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Project Fuel Injected Turbo FZR250, half

Discussion in 'Your 250cc Projects' started by Mike Green, May 9, 2020.

  1. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Mike, question for you, I'm trying to accurately measure the spring installed height (This is for fitting CBR250RR valves).

    How do you lift out the spring seats?
    I'd like to measure their thickness
    They rotate nicely on the oil film using a small dental pick.

    ...and as I'm writing this I realise that I haven't squirted some degreaser down there to see if that helps...
     
  2. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    Magnet perhaps???
     
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  3. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Good idea - I'll try that, I've for some little 6mm neodymium discs that I'll have to try and hold somehow on edge and keep them off the valve guide - space down there is very tight
     
  4. Mike Green

    Mike Green Well-Known Member Premium Member

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    Magnet is a good suggestion. Or maybe circlip pliers that can grab the lip. I found tipping it upside down and banging it on the bench can also work. Blowing compressed air around can also loosen it by blowing it off the seat. You don't need to remove the seats if you are measuring spring installed height. Only if you need to make new spring bases. The attached drawing is what I ended up making. I may have also made a couple with even thicker base, maybe 1.4-1.5mm thick. I just measured a standard one and it has a .98mm thick base. upload_2024-1-31_13-23-58.png
     
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  5. Mike Green

    Mike Green Well-Known Member Premium Member

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    I've fitted the spring I modified and put the shock in the bike. With standard dogbones it sits low at the back. If I hold the bike at the same height as the original shock and dogbones, with the new shock I need 10mm shorter dogbones which I'm going to have to make. Once fitted I'll go through measuring shock travel versus wheel travel again to check for differences and also to record exactly what I end up with if further changes are required. The comparison with the original setup will be interesting as well.
     
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  6. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Love you for this, big man hug - I've for a piece of thin wall brass tube with two 6mm neodymium magnets taped to it (looks a little like Princess Leia's hairstyle from Star War) - down over the guide, click and up she pops.

    Measured the first spring seat, it's 1.02mm thick.
     
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  7. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    That's really impressive work with that attention to detail - I'm coming at it from a different direction - CBR250RR valves are longer ~2.5mm - for far too long I've wrestled with removing valve length from the tip, then thinner shims, and I got as far as contemplating grinding the buttons on the underside of the buckets.

    Still wasn't going to get me to something which fits under the standard cam without grinding..

    I don't know if my springs are over length as yours were just yet, but...if they are as is to be anticipated

    This was before I bought a lathe mind you...

    The sensible solution is to cut new collet grooves, so now that I know there's some potential to thin the spring seats (within reason) I've got enough material between the old and new collet grooves to be able to do that sensibly
     
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  8. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Give that you've got the original suspension geometry mapped it will be very interesting to know if the near linear ratio is altered significantly by dogbone length - it's likely not something us mere mortals would detect.
     
  9. Mike Green

    Mike Green Well-Known Member Premium Member

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    Yeah, agreed. I doubt I would notice a small change. I have seen comment about different length dogbones making the suspension harsh which might be pointing to a rapidly rising rate. Originally it does rise but it's pretty linear as far as I measured which was up to about 90mm wheel travel
     
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  10. Mike Green

    Mike Green Well-Known Member Premium Member

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    [QUOTE="ruckusman, post: 152728, member: 2491 "I got as far as contemplating grinding the buttons on the underside of the buckets."[/QUOTE]
    I'd be very careful. The force to open the valve needs to be applied to the valve stem. Reducing things like this, or more particularly stem height above the collets, can result in the force being applied to the spring retainer or collets. The other thing is that the inside of the buckets has a radius. Reducing the button will move that radius closer to the edge of the spring retainer. New cam grind would be the safer option but then you run into lobe clearance problems and as I recall it's already snug.
     
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  11. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Fortunately I've ditched that idea - the button in the centre of the underside of the buckets sits proud by 1.45mm, but even with valve tips ground, thinner shims and potentially the buttons in the buckets ground, I still couldn't get it to where there was adequate the clearance - so you correctly spotted the issues, shims getting closer to the colets, potentially the spring retainer touching the underside of the bucket - that would end in disaster.

    A lathe has allowed me to rethink ideas

    I do plan on getting the cam reground as per @gregt bucket motor build, but I wanted to do stepwise changes to see what changes what - valves being first - then I can make up any difference with clearance on a reground cam with thicker shims, keeping it all within spec.

    Concept is to be able to say CBR250RR valves, recut collet grooves, recut seats = this change/increase
     
  12. Mike Green

    Mike Green Well-Known Member Premium Member

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    Short dogbones made and fitted. The bike is setup and ready to remap the shock ratio
     
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  13. Mike Green

    Mike Green Well-Known Member Premium Member

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    I just thought of something else. Going the cam grind route will have the buckets 2.5mm further out of their bores. Is there a danger of them being canted over as the cam lobe starts to push down?
     
  14. Mike Green

    Mike Green Well-Known Member Premium Member

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    The results may be indicating a bit of flex in my setup for measuring the ratio. I will get a chunk of steel and do it once more to see. the jig also bottomed out at 65 turns
    I have made another set of measurements as per the attached image. The ratio was reasonably close apart from initially. That initial movement would normally be covered by rider sag. There's nothing really horrible about it from my perspective. I set the first values to "10" so the "Y" axis scale remained reasonable.
    Ratio1 is with standard dogbones, Ratio2 is with 10mm shorter dogbones.
    upload_2024-2-2_12-15-42.png
     
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  15. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    The harsher sensation that has been described to you makes sense with it graphed out like that - Imagine braking into a corner, front loaded, back unloaded, until the back sits back down past ~30mm at the wheel, small bumps will encounter a high ratio at the shock travel
    I feel all knowledgeable all of a sudden - thank you
     
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  16. Mike Green

    Mike Green Well-Known Member Premium Member

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    As I understand it, the effect will be the opposite of what you suggested. Wheel movement results in hardly any shock movement early on.
    Anyway, I made the rig a whole lot stiffer, made sure to remove slack before starting measurements, and got a completely different answer.
    I'm going to set it up with the standard shock and dogbones and do those measurements again.
    The ratio is closer to 1:1 with the short dogbones so that would make the suspension harsher if nothing else changed, but I have a much softer spring and the shock is adjustable so hopefully will allow the damping to be tweeked for a decent ride.

    Living and learning.

    upload_2024-2-2_15-2-17.png
     
  17. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Got it, I read the ratios backwards - looking at what you've got there, in a realm of infinite adjustment, if you were able to vary both the shock length and dogbone length you'd be able to fine tune for a given spring rate within reason &/or make small changes to the steering head angle by moving the back wheel up and down - it would take more than a mere mortal to know what to change I suspect.
     
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  18. Mike Green

    Mike Green Well-Known Member Premium Member

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    Right. I have put a bit of thought into this. The ratio I have been graphing is the overall ratio at each increment of wheel movement of 1.25mm. So the graph is of total wheel movement up to that point divided by total shock travel at the same point. Technically I should be plotting the instantaneous ratio, or in other words the amount the wheel moved from the last point divided by the amount the shock moved from the last point. I did have a look at this but it's quite noisy which is likely to be a limitation of my setup. It showed a trend and if I was a bit better at this I probably could have smoothed the line.
    I did go ahead and record the movements using the standard shock and dogbone layout. The end result was a bit better start to the graph but generally the same. Ignoring the first 10 data points the graphs have a similar slope. Both have a slightly rising rate. The difference is that the short dogbones result in a lower ratio throughout, there's less leverage on the spring and damping.
    Where I see this being a problem is if you simply put an R6 shock into a 250 and make 10mm shorter dogbones to retain ride height, the slightly heavier R6 spring in conjunction with the reduced ratio, could result in the bike being over-sprung. I haven't tried it yet but I'm near certain that the 250 would benefit from a lower spring rate. Definitely for solo riding, maybe!
    upload_2024-2-2_17-53-7.png
     
  19. Mike Green

    Mike Green Well-Known Member Premium Member

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    Dogbones being lightened a bit. I might decide to take a bit more off. It'll be easy with the jig I made. Just bolt each one back on and run a cutter across.
     

    Attached Files:

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  20. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Looking at that, you could probably make them hollow through the centre as te material left is equal or more than the amount of material around the bolt head, the bone shaped dog bones are the same, just in reverse.

    What mill do you have BTW?
     

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