Ton up SR250: 100mph, 100kg, 30hp

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.
 
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.

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?
 
Anyway, here are the results of testing the stock SR250 fork spring so I know more about how it looks and behaves in use. Interesting! Now I can start experimenting.
 

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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?

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!
 
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!

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.
 
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.
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.
 
Have been spending a lot of time in CAD and prototyping trying to get the adjustable preload caps to fit and work within these unique constraints of this fork. Remember, all other adjustable caps are screwed in place! After a few attempts and failures, I think I have something that is close enough it is worth doing some engineering drawings and getting some metal prototypes machined up.

The idea is that the caps are installed easily as in my previous prototype, then locked in place with the circlip. The adjustment rod has an M10 thread and provides 20mm of adjustment. It is also completely removable so that fork oil can be added or removed with a 6mm tube (through the hole in the adjustment plate/spacer), allowing fork oil height adjustment without needing to remove the entire fork cap. I have found suitable sized orings for each component as well and adjusted the parts to suit. I still need to order them though - hence the prototypes missing them.

Thoughts?
 

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Make sure that when you get real ones, the circlip is installed with the sharp edge UP (away from the spring).
 
pidjones said:
Make sure that when you get real ones, the circlip is installed with the sharp edge UP (away from the spring).

Yes! Great point. Will provide a healthier land/contact patch for it against the groove in the stanchion.
 
YSS fork emulators in the correct size (almost!) arrived this week! Can't wait to install and test them. Like any curious engineer, I took them apart to see how they are built up. Pretty nice stuff. Good quality and impressive fitment/tolerances.

Only thing is, they don't quite fit into the SR's damper rods like they are supposed too. I'll turn that small piece of aluminium down on them so they fit nice and snug there.
 

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I did end up ordering both a set of springs for the RD250 and CB400 to see if they fit. They were second hand and cheap and they should arrive this week.

I also bolted up the wheels to get this project's first rolling chassis! Woohoo! Haha, no, now I will draw the basic geometry into CAD and work out what specifications I want and how I can achieve that - fork height, rear shock length, ride height, rake/trail and different to the other bikes I have built... ground clearance! This will be worked out with rearset bracket position as well. Something that has frustrated me with previous SR250 builds is the lack of ground clearance when cornering hard. I don't think these bikes were designed to corner hard but they should be able to with the right upgrades and some better foot clearance - the pegs are always the problem.
 

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When I was 165lbs I was around 13 years-old. Of course when I was in 6th grade I was the tallest person there - including the teachers and principals.
 
I weigh 165 now, but remember a couple years ago weighing, as phife dawg from A Tribe Called Quest says; 'A buck fifity'! So the RD400 springs should be good! I'll be able to work out their rate from the dimensions and number of coils and apply that to the SR spring (hopefully).
 
Seasons greetings all!

I took the couple days off this week between Xmas and NYs while my GF is at work so I can be in the workshop ;D

Just before Xmas I received the machined brake perch repositioning bracket. Which I have been pretty quiet about since the last time I mentioned it - that is because it took an embarrassing amount of prototypes to get it right! The angle and position on the fork was just doing my head in. Anyway, it came up mint and works a treat. Now I just need to make a small eyelet for the brake cable to move through and specify a custom cable for Venhill to make.

You can see I took some advice from a few of you with the way that it is mounted. The only difference really is that the bolt does not act like a pinch bolt (because of the 3 degrees draft on the leg boss), rather it is just done up tight enough with a lock nut so that it holds itself in position and stable, and takes the braking loads together with the bottom piece of the opening in the bracket.
 

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