the Race Tech web site.
 
We don't know if that is the initial rate or rate after rise. It is soft for riders over 160-165 lbs. There is room to add preload and it is easy to get static sag and free sag to target numbers if you are at our under this weight.
 
Just to muddy the waters a bit more, aren't the OEM springs progressively wound?
 
wjbertrand said:
As a comparison, the straight rate spring recommended by Sonic for the ST1300 is a 1.2 Kg/mm rate, almost 40% stiffer than OEM!
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That is why I think the Sonic Spring calculator is whack. If you choose Sport Tour, Aggressive Street, bike weight 721 lbs, it indicates 1.2 springs for the 112 lb rider on up to whatever weight you could possibly be. In effect, all riders should be on a SS 1.2 if you believe the calculator.

OTOH, Race Tech recommends a .870 kg/mm spring for my weight which is very close to OEM of .860. If this bike had preload adjusters one would logically try and set sag with them first. I was able to set sag to what available tuners recommend with 16 mm of preload. My sag numbers are very good, it has more than 10 mm of travel in reserve under full on braking, and handles very well for me, better than the stock set up. If I simply swapped for 1.2's I wonder how that would measure out seeing as .860s hit target values?
 
I just got the Hyperpro springs from Klaus at EPM. Hyperpro are variable rate springs. I don't know the actual rate range, they are VERY smooth. Instant improvement. Install was very easy, no messing with spacer changes. Just remove the caps, lift out the springs, pour/pump out the old fluid. Drop in the new spring, fill with the EPM-supplied 7.5w synthetic fluid to the level in their instructions. I also got the Hyperpro rear shock at the same time. That also has a variable rate spring.

I now have proper sag on both ends. I was going to do the gold valve kit for the front, from Racetech at the same time. Klaus suggested I try the spring change alone first and see how it performs. I could always do the valve change later. Happy to say the damping is just fine. With a proper spring, the damping isn't overworked. Although I am happy with the custom rear shock, in retrospect I would probably have been just fine buying a new spring for the stock shock.

Any higher rate spring in the front is going to raise the front ride height from where it is now with the soft spring. If you don't do something with the back, the bike's handling will be affected. The turn-in will be slow, not sporty. I suggest you do something with both ends at the same time, preferably using springs from the same vendor so they work well with each other. Over the years I've bought shocks or springs from Klaus several times, including when he handled different brands. I don't think the brand is as important as proper rate selection, so pick a vendor you trust.
 
I have about 3/4" front and rear, measured all the way from fully extended, to with a rider. That would be called "rider sag". The static sag, without a rider, is in the middle as it should be. That is with about 1/2 the rear preload cranked in. I still have more rear preload available for when I carry a passenger. Some will have other opinions on the right amount of sag, but IMHO this is just right. The bike sits at least an inch higher than before, which means the OEM springs had at least 1.75" of sag, probably more in the rear. That extra inch is now available for the suspension to work without bottoming.

My seat height is on the highest front and middle rear setting, and I still have no trouble reaching the ground with my 30" inseam and riding boots.
 
CarSalesman said:
I have about 3/4" front and rear, measured all the way from fully extended, to with a rider. That would be called "rider sag". The static sag, without a rider, is in the middle as it should be. That is with about 1/2 the rear preload cranked in. I still have more rear preload available for when I carry a passenger. Some will have other opinions on the right amount of sag, but IMHO this is just right. The bike sits at least an inch higher than before, which means the OEM springs had at least 1.75" of sag, probably more in the rear. That extra inch is now available for the suspension to work without bottoming.

My seat height is on the highest front and middle rear setting, and I still have no trouble reaching the ground with my 30" inseam and riding boots.
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If I understand what you wrote - you have more sag without a rider than with a rider? That is odd.

Static sag is generally measured with the bike in normal loaded configuration which includes a rider. Free sag is measured in the normal loaded configuration but with no rider.
 
No, maybe my choice of words is incorrect or not clear. Sag of the bike alone, no rider, is about 1/2" on both ends. Rider gets on, sag is about 1/4" additional (total 3/4"). The front might be a hairs width less than those numbers.

I dropped the front forks about 1/4" to restore the quick turn-in that I like. That was not surprising, considering the increase in spring rate on both ends, but no easy way to adjust preload in the front.
 
You are right, the amount of sag one wants is what one wants but 3/4" sag with rider isn't much. What happens when you ride over a bump or hole in the road and the suspension cannot extend enough to keep traction on the wheel? Most set ups allow the suspension to operate normally in the middle third of available travel leaving 1/3 for suspension extension and 1/3 for compression over and above normal undulations and braking forces.
 
Dave - I checked my notes again. I have the rear set to 1" rider sag measured total from fully extended. My brain must have been a little tired when I wrote 3/4". Not much different, but significant.

These small differences can have a huge impact on the handling of the bike, which is why I always tell people to tune and test by adjusting the front fork tube height. Since we don't have a rear ride height adjustment, adjusting front height is the only way to tune the turn-in handling. A proper spring on the rear can raise the rear height up to a full inch compared to the stock spring, depending on the load. That has to be compensated for in front height. Likewise, I often see members here talking about replacing the front springs with no discussion of what that single change does to handling. There is a substantial amount of up and down height adjustment available in the front tubes. An ST can have really sweet and easy handling if set up right.
 
1" still isn't much but if it works for you than that is all that matters. The ST1300 does not have much adjustment in the fork clamps - a bit less than 5mm. Any more than that and the fork caps are bearing against the underside of the handlebars. When I changed the spacers I lost 11mm of fork sag (47 to 36mm). Since I was able to reduce rider sag of the rear shock to less than 14mm with all the preload cranked in this did not present the problem of not being able to adjust rake sufficiently after raising the front ride height 11mm and still using the stock rear spring. I set rear sag to about 30 mm most of the time now. This works great with 36mm up front. When I changed from Pilot Road 2s to the 3s I dropped the fork back down 4mm to where it was when I ran Avon Storms. When it was dropped with PR2s it made the turn-in and mid corner stability too squirrely for my taste.
 
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