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Author Topic: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp  (Read 24830 times)

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #150 on: Aug 30, 2018, 12:13:45 »
Last night I tried experimenting with the idea of getting the front brake cable to run parallel with the fork leg - so that the lever is at right angles.  This removes cable length and bends in the system and 'should' stiffen up the brake feel at least.  Whether or not it improves the actual function or not, I don't know.  I have another couple tricks up my sleeve for that so we'll see if I can experiment with those too.

This first operation requires rotating the drum housing/pivot arm around the axle.  To do this, the locating boss on the fork leg needs to be modified (in this case I cut it right off in order to be able to measure and prototype a solution) and a new bracket/boss will take its place - one that fits in the slot and can be adjusted in angle increments about the center of the axle.  Prototypes coming soon...

Offline jpmobius

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #151 on: Aug 30, 2018, 12:45:07 »
What's the plan for the cable as the suspension moves?  The brake cable could telescope through guides and bend where it needs to go to the lever, but the speedo cable is a pretty straight shot with no place to curve away when the suspension compresses.
Mobius


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Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #152 on: Aug 31, 2018, 03:37:22 »
What's the plan for the cable as the suspension moves?  The brake cable could telescope through guides and bend where it needs to go to the lever, but the speedo cable is a pretty straight shot with no place to curve away when the suspension compresses.

Yeah exactly, the cable will run through an eyelet - like on early motocross bikes.  The speedo cable is however more tricky.  If I was keeping it, I would need to create an extra loop somewhere I guess?  Or a universal joint?  Ahahah

But for this bike it won't matter, I'll be plugging that hole with one of the tach plugs I designed and will use an electronic speedo with a pick up instead - hoping this will be more accurate for speed. 

Offline der_nanno

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #153 on: Aug 31, 2018, 07:56:57 »
Last night I tried experimenting with the idea of getting the front brake cable to run parallel with the fork leg - so that the lever is at right angles.  This removes cable length and bends in the system and 'should' stiffen up the brake feel at least.  Whether or not it improves the actual function or not, I don't know. 

Regarding your question: This improves the front brake to no end and not only the lever feel, but the actual and measurable performance. But you have to set up the lever so it only hits 90degrees at full pull of the cable. Going over this point will decrease the length of the lever and thus make the brake feel worse again.

(It's a common thing on XT500s to tune the front brake like that and same goes for the rear drums of both  my 4-valve xt and my TR1.)
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Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #154 on: Sep 03, 2018, 03:48:59 »
Great thats good to know nanno!  The bracket will be a bit adjustable to I am hoping to be able to play with that angle. I always think it's much cooler to improve on something already there with a bit of ingenuity, rather than just swap it out entirely.  I think that's the kiwi in me.

While on this topic, I was thinking of designing a billet lever arm - a bit like the ones for the SR and XT500.  But when holding the stock pressed metal one in my hand and looking at it, I just can't imagine a new arm would or could be any stiffer - the stock one has all it's stiffness in the right direction.  Plus I don't think there would be any weight savings to speak of.  Thoughts?


Offline der_nanno

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #155 on: Sep 03, 2018, 15:03:39 »
These look nice. That's about it. Look at the rear (ally) brake lever of a 1984 to 1986 XT600, basically the same bit except the fancy nut.
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Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #156 on: Sep 07, 2018, 05:57:24 »
I dismantled the forks this week to get a closer look at the damping rod, spring and circlip groove. 

I tested the circlips I ordered and they work a treat.  This is definitely what I will be using in the future and will specify with any eventual kit I might develop.  Now it would be a one hand job to remove the clip (with circlip pliers of course), instead of 3 hands a lot of swear words.

Offline zap2504

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #157 on: Sep 07, 2018, 12:10:12 »
I think circlips is a great idea! Don't know why they were never used in the first place as they can't cost that much more and would greatly improve assembly/repair times.
Also looking forward to any quantitative measurements on the front brake mods as I cannot imagine that ventilation holes and lever repositioning will beat replacement with a disk brake (and its subsequent improvements on both swept square inches and cooling).

Offline Sav0r

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #158 on: Sep 07, 2018, 13:56:41 »
I use circlips on all the dampers I build, no threads at all. Makes rebuild like a 10 minute process. The downside I think is that they are a little more finicky and they are more tempting for people to mess with. They also have less shear than a threaded item, but in many cases they are good enough. Machining the grooves is generally much easier.
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Offline goldy

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #159 on: Sep 08, 2018, 09:01:24 »
The way the original clip works, they are basically locked in place by the relief cut in top slug and cannot be easily dislodged. The snap ring you are using isn't locked in place at all. The whole mass of the front end is being held on that snap ring. Having said that, I am currently working on a set of GT 750 forks with exactly the same modification...they have been in service for a very long time with no adverse effects and like you mentioned, they are a heck of a lot easier to get apart!
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