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

Asides from the speed sensor bracket, the last parts I want to get right for the bike are the mirrors, which are a bit of a side project in themselves. I have been thinking that if I get them right I may want to make a production run of them. The plan was to make the smallest mirrors possible that are still usable - this is how I landed on a 50mm diameter. I tested both flat mirrors and convex. The idea was to be able to mount them on stock brake lever perches and use them as 'underarm' mirrors. After a whole lot of testing and prototyping (mostly fails), I quickly realised that this was just never going to work and it's no surprise no other manufacturer does this. No matter how wide your bars are, or narrow your body is, you just can get a decent view behind you looking under your arm and down the side of your body. Check out a few of the different mounts I tried to get them to work! haha. I am happy with the mirror design itself, it allows for a lot of adjustment in the ball joint. It is just the mounting/placement of the mirror I need to sort... and it is looking like bar end mounting is the only option to be able to see anything behind the bike, even though bar end mounting was the last thing I wanted to do.
 

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Asides from the speed sensor bracket, the last parts I want to get right for the bike are the mirrors, which are a bit of a side project in themselves. I have been thinking that if I get them right I may want to make a production run of them. The plan was to make the smallest mirrors possible that are still usable - this is how I landed on a 50mm diameter. I tested both flat mirrors and convex. The idea was to be able to mount them on stock brake lever perches and use them as 'underarm' mirrors. After a whole lot of testing and prototyping (mostly fails), I quickly realised that this was just never going to work and it's no surprise no other manufacturer does this. No matter how wide your bars are, or narrow your body is, you just can get a decent view behind you looking under your arm and down the side of your body. Check out a few of the different mounts I tried to get them to work! haha. I am happy with the mirror design itself, it allows for a lot of adjustment in the ball joint. It is just the mounting/placement of the mirror I need to sort... and it is looking like bar end mounting is the only option to be able to see anything behind the bike, even though bar end mounting was the last thing I wanted to do.
Some good testing going on there. I had some convex 60mm bar ends I found good but ended up getting some legal e-marked mirrors just trying not to draw the wrong type of attention.
 
Some good testing going on there. I had some convex 60mm bar ends I found good but ended up getting some legal e-marked mirrors just trying not to draw the wrong type of attention.
Probably a good call! I think the E-marked mirrors, at least in Europe need to be a whopping 90mm in diameter or corresponding area.
 
From previous post: 'and it is looking like bar end mounting is the only option to be able to see anything behind the bike, even though bar end mounting was the last thing I wanted to do.'

And this is where I am at... Will most likely order some machined aluminium prototypes shortly. Seems to look pretty ok and works quite well - a decent field of vision behind!
 

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From previous post: 'and it is looking like bar end mounting is the only option to be able to see anything behind the bike, even though bar end mounting was the last thing I wanted to do.'

And this is where I am at... Will most likely order some machined aluminium prototypes shortly. Seems to look pretty ok and works quite well - a decent field of vision behind!
I'm probably one of the very few who like the look of the "Snail Eyes", but I've always wanted to mess with bar-end mirrors also. Hope those work for you.
Regarding the weight issue, the only thing I can imagine that could be further pared or whittled on would be the rims/tires. The engine, which is obviously essential, and the heaviest, really can't be messed with. I weighed my very stripped down and incomplete "thing" and it came out to 220lb/100kg as it sits. This is with it still awaiting battery, tank, lights, muffler and fenders. (When I cut off the rear subframe, I was shocked at how light it was.)
So, with you getting your bike down to 115.5kg, that's pretty much the practical limit for it to retain its identity as a good-looking SR250! Pat yourself on the back! May I also THANK YOU profusely for the incredible side-on pix you've posted of your stockers- it REALLY has helped me "lay out" what I want to do without actually hacking away on the corpse.
 
I'm probably one of the very few who like the look of the "Snail Eyes", but I've always wanted to mess with bar-end mirrors also. Hope those work for you.
Regarding the weight issue, the only thing I can imagine that could be further pared or whittled on would be the rims/tires. The engine, which is obviously essential, and the heaviest, really can't be messed with. I weighed my very stripped down and incomplete "thing" and it came out to 220lb/100kg as it sits. This is with it still awaiting battery, tank, lights, muffler and fenders. (When I cut off the rear subframe, I was shocked at how light it was.)
So, with you getting your bike down to 115.5kg, that's pretty much the practical limit for it to retain its identity as a good-looking SR250! Pat yourself on the back! May I also THANK YOU profusely for the incredible side-on pix you've posted of your stockers- it REALLY has helped me "lay out" what I want to do without actually hacking away on the corpse.
Thanks, let's see how the billet prototypes come out!

I agree with you on the wheels. The wheels that I build up were on a couple hundred grams lighter than the stock ones, so there is absolutely an opportunity there to reduce weight. Thanks for weighing your bike, it is nice to have another reference!

Haha no probs, side profile pics of bikes is where it's at!
 
Thanks, let's see how the billet prototypes come out!

I agree with you on the wheels. The wheels that I build up were on a couple hundred grams lighter than the stock ones, so there is absolutely an opportunity there to reduce weight. Thanks for weighing your bike, it is nice to have another reference!

Haha no probs, side profile pics of bikes is where it's at!
Hi,

Congrats on the top speed run - I’m messaging from NZ and am tuning my 89 sr250 ( I believe it’s actually a 239cc)
Messing around with intake stacks and jetting / basically copying your work.

I was hoping to get a few pointers from you if you don’t mind?
 

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Hi,

Congrats on the top speed run - I’m messaging from NZ and am tuning my 89 sr250 ( I believe it’s actually a 239cc)
Messing around with intake stacks and jetting / basically copying your work.

I was hoping to get a few pointers from you if you don’t mind?
Cheers!

Nice bike! And yes, it will be a 239cc version - you will see it cast on the cylinder.

Most of the work I did was standing on the shoulders of giants. I read a lot and found some handy online calculators for intake size/volume/length and bell mouth design. Then beyond that it was balancing an optimal design with packaging and manufacturing constraints.

My biggest tip would be to tune for the 3rd harmonic. This is from personal experience - testing, but also from what 100s of others have said/done/tested. The 2nd harmonic is too peaky and if you get the 3rd right, you will also automatically get the 4th, lower in the rpm (weaker however, but still significant).

Best of luck!
 
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was your speed testing done with the bike set up as in picture? (img_0900) or did you have lower bars on it?
It won't look as good but some alloy or even steel sheet filling in the triangle to rear shock will give better aerodynamics, plus a degree of 'ram air' if you get it right
 
was your speed testing done with the bike set up as in picture? (img_0900) or did you have lower bars on it?
It won't look as good but some alloy or even steel sheet filling in the triangle to rear shock will give better aerodynamics, plus a degree of 'ram air' if you get it right
The tests were done like that yes. There was no special configuration for the speed tests apart from my body position haha.
That is a great idea for some simple aero! I did everything I could to make this attempt with the bike as naked as possible, but in futher testing, there a definitely some low hanging fruits that would help if I did some simple mods like that.
 
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