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Author Topic: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp  (Read 24890 times)

Offline Sav0r

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #120 on: Apr 08, 2018, 16:09:20 »
YSS makes emulators, there are also Taiwanese knockoffs. I have tired the YSS, the RT's, and the knockoffs on my RD 350 (FZR600 forks). I prefer the knockoffs over all of them. That said, one of my long brewing projects has been to make a set of my own design, albeit highly derivative of the existing designs. There's not really a lot of room left of innovation with this type of damping arrangement, emulators pretty much top them out performance wise.
Sav0r the adventure.

Visit www.chrislivengood.net to get more information on my RD350 dubbed Mia Wallace as well as my other projects and snafus.

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Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #121 on: Apr 11, 2018, 16:29:04 »
Thanks for the feedback and tips about other emulators.  Good to know.  Which measurement is important for the emulators to fit?  Is it the stanchion inner diameter, the spring diameter, or do I need to disassemble the forks and measure up the damper rod and get it's dimensions?

I have taken it upon myself to start designing an adjustable preload fork cap.  First things first, measure up the fork tubes, caps, c-ring, oring etc.  I have also ordered some custom stainless spacers for the black Jadus SR because I want to sell it and the forks are the only thing letting it down.  I ordered several 5mm thick spacers and will experiment with how much preload feels good.  Then I will use that as a basis for the adjustable ones.

Offline zap2504

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #122 on: Apr 11, 2018, 16:50:49 »
I think an adjustable preload cap would be a lot of work with very little benefit. Most preload devices are some sort of tube (usually PVC plastic) that partially compresses the spring. Cut the tube for the amount of preload you want. Some trial and error, but fairly easy and real cheap - but you lose suspension travel. To minimally improve the OEM forks you need to get the right springs for the expected rider/vehicle weight (the OEMs are typically undersprung). Correct weight springs means minimal need for preload. Then experiment with extension/compression circuits, bleed holes and fork oil weight.

To radically improve the forks you need larger tubes for stability at the triple trees - either upside down (real sturdy in the triple trees) or right-side up with a fork brace. Still need the right springs and probably need to experiment with bleed holes/oil weight - unless you swap-in a cartridge fork. Complete front end swap (with front disk brake) usually makes the most sense.

Offline Sav0r

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #123 on: Apr 12, 2018, 11:47:44 »
The spring sits on top of the emulator and the emulator sits on top of the damper rod. Generally the emulator is the same OD as the spring. The emulator valve preload spring (the part that actually does the work) is inside the fork spring.
Sav0r the adventure.

Visit www.chrislivengood.net to get more information on my RD350 dubbed Mia Wallace as well as my other projects and snafus.

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Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #124 on: Apr 14, 2018, 14:56:31 »
Ok well if the emulators sit like you say, I should order the 23.8mm YSS emulators and give them a crack (internal diam of SR forks is 23.95mm roughly).

After working on the forks on the black Jadus bike and the orange one, I would say preload would do a lot.  There is so much sag in the fork already!  I know that when you add preload you loose suspension travel.  But the SR already has 140mm, which by today's standards is excessive for a road bike.  I think if the result of a well sprung SR fork ended up being 100mm travel, it would be plenty. 

See that is the thing with those plastic PVC spacers, it is trial and error and if you cut too short there is no going back.  If I design the preload adjusters to have say 10mm preload in them already, they could have a further 30mm of adjustment travel and this would make a big difference.  Then if the ride ends up being too harsh or oversprung, the adjusters could be backed out a little.

I know all the other paths to improve the forks involve larger, stronger/stiffer fork tubes and front end swaps, but call me old fashion, I just think modern forks look downright silly on vintage bikes.  I know it's a fad right now and it is a taste thing, but the forks will always end up outperforming the bike.  Besides, how many SR owners would be prepared to do a front end swap?  Not many I think.  I had a guy contact me on insta wanting to sell me a custom set he had made from Cognito Moto for 1500USD for his project.  He of course had since abandoned the project and was looking to get some money back on the parts.  1500USD would get you an entire SR250!!!  Mental imho.

Offline Rat_ranger

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #125 on: Apr 14, 2018, 16:58:34 »
Preload caps should be pretty simple to make.  I mean the xs650 had fork caps that allowed 3 different preload options. 

 As far as fork swaps, I think a bunch of people don't look at how much it'll cost in the end.  I mean its what $150 for a stem, $400 for a hub, whatever the forks cost you, maybe triple clamps.  Then there is the rim and spokes and lacing.  Even my extreme budget swap was about $700, that was making or modding parts and using scrap bin materials where I could. 

*edit* basic ones look pretty simple to me. https://www.economycycle.com/wp-content/uploads/2014/12/RDPreloadAdj2.jpg
« Last Edit: Apr 14, 2018, 17:00:36 by Rat_ranger »
Xs650

Offline zap2504

  • Posts: 363
Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #126 on: Apr 14, 2018, 18:13:07 »
Ok well if the emulators sit like you say, I should order the 23.8mm YSS emulators and give them a crack (internal diam of SR forks is 23.95mm roughly).
I think this is the most promising direction yet, as you already have the fork brace.

After working on the forks on the black Jadus bike and the orange one, I would say preload would do a lot.  There is so much sag in the fork already!  I know that when you add preload you loose suspension travel.  But the SR already has 140mm, which by today's standards is excessive for a road bike.  I think if the result of a well sprung SR fork ended up being 100mm travel, it would be plenty. 
I think I saved some info from CrazyPJ where he recommended Honda CB360/400F springs as they were a little loose for the Honda's 33mm forks and fit the SR250's 32mm (also mentioned XS250/400 springs). Probably could get custom springs wound but it would also be pricey.

See that is the thing with those plastic PVC spacers, it is trial and error and if you cut too short there is no going back.  If I design the preload adjusters to have say 10mm preload in them already, they could have a further 30mm of adjustment travel and this would make a big difference.  Then if the ride ends up being too harsh or oversprung, the adjusters could be backed out a little.
True, but PVC pipe is real cheap and easy to cut.

I know all the other paths to improve the forks involve larger, stronger/stiffer fork tubes and front end swaps, but call me old fashion, I just think modern forks look downright silly on vintage bikes.  I know it's a fad right now and it is a taste thing, but the forks will always end up outperforming the bike.  Besides, how many SR owners would be prepared to do a front end swap?  Not many I think.  I had a guy contact me on insta wanting to sell me a custom set he had made from Cognito Moto for 1500USD for his project.  He of course had since abandoned the project and was looking to get some money back on the parts.  1500USD would get you an entire SR250!!!  Mental imho.
Mental - agreed
Along this line, I know that Cosworth had a somewhat interesting fork mod that he swears by. I believe it is detailed in his SR250 Café Racer build.

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #127 on: Apr 29, 2018, 04:54:36 »
Yes, I also think custom springs, or even progressive springs (available online) would be too pricey for this bike.  Yes, PVC is cheap - I actually got a message from a customer the other day to say he completely cut out the soft part of the SR spring (it is two stage rather than progressive) and replaced that with a spacer.  He had good results with that but needed to heat up the cut end of the spring, bend it over and then linish it flat to get a decent seat for the spacer to sit on.  I figure not every weekend mechanic has a blow torch and a linisher!  So I'll shelve that idea.

Yes, I have saved all the posts from Cosworth about his fork mods - they seemed to work really well.  I'll be using some of these ideas too.

Anyway, I received the prototype spacers I mentioned in a previous post.  I'll install them along with a little extra fork oil and see how that rides.

My latest idea is to somehow design the adjustable spacers to have a through hole in them when the preload adjuster is backed all the way out - so that you can remove and add fork oil with a small tube without removing the entire cap.  This then gives adjustment on the spring (with the preload adjuster) and on the dampening - more oil, less air in the fork, harsher damping, less oil, more air in the fork, softer damping.

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #128 on: May 22, 2018, 04:27:05 »
I tested these spacers in the black Jadus bike along with 170cc (168 in manual) of 20W fork oil in each leg.  It improved the forks about 10%, barely noticeable.  I have tried 200cc in each fork leg in another SR and that seemed to help a little more - it probably didn't make the fork perform any better, just made it feel less mushy.

I am convinced that for this build, I will use emulators for improved damping and then spacers with adjustable preload for spring control.  However, I will cut out the soft part of the springs and replace with a long spacer.  The fork is just always going to be mushy with that first pathetic spring rate.

As for something I will be able to offer customers, that is a tougher one - it is a balance of cost vs benefit and ease of installation!

Offline Pod70

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #129 on: May 22, 2018, 12:27:58 »
Really following this with interest and I like your approach to upgrading the bike.

As you say the preload adjusters will just reduce the sag in the forks but not eliminate the mushiness in the forks, only stiffer springs will solve this.

I'm guessing the forks on the SR250 are 32mm which could open up the possibility of using RD250/350LC springs which given the racing heritage of these little bikes, springs should be readily available in a range of rates. As to what rate you require, I would guess you're looking at around 0.8kg/mm at a rough guess which Sonic Springs are listing for around $85 which I don't think is too bad. Add in the emulators and with a bit of setting up you will totally transform the bike
« Last Edit: May 22, 2018, 12:37:05 by Pod70 »