I'll need some kind of spring to hold up the back of the bike. As I mentioned in an earlier post modern off-road bikes have progressive suspension systems where the damping force and spring rate increase as the shock compresses. Most bikes use a linkage mechanism to achieve this, but some have a linkless design where the progressive effect is accomplished with progressive shock valving and progressive rate springs.
Here's a nice spring force diagram from Race Tech
With some searching I found two progressive springs on the market that I thought would fit my TTX44 shock. One is the Race Tech 6326 Series
The other is the Langston Racing Super Progressive
Just based on these two photos the Langston spring seems to have a much more progressive wind than the Race Tech.
Here's a comparison of the recommended springs for a 250 SX-F from Ohlins, Race Tech and Langston. The stock spring rate for the 250 SX-F is 62 N/mm.
rider weight Ohlins Race Tech Langston
kg (lb) N/mm (lb/in) N/mm
64 (140) - 6326P05
69 (152) - - LRS-01 64-132
70 (154) 60 (343) -
75 (165)* 62 (354) -
76 (167) - 6326P10
80 (176) 64 (365) -
85 (187) 66 (377) -
90 (198) 68 (388) 6326P15
95 (209) 70 (400)
105 (230) - 6326P20
109 (240) - - LRS-02 83-176
120 (265) - 6326P25
136 (300) - 6326P30
To get a handle on which of these would work with my custom rear suspension I considered how the ORGS rear compares with some linkless KTMs.
bike style travel(mm) ratio weight(kg) rate(N/mm)
KTM 250 SX-F MX 335 3.07 100 62
KTM 450 SX-F MX 335 3.07 105 68
ORGS trail 280 2.57 200 -
This bike will be much heavier than a 250cc MX bike, but it will mainly be used on trails, and the shock leverage is about 80% of the KTMs. I thought I'd rather have the back end a little too soft than too stiff, so I decided on the Race Tech 6326P10. The LRS-01 was another choice, but I thought it might be too stiff near bottoming. I put in an order and this arrived.
Here's what I measured it to be:
Race Tech 6326P10
length 260mm 10.35"
ID 63mm 2.48"
wire 13.0mm 0.51"
Pretty close to 2.5" x 10", a common size of off-road racing truck coil-over springs. I've never seen any progressively wound truck springs though.
To get the spring on the shock I needed to make a spring compressor. Here's an updated version of my ohlins spring plate drawing
The design is similar to my drive shaft spring compressor. A base plate has two arms welded to it.
And two bolts are welded to the arms that pass through the plate.
Here's the unit in action. I made the arms long enough to allow a block of plastic at the bottom to keep the shock from getting scratched.
This photo shows how the 36.5mm radius cut-out allows the spring clip to be placed in position while the spring is compressed.
And the shock with spring installed. I noticed that the spring hits the corner of the battery. I thought that it might after I got the new tray done. There's enough room to move the battery to the left a little. I just need to drill another set of mounting holes in the tray.
With the shock installed I checked the free sag and ride height. For the 250 SX-F Ohlins recommends 30mm and 110-115mm for those. Converting by 109/335 gives 9.7mm and 35.8-37.4mm at the shock shaft. I measured 10mm and 28mm, which would seem a little too stiff, but I think it will be OK with the heavy bike, and the next lighter spring in the 6326 series is considerably lighter. After I get some trail riding time on the bike I'll be able to judge how well this spring works. I guess I'll need a stiffer spring while carrying traveling gear.
I spent a lot of time researching springs, studying the data, and writing up the report, but the rear suspension is a big part of what this bike is and so I wanted to give selection of the spring proper coverage.