zap2504 said:Very interesting work! I did read that the CB400F/360 fork springs may need to be cut (that's if they do fit in the SR250 tubes) so your cut spring adapter may come in real handy. Much easier than trying to grind off the cut end.
JadusMotorcycleParts said:Yes, it could work with different springs, so long as the dimensions were somewhat similar.
I ask the question though, why change springs if the stock one is up to the job when it is cut and stiffer? As long as it is within its working range when fully compressed... I looked into springs from the RD250/350s and the CB360/400s and yes, there are many available in different rates, but they range in price from 110usd for straight rate to 180usd for progressives. Perhaps the SR spring can provide the same performance if the right math/calculations are done for rider weight, length - resulting in rate, and finally preload?
zap2504 said:Because the original springs (both front and rear) were specified as such to support a rider/passenger of a specific weight. If your projected load is not fairly aligned with that design parameter, then it will not matter much how much preload you introduce (until you fully compress the spring). Being somewhat self-serving here as I weigh somewhere in the neighborhood of 260-270 lbs when suited up (still way under the stated max load), as are many USA riders judging from the many discussion threads on suspension mods of all sorts of bikes. This is the reason of my interest in different springs with stiffer rates for the forks. Keep up the good work!
You are correct of course - it is the reason that adding preload helps to hold up more load weight. But at the cost of suspension travel. I would be looking for the same travel but able to hold a (I suspect much) higher load weight without additional preload.JadusMotorcycleParts said:Great input zap, thanks. Very good points. I might, for the sake of research, buy a cheap second hand set of springs from these mentioned models of bikes just to see if they fit and could work.
One important point about the stock SR springs though; if they are cut, the rate increases with every coil removed. By the time you remove 5 of the working coils of the main spring, the rate is already up past 5N/mm - which is more suitable for a 90kg rider. I will need to test this of course with the threaded rod jig to see how much stress is put on the cut spring when it is compressed the 140mm (plus maybe 10-20% preload) of travel. Perhaps this much movement with a shorter spring will put it outside of its working range and begin to deform it over time.
pidjones said:Make sure that when you get real ones, the circlip is installed with the sharp edge UP (away from the spring).