Thread: ORGS build up
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Old 12-05-2010, 11:46 AM   #105
x3300 OP
geoff
 
Joined: Mar 2005
Location: San Jose, CA
Oddometer: 170
Rear Spring

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:

http://stuff.gotdns.org/bike/orgs-build-up/34-rear-spring/race-tech-image002.jpg

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.

http://stuff.gotdns.org/bike/orgs-build-up/34-rear-spring/SRSP6327P25.jpg

The other is the Langston Racing Super Progressive.

http://stuff.gotdns.org/bike/orgs-build-up/34-rear-spring/LRS02-OR_CN400.jpg

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.

Code:

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.

Code:

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.

http://stuff.gotdns.org/bike/orgs-build-up/34-rear-spring/new-spring.jpg

Here's what I measured it to be:

Code:
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.

http://stuff.gotdns.org/bike/orgs-build-up/34-rear-spring/ohlins-plate-drawing.jpg

The design is similar to my drive shaft spring compressor. A base plate has two arms welded to it.

http://stuff.gotdns.org/bike/orgs-build-up/34-rear-spring/welding-arms.jpg

And two bolts are welded to the arms that pass through the plate.

http://stuff.gotdns.org/bike/orgs-build-up/34-rear-spring/compressor.jpg

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.

http://stuff.gotdns.org/bike/orgs-build-up/34-rear-spring/compressing.jpg

This photo shows how the 36.5mm radius cut-out allows the spring clip to be placed in position while the spring is compressed.

http://stuff.gotdns.org/bike/orgs-build-up/34-rear-spring/ring.jpg

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.

http://stuff.gotdns.org/bike/orgs-build-up/34-rear-spring/installed.jpg

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.

-x3300
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