Author Topic: What rear shock spring rate needed for 4" travel? Any for sale or swap?  (Read 606 times)

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Offline John Orchard

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What would I do without this place  ;-)

Me never had any experience with respringing a 4" travel bike, can someone give me an idea what spring rate I am after for a 90kg rider (I'd like to say 'fast' rider but time will show)?

I am wanting straight-rate springs, bike currently has 'progressive springs', not interested in the effect of little bumps when I have to hit big bumps at speed with 4" !!!

The shocks on the bike are 340mm gas Ohlins STX36 CC607's, the spring dimensions are:

Length:  228mm.
Coil ID:  46.50mm both ends.
Wire thickness:  6.9mm
Active coils:  11
Total coils:  13

Thanks.   :-)
« Last Edit: April 11, 2019, 03:59:03 PM by John Orchard »
Johnny O - Tahition_Red factory rider.

Offline bazza

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There is a sight on net, put in spring thickness,length etc and tells you what your are for a starting point, im 105kg think my springs were 90ib? will try to find notes
« Last Edit: April 13, 2019, 09:31:38 PM by bazza »
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Offline John Orchard

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The Ohlins springs fitted are printed:  00180/90.
Johnny O - Tahition_Red factory rider.

Offline sleepy

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Those Olins spring are 68/82 pounds. Back in the day almost all Koni shocks came with 60lb spring for 250 MX bikes. You should be close with those as long as you get the pre-load correct. Sitting on the bike it should only sag about an inch. The last set of those I checked didn't get to the 82 lb/in until a bit over 3" of compression which is pretty close to bottoming as to get 4" at the wheel the shock travel is only around 3.5" or less.

Offline John Orchard

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Yep you just beat me, l just did the conversion 12 N/mm / 14.3 N/mm convert to 68.5 lb/inch / 82 lb/inch :-)

So you say 60 lb springs would be the go?

l measured the progressive springs fitted, after 4" of travel, only the soft portion of the spring will ever get used, so basically my springs are 68 lb/inch, so your suggestion of 60 lb sounds good.

l wonder if a 65 lb/inch straight-rate spring will be lighter, l think so, think that's what l will look at trying.

Thanks for your knowledge & experience :-)
Johnny O - Tahition_Red factory rider.

Offline sleepy

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Getting the damping correct if far more important. Changing to a 65 lb spring wouldn't even be noticeable. I remember years ago helping an engine customer of mine with some type of open wheel speedway car that had front suspension problems. The front end would bounce over the bumps making it almost impossible to steer. They thought springs to heavy so they had softer coils made despite me telling them that it was most likely the damping. They finally got better shocks with much less compression damping and then went back to the heavier springs and the wheels stayed on the ground for once.   

Offline John Orchard

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Getting the damping correct if far more important. Changing to a 65 lb spring wouldn't even be noticeable. I remember years ago helping an engine customer of mine with some type of open wheel speedway car that had front suspension problems. The front end would bounce over the bumps making it almost impossible to steer. They thought springs to heavy so they had softer coils made despite me telling them that it was most likely the damping. They finally got better shocks with much less compression damping and then went back to the heavier springs and the wheels stayed on the ground for once.
 

Yep l know what you are saying, these shocks are not externally adjustable for damping, but even so the rebound damping is too weak for these springs currently fitted, l'm guessing a softer spring will not fire the shock back so fast. l'll look for some straight-rate springs in the lower 60 lbs to try, l'm hoping travel & damping rates will be in the ballpark.
Johnny O - Tahition_Red factory rider.

Offline topari

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Maybe the resident YSS specialist and chime in. Though if the shocks are rebuildable, the internal adjustments can be undertaken.
topari

1979 IT400F, 1984 kdx200

Offline sleepy

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I know of one bike with those shocks and they are using the stock springs but they came apart several times before the damping was correct and are now very good. Can't tell you what they changed but the bike is a Title winner.

Offline bazza

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Will be interesting old TY trials had about 45Ib so 60 may be light, will wait your test results
https://www.falconshockabsorbers.co.uk/classicpre75-applst.htm   looking at this seems 70IB is norm?
« Last Edit: April 13, 2019, 09:31:02 PM by bazza »
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Offline ralph311

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Re: What rear shock spring rate needed for 4" travel? Any for sale or swap?
« Reply #10 on: April 14, 2019, 09:39:13 AM »
With the amount of preload you'll need to set the ride height with a 90kg rider my guess is those dual rate/progressive ohlins springs (horrible things imo) will be basically running as 82lb springs, but with tons of preload. I'm 75kg and set of straight rate 80lb springs feels pretty good to me on a '74 husky. :-)

Offline John Orchard

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Re: What rear shock spring rate needed for 4" travel? Any for sale or swap?
« Reply #11 on: April 14, 2019, 04:59:18 PM »
With the amount of preload you'll need to set the ride height with a 90kg rider my guess is those dual rate/progressive ohlins springs (horrible things imo) will be basically running as 82lb springs, but with tons of preload. I'm 75kg and set of straight rate 80lb springs feels pretty good to me on a '74 husky. :-)


Good to know, l agree, thanks.
Johnny O - Tahition_Red factory rider.

Offline John Orchard

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Re: What rear shock spring rate needed for 4" travel? Any for sale or swap?
« Reply #12 on: April 16, 2019, 09:05:52 AM »
Will be interesting old TY trials had about 45Ib so 60 may be light, will wait your test results
https://www.falconshockabsorbers.co.uk/classicpre75-applst.htm   looking at this seems 70IB is norm?


Thanks Baz, that's exactly what l was originally after :-)
Johnny O - Tahition_Red factory rider.

Offline YSS

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Re: What rear shock spring rate needed for 4" travel? Any for sale or swap?
« Reply #13 on: April 17, 2019, 02:21:32 PM »
Maybe the resident YSS specialist and chime in. Though if the shocks are rebuildable, the internal adjustments can be undertaken.

Topari , except for road racing and observed trails, we always use progressive springs . For the DT and up to 90 kg rider I fit 10-15 N springs . Progressive springs are a bit more difficult to produce , so not everybody can offer them . There are a few riders (not many ) that do  insist on them . I do have som 10 , 12 , 15 and 16 N linear springs that  I use for testing new model bikes ( as base to construct the right progressive spring later )
362 shocks ,only  pay for performance and not for names