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Author Topic: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp  (Read 20683 times)

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #170 on: Oct 01, 2018, 03:34:41 »
If the cam that pushes the shoes apart is symmetric, it does not matter which way it turns so you could flip the arm over and have the cam rotate backward.  However, you would have to make a new mount for the cable housing, and have an all new design for a plate to keep the brake plate from turning.

Just what I thought.  Thanks.

Isn't that a speedo drive in the brake hub?  Flipping it won't for that, as much as it wouldn't work for the stay.

In this case, the speedo drive will not be used anyway, so it's position is not considered.

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #171 on: Oct 01, 2018, 03:37:59 »
This is my latest idea.  Why not try to replicate the what the stock brake plate does, but just reposition it?  So now the idea is to bolt the bracket to the plate, then have it slide up onto the fork boss - like the stock one - as you install the wheel.  Then the last step is to install a bolt, not necessarily a pinch bolt, rather an encapsulating bolt that just holds it in position and increases the strength of the bracket.  I'll give it a shot with a prototype anyway.

Offline Pod70

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #172 on: Oct 10, 2018, 12:42:31 »
Please take this as a constructive observation rather than criticism but I think you may going in the wrong direction with the brake cable mod. As I understand it the 2 key points to an effective cable operated brake are:

1. The angle between the cable and the cam lever should be kept as close to 90 degrees at the minimum and maximum points of the pull (brake off & on) this is governed by the position of the cable retainer on the drum and the length of the cam lever. Rotating the drum around as you have done is not going to alter this but correct setting up of the cable, lever etc. will

2. Sharp bends in the cable need to be avoided to prevent the inner cable binding against the outer cable and you must check this at max & min suspension travel as well as from lock to lock. Having the brake lever on the right and the brake plate on the left ensures that the cable doesn't bind or twist in the cable adjuster at the lever thanks to the straight run of the cable along the handlebars and then a gentle curve down the left fork leg followed by a gentle curve towards the cable attachment point on the drum. The first curve allows for the movement of the handlebars and the second allows for the compression of the forks.  Removing the 2nd curve could cause binding of the cable when the suspension is compressed especially on a road bike with less suspension travel than the off-road forks shown earlier in the thread.

Adding the bracket will only increase the unsprung weight (not a good thing) and potentially introduce more flex into the system (definitely a bad thing)

I hope my points are helpful by coming from a (literally) different direction and keep up the good work
« Last Edit: Oct 10, 2018, 12:46:05 by Pod70 »

Offline zap2504

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #173 on: Oct 10, 2018, 15:11:26 »
WRT fork springs - were you able to locate any CB360/400F or RD250/350LC fork springs for experimentation? I've seen some new CB360/400F fork springs on ebay in varying rates so would be very interested in fitment to the SR250 forks.

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #174 on: Oct 16, 2018, 03:43:56 »
Please take this as a constructive observation rather than criticism but I think you may going in the wrong direction with the brake cable mod. As I understand it the 2 key points to an effective cable operated brake are:

1. The angle between the cable and the cam lever should be kept as close to 90 degrees at the minimum and maximum points of the pull (brake off & on) this is governed by the position of the cable retainer on the drum and the length of the cam lever. Rotating the drum around as you have done is not going to alter this but correct setting up of the cable, lever etc. will

2. Sharp bends in the cable need to be avoided to prevent the inner cable binding against the outer cable and you must check this at max & min suspension travel as well as from lock to lock. Having the brake lever on the right and the brake plate on the left ensures that the cable doesn't bind or twist in the cable adjuster at the lever thanks to the straight run of the cable along the handlebars and then a gentle curve down the left fork leg followed by a gentle curve towards the cable attachment point on the drum. The first curve allows for the movement of the handlebars and the second allows for the compression of the forks.  Removing the 2nd curve could cause binding of the cable when the suspension is compressed especially on a road bike with less suspension travel than the off-road forks shown earlier in the thread.

Adding the bracket will only increase the unsprung weight (not a good thing) and potentially introduce more flex into the system (definitely a bad thing)

I hope my points are helpful by coming from a (literally) different direction and keep up the good work

Thanks for the input Pod!  This mod is inspired by these guys here: https://www.klemmvintage.com/bighorntech.htm  They are serious vintage racers and don't usually do stuff just because.  The mod makes sense to me and it seems like a simple way to improve brake 'feel'.  How much it actually improves mechanically is up for debate  ;)  But eliminating big sloppy loops in the wire is the number one goal (pretty much what you're getting at in point 2 of your reply).  With the set up using this bracket, there will essentially be one soft 90 degree bend in the wire (at the handle bar just after the lever).  Otherwise it will be running straight up and down with the fork leg.  I also don't understand why all offroad bikes from this era with front drum brakes have this set up, road bikes didn't?  Something to do with the speedo cable drive outlet?  Offroad bikes didn't have this but road bikes do/did?

Point taken about the unsprung weight.  I agree.  But hopefully the 'drillium'  ;D will have helped here plus the fact I have removed half of the boss on the fork leg - so maybe I am back to plus minus 0.

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #175 on: Oct 16, 2018, 03:47:01 »
WRT fork springs - were you able to locate any CB360/400F or RD250/350LC fork springs for experimentation? I've seen some new CB360/400F fork springs on ebay in varying rates so would be very interested in fitment to the SR250 forks.

I haven't come this far sorry Zap.  What I will do, both for my own sanity and for others wanting to improve the SR's front end, is create sort of a cost vs benefit spreadsheet - where I included things like new springs, spacers, adjustable fork caps, valve emulators etc and see where is all ends up.  Maybe none of it is worth it, maybe all of it is, but probably there is a nice balance.  Also see this next post about what I have been drawing up for fun...

I have also got stuck with this damn brake perch, the angles and fitment have been baffling me.  I am (embarrassingly) on like the 10th prototype  :o  I'll put up a post when it is all sorted.

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #176 on: Oct 16, 2018, 04:07:28 »
I will hopefully picking up my wheels this week - with the tyres mounted.  This will give me a rolling chassis where I can start working out my ride height, suspension travel, rake and trail.  Very exciting.

This got me inspired to finally start on something for the forks.  At this stage, I am planning on testing two routes, a budget route and an expensive route and I will then test the performance difference (if not too subjective and hopefully measurable???). 

Cheap route - little like the Minton mods for the XS650 forks:

1) Investigate the use of 33mm fork seals (instead of the SR's 32mm) to reduce stiction - read about the Minton mods before calling me an idiot
2) Drill up the damper rod holes a size (just a little) and countersink the holes (better oil flow in and out of the orifices)
3) Cut out the soft part of the stock SR spring (remembering this is a dual rate spring, not a progressive spring) and replace that length with a spacer
4) Install custom fork caps with a circlip that are easy to remove
5) Play with preload by installing different length spacers under this cap - the ones that sit on top of the stock cut spring
6) Play with fork oil viscosity and level (perhaps via a convenient hole in the fork cap that oil can be added and removed from, them plugged - without removing the entire cap).

Expensive route:

1) Drill out the damping rods so they are in essence, useless
2) Install valve emulators (correct size exists, just need to order)
3) Investigate either custom progressive springs for the SR or as Zap mentions, CB360/400 or RD250/350 springs
4) Install custom designed adjustable fork caps (with 20mm preload adjustment)
5) Still play with fork oil viscosity and level

This will be fun to mess around with and likely improve the front end a lot, even by doing little - because it is so useless stock.

There is one more possibility on the table that I haven't figured out yet - that is designing the caps to have an air valve in them to be able to convert the forks to air dampened assist (same as my mountain bike).  I have little knowledge of this so am less inclined to test.  But it wouldn't be too difficult to implement. 

Attached are a couple ideas I am working on for the adjustable cap version.  Whether or not this works out from a cost benefit point of view, I don't know.  But for me it is a lot of fun to draw up concepts.

Offline Pete12

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #177 on: Oct 16, 2018, 13:04:18 »
Not sure whether you can do this on your forks or not but on my RD flat tracker I used Kawasaki forks that had a circlip holding the caps in place, so I just machined an internal thread in them and used conventional fork caps.

Offline XS750AU

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #178 on: Oct 16, 2018, 23:06:09 »
Quote
This got me inspired to finally start on something for the forks.  At this stage, I am planning on testing two routes, a budget route and an expensive route and I will then test the performance difference (if not too subjective and hopefully measurable???). 

So how are you going to measure success?
I have bought a few bikes, the Husaberg in particular, where the PO had wound up the preload, compression dampening and rebound to a point the bike was unsafe. It would spit you off at the first bump, no wonder he sold it so cheap! Backed it all off so the wheels could actually react and now it is beautiful bike to ride! It is much softer, but it does not bottom out and the wheels stay engaged with the track (except when airborne)  ;D.
“Engineering is the art of being approximately right rather than exactly wrong.”

Yamaha XS750-2D
Yamaha XS896
Husaberg FE550
Yamaha TT250
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