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1983 V45 Sabre Cafe Racer

This is a discussion on 1983 V45 Sabre Cafe Racer within the Project Builds forums, part of the Caferacer.net Forums category; Originally Posted by BalkanMoto ....Another thing to put to rest is i just measured the uncompressed trail. It is roughly ~6.5". Which is plenty for ...

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  1. #71
    Senior Member 8ball's Avatar
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    Quote Originally Posted by BalkanMoto View Post
    ....Another thing to put to rest is i just measured the uncompressed trail. It is roughly ~6.5". Which is plenty for stability and agility. Given that the stock fork was 2" longer that would have resulted at a longer trail that would have made the bike less nimble but more stable and high speeds.

    ....So now that I have answered your questions, could someone enlighten me on what i have done wrong. If anything.
    Considering the stock trail was 4.6", the first clue would be that it is inconceivable that you added 2" of trail by shortening the forks. You don't use the fork line to determine the rake. The line is drawn along the head tube. Maybe that is what you did wrong?

    Name:  Rake Trail.jpg
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    Also, I am trying to follow your improvise diagram of your spring modification. The "rebound spring" is a top out spring, and that is just to keep the fork from having a hard THUNK if/when the fork goes to full extension. Your fork travel is not limited by the fork spring length or # of coils. It is usually limited by the length of the damping rod (that tube that the top out spring slides on). Whatever travel that piece allows has to be available on the fork tube between the bottom of the triple tree and the dust seal...or top of the fork bottom. If the fork tube sits 2" lower in the slider, then you have 2" less travel.
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    .... same thing with furniture, it doesn't have to be straight or not wobble, or keep thing from rolling off it, or not collapse when sat upon, it's personal taste man... if I want a dining room set that endangers the life of my dinner party guests, then it doesn't need second hand approval

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  2. #72
    Member BalkanMoto's Avatar
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    I am positive that i measured it exactly as you have shown on the diagram. I will double check it tomorrow, and will provide some photos. What you are saying makes sense. Just drew it out on a piece of paper and you are right, the piston at the bottom of the fork is indeed the limiting factor. By moving the tubes resting position 2" down i have indeed reduced it by 2" of travel. Thank you for pointing that out.

    So then this would mean i have roughly 3"-3.5" of travel available, and a slightly softer spring, now that its shorter. Which i would imagine would absorb smaller bumps and things quite ok, bottom out pretty quick on bigger ones, and dive under breaking easier then before. Interesting will see how that turns out. Still don't see much wrong with it. Other then the obvious comfort issues. These forks aren't the greatest things in the world in terms of performance.

  3. #73
    Senior Member woodsman's Avatar
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  5. #74
    Member BalkanMoto's Avatar
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    uuuuu thats interesting, and he will deliver anywhere in GTA for $50 extra. i might actually take him up on that.

  6. #75
    jcw
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    https://rbracing-rsr.com/rakeandtrail.html

    https://rbracing-rsr.com/advchoppercalc.html

    There’s an (in)clinometer app for smart phones that gives you a digital readout that I use to measure swing arm angles. Could use it on the steering head as well.

  7. #76
    Senior Member XB33BSA's Avatar
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    Quote Originally Posted by BalkanMoto View Post
    I understand that any sort of modification of the internals of a fork, deviate from the engineered specs and 9/10 ruin performance. And i understand that by definition (on this forum, and any other forum where people care about performance) doing so is completely wrong and pointless. I am not arguing the correctness of that, in fact i do agree. Unless you are an engineer or have enough experience to know exactly what a different component or one modified will achieve, its more or less just an educated guess at best.

    So moving past the impacts of that on the performance of the fork, the affects on the handling and stability, and the aesthetics. Also lets remove the bike as a factor completely. You have a pair of forks and want to shorten them by 2 inches. Now in that context what in my approach of cutting things the way i described above is incorrect? I am trying to learn here if there is indeed something about that process and the math of my measurements that is completely wrong. I guess it would help if someone that understands them would be able to explain what the effect on the fork would be if left like this, or modified further.
    what you should have done first if you really must lower the front is remove fork springs and gently let the front down until bottom metal contact
    then you observe and mesasure the gap to the bottom triple from top of fender/brace
    whatever that gap is then that is how far you can slide the stanchion tubes up,say it is 1-`1/4''
    then you should lower the rear the same that is a little more involved, but the same ,first lets observe what we have now,methodology is the only way to begin a modified design
    take pictures make notes
    why not just carry a pump so when you park it at the mall to hang out with friends you can let the air out of the rear shock and forks for that sweet low badass mean tough burly ass stance
    when you come out the mall

  8. #77
    Senior Member brad black's Avatar
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    because all this is maths based, and i like doing the math and spreadsheets, i'll try to point out why your method is a crap idea in theory.

    firstly, i'd pull your piece of cut off spring out and chuck it to the shit house and refit the original top out spring. use a spacer to shorten them 2" if you must, but don't make it an unknown variable.

    anyway, making a heap of assumptions and some guesses, an example.

    i'd guess your original springs are about a 0.5kg/mm rate - most std non sport things tend to be sort of that. if you take measurements from the spring we can work out an approximate rate. from your info they've got about 140mm travel, and i'd guess they've got 20mm or so of preload.

    spring rate is directly proportional to the number of coils, so assuming they're linear springs if you cut them 15% they're 15% stiffer now, or 0.575kg/mm. you've reduced the travel to 90mm, so the spring force fully compressed is going to be quite a bit less than it was before, and it will bottom out at a load that would have left 30mm or so of travel originally. you can increase the preload a lot, but that makes them very harsh and very crap, and will reduce the sag massively, possibly to 0.

    fitting a heavier spring with less preload will help you hit your target - in this example a 0.80kg/mm with 10mm preload.

    graph shows the load versus compression curves for the above. blue is the original, red is your cut down and shortened crap, orange your cut down crap with an extra 30mm of preload, and purple the heavier rate spring with 10mm preload.

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    the actual rates will depend greatly on oil height (how much oil you put in the forks), as this effects the air spring effect and can give a marked progressive effect on it's own. you can play with this as well, and it might also help you hit a target to compensate for the travel reduction.

    some examples of that are on my blog.

    https://bradthebikeboy.blogspot.com/...nd-impact.html
    https://bradthebikeboy.blogspot.com/...nd-impact.html

    i played with some late 80's 750 sport forks a while ago, cutting the springs down from 40 to 32 coils (they're a dual rate sort of spring, and the 8mm cut off was the "soft" end) and setting the oil height. std spec was 150mm or so, and i usually test how they feel once assembled by putting the bottom on the floor and bouncing down on them as hard as i can. most things i can almost bottom, but the sport forks with the oil at 150mm were nowhere near bottoming, so i ended up dropping the oil to 200mm, and they felt a lot better.
    Last edited by brad black; 10-26-2018 at 07:00 AM.
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  9. #78
    Senior Member brad black's Avatar
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    Quote Originally Posted by XB33BSA View Post
    what you should have done first if you really must lower the front is remove fork springs and gently let the front down until bottom metal contact
    then you observe and mesasure the gap to the bottom triple from top of fender/brace
    whatever that gap is then that is how far you can slide the stanchion tubes up,say it is 1-`1/4''
    then you should lower the rear the same that is a little more involved, but the same ,first lets observe what we have now,methodology is the only way to begin a modified design
    take pictures make notes
    why not just carry a pump so when you park it at the mall to hang out with friends you can let the air out of the rear shock and forks for that sweet low badass mean tough burly ass stance
    when you come out the mall
    like this.

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    then, instead of guessing, you know exactly where you are. just like doing the math.
    8ball and BalkanMoto like this.

  10. #79
    Senior Member TrialsRider's Avatar
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    Did you price out insurance yet? If not, get ready for a shock.
    You also might want to locate a motorcycle mechanic who will certify it after it's modified, or plate it in stock form first.
    woodsman likes this.

  11. #80
    Senior Member Cyorg's Avatar
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    Not suggesting you do this, but you can confirm trail etc if you want without breaking the bank. Laser sights and bore sights can be had for cheap. They don't have to be super accurate because you're not working at 1000 meters. One laser mounted in a piece of stock. Mild steel will do and then your magnetic protractor will stick to it. Slide it through the centre of the triple tree. Another one hanging from the front axle (wheel removed). The front axle has to be the proper distance from whatever you are shooting the beams onto. You could just use a string and a weight, but the weighted laser bracket hung from bearings is self centring, so it remains accurate when farting around trying different settings etc. Ignore the level....

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    "Non urinat in ventum"

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