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Author Topic: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp  (Read 33898 times)

Offline zap2504

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #200 on: Nov 18, 2018, 19:49:15 »
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.

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #201 on: Nov 21, 2018, 04:11:21 »
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? 

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #202 on: Nov 21, 2018, 04:13:38 »
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.

Offline zap2504

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #203 on: Nov 21, 2018, 19:52:10 »
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!

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #204 on: Nov 22, 2018, 04:21:09 »
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.

Offline zap2504

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #205 on: Nov 22, 2018, 22:16:26 »
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.

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #206 on: Nov 26, 2018, 05:03:12 »
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?

Offline pidjones

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #207 on: Nov 26, 2018, 08:00:57 »
Make sure that when you get real ones, the circlip is installed with the sharp edge UP (away from the spring).
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Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #208 on: Nov 26, 2018, 09:45:01 »
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.

Offline der_nanno

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #209 on: Nov 27, 2018, 06:38:35 »
I really like what you did there.
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