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Author Topic: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp  (Read 22255 times)

Online JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #160 on: Sep 10, 2018, 11:36:01 »
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.

Online JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #161 on: Sep 13, 2018, 14:13:38 »
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.

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #162 on: Sep 16, 2018, 14:43:15 »
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.

Offline der_nanno

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #163 on: Sep 17, 2018, 01:55:34 »
The angles are looking good!
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Offline doc_rot

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #164 on: Sep 18, 2018, 11:57:12 »
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.

Offline jpmobius

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #165 on: Sep 18, 2018, 14:05:28 »
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.
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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #166 on: Sep 28, 2018, 12:07:31 »
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!

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #167 on: Sep 28, 2018, 12:11:28 »
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...

Offline jpmobius

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #168 on: Sep 28, 2018, 13:50:45 »
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. 
Mobius


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Offline irk miller

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #169 on: Sep 28, 2018, 15:59:08 »
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.