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

[JadusMotorcycleParts]

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).

Yeah, will be good to see if any noticeable difference. Might be hard to quantify though - the human bias to change (seat of the pants) is pretty strong. I guess some brake test scenario could be laid out and back to back tests with original set up and new set up could be done - if I get to it.

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.

Great to hear! Yes, I was/am concerned about sheet as well... I guess I could have a buddy from work to an FEA on the circlip and see what forces it takes to deform it based on the way it is held in the groove and the material specs.

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!

Very good to know, yes, am quite concerned about this. I wonder if I could get around the issue of 'locking it in place' by designing some recess groove in the new fork caps I will design - so that the circlips can only be removed in one orientation, otherwise they are trapped in the stanchions undercut and cap groove. But the best is that you have had the same solution in operation for an extended period! This gives me faith.

 

[JadusMotorcycleParts]

A completely random item... The oil filter drain screw. I wanted to offer these to my customers and when I drew it up I decided to get a quote in Ti as well as steel. The price difference was around 20% (most of the cost is programming and machine time, then material is marginal). The part is really nicely made and great quality! Astonishing that the weight is nearly half the stock item. This is negligent in the scheme of things, but I figure after this wee experiment I will try to put together a Ti engine bolt kit. One of Cosworths suggestions from the last build. I figure if I could offer them in black, silver and gold that would be a cool detail item that could appeal to many and give some weight savings to boot.

 

[JadusMotorcycleParts]

Here is some of the development of the re-positioned brake lever/perch/arm. At first I thought it would be logical to do like I have seen on other bikes with a control arm located on one of the guard bosses (which then goes down to a boss to hold the brake rotor in place) - as seen in the first couple of pictures.

But then I thought again and decided there must be a way to design a special bracket to use the existing boss in some way - so instead of cutting it off entirely, I modified it so the brake rotor/drum brake housing could rotate freely without interfering with it. Then when sketching up the idea I saw that it could be adjustable as well - to be able to achieve the correct brake lever arm angle with any given/decided tension/brake trim. The pictures explain most of the idea. This way, the parts can be smaller, weigh less and be less of an eyesore. Plus, it will allow me to shave the fork legs like I want to do (like the other SR I built) and just have the fork brace there holding the guard instead.

The bracket could also be further developed to include a boss that extends rearward and holds the brake wire itself - in cases where the stock brake cable perch is damaged. But this will not be examined this time round.

The last prototype works relatively well/as intended. However it does not sit on the fork leg boss securely enough. I am designing a further prototype now where the boss gets bolted to the brake housing through the backside - removing as much play in the system as possible. Hopefully I'll be able to print and test this during the week.

 

[der_nanno]

The angles are looking good!

 

[doc_rot]

what if you made the opening around the locating boss on the fork slotted so you could add a pinch bolt there to remove slop even further.

 

[jpmobius]

what if you made the opening around the locating boss on the fork slotted so you could add a pinch bolt there to remove slop even further.

I think this is a good idea. In all things brake related, it is wise to consider what will happen if a critical component fails. This is such a part where failure would be considered catastrophic. I think there is no issue that the component will be strong enough, but keeping it in place is another matter. One of the issues I see is the draft in both the boss and the recess in the backing plate. Regarding the boss on the fork leg, I think adding a thick section to the plate that would fit between it and the backing plate would prevent the plate from slipping off the boss. Not sure how that would affect being able to assemble the parts. Machining the boss so that the working faces are parallel and perpendicular to the load would remove the propensity of the existing draft be a ramp and allow it to slip off of the boss, as well as greatly reduce the wear and make for a much improved capacity for being clamped by a pinch bolt. The other side where it fits into the notch in the backing plate has the same issue, but that would seem to be an easy fix by simply using longer bolts that can thread into the backing plate. I don't see any value in the adjusting slots - just drill holes in the right spot - the slots are just another thing to come loose and cause trouble.

 

[JadusMotorcycleParts]

Thanks for all the input guys! It means a lot. I think what I will do now is a combination of your ideas. I will remove the slots - jp is right, if it is in the correct position from the beginning, it shouldn't need adjustment. Then if I do that, why not combine the boss into the bracket (to make that one piece) - to eliminate any movement there too (if this still allows the wheel to be mounted). But I will also make it so that the drum plate is fastened to the bracket completely - both at this boss, but also at the bracket. This will eliminate the slop we are concerned about. I think this is the best compromise to fasten it to the fork boss - because yes, as jp points out, that boss has draft on it and would be impossible to add a pinch bolt to - without machining it flat ofc. I'll draw up a new prototype over the weekend and see if it'll work!

 

[JadusMotorcycleParts]

I have a theoretical question while I'm at it... Would there be any difference if the brake arm was pointing forward or rearward? Still so it is perpendicular to the fork leg, just that the wire is up front or behind the fork. I can't see how it would be any different, but may allow for even easier cable routing from the brake lever on the handle bar...

 

[jpmobius]

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.

 

[irk miller]

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.

 

[JadusMotorcycleParts]

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.

 

[JadusMotorcycleParts]

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.

 

[Pod70]

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

 

[zap2504]

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.

 

[JadusMotorcycleParts]

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.

 

[JadusMotorcycleParts]

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 I'll put up a post when it is all sorted.

 

[JadusMotorcycleParts]

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.

 

[Pete12]

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.

 

[XS750AU]

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.

 

[JadusMotorcycleParts]

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.

Thats a great idea! Unfortunately I don't have the equipment for that