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Old 03-16-2013, 11:36 PM   #4
DisTech OP
Studly Adventurer
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Joined: Sep 2008
Location: Seattle
Oddometer: 616
Fork Springs

So how do you figure out what rate your ebay fork springs have? Not being the first person to ask this, Google lead me here
My variation on the ThumperTalk designs for putting load on the spring. 1" EMT in vise, 5/8" ATR, nuts and washers.

One of the calcs wants a measurement at 20% deflection so 2.5" of 12.2" travel works for me.

2.5" deflection was from a BOSS 302 head (50.5lb) and 10lb dumbbell.

Zip tied a caliper to a stick and zeroed it at this point.

Changed the 10lb out for a 50lb.

Caliper reads 39.28mm difference.

The stock 2006 RMZ450 fork spring rate is .47kg/mm.

In the red shaded section you don't need to weigh anything or build the contraption and that calc look like it worked pretty well. Depending on what modulus of rigidity you use, either side of .47kg/mm.

The blue shaded section requires weighing and measuring via the contraption. calc used the delta between two measurements showed a rate of .4619kg/mm. ThumperTalk simple rate calc's suffered from my ability to accurately weigh things. I ordered a digital fish hand scale that can measure down to the ounce and will update the sheet with accurate weights.

Anyway, it looks like the forks have stock springs in them. Ordered .52kg/mm springs, seals, wipers and 5wt oil today.

The fish scale arrived so I re-weighed the weights and head for more accurate data. The spring rate calc sheet has been updated and now the simple rates are close to the rate of the springs I ordered ... I'm betting the rate calc that uses a delta between two points is correct, but we'll see when the new springs arrive.

Update! Update!
RaceTech .52 springs are finally here!

Here is the exciting fish scale! From Cabellas, works pretty good, pounds and ounces or kilograms, 50lbs max.

The rate calc sheet now has accurate numbers from both pair of springs.
  • Red: No weighing neccissary, easy and accurate.
  • Blue: Two sets of measurements required.
  • Tan: One measurement, not so accurate. This calc is essentially the same as the two measurement method with one measurement assumed to be at 0". This means you include the end portion of the spring which is much stiffer and fouls your results.

So the ebay forks had the stock .47 springs, good! Ordering RaceTech .52's was correct, back to the build!
DisTech - Suzuki (1993 DR350S & 1994 DR350, 1996 DR650/DR790), BMW (1999 R1100GS)

DisTech screwed with this post 04-13-2013 at 10:37 AM Reason: Fish scale! Springs arrived!
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