Cafe Racer Forum banner

61 - 80 of 137 Posts

·
Registered
Joined
·
1,512 Posts
shortening the actual usable travel always requires a firmer spring rate,which cannot be achieved with preload
if the main spring is a straight constant rate then cutting to remove active coils will firm up its rate
 

·
Registered
Joined
·
2,107 Posts
Here's a gift for you (includes Sabre). Aftermarket manuals aren't always up to snuff. Read it from cover to cover. Take an iPad to the shitter with you to expedite the process. Just tape off the cameras so your junk doesn't show up on a web site somewhere in the Balkans.

Ps. quit dicking around spending money and figure out what you are going to do with those cases and "dented" frame.

VF750C Shop Manual
 

·
Registered
Joined
·
38 Posts
Discussion Starter #63
Thanks for the indexed service manual. I had a copy of it already but this helps with finding things quicker.
 

·
Registered
Joined
·
1,512 Posts
and i might add that taking 2'' of travel out of a 5-6'' travel fork,will give you a terribly bad mannered motorcycle
why are you lowering it in the first place ? you do realize that besides completely ruining the comp[liance of the forks that you are moving the steering geometry closer to that of spontanious instability
and if you lower the rear 2''
to match the front and spring it stiff as fuck as well to keep it off the bottom bumpers
then the only roads you will be able to play on are the very smooth ones because i defy you to miss all the bumps and holes
you know those litle used roads that are narrow and twisty and curvy
the best riding roads also used by log trucks.... the smooth chuckhole free logging roads lol
back to the main question why are you fucking up the handling and suspensioon ?
because you have the oportunity here to listen to us and learn what will work to improve the machine
if you are lowering it for a look well then dont waste my time tell me now
 

·
Registered
Joined
·
38 Posts
Discussion Starter #65
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.
 

·
Registered
Joined
·
1,588 Posts
Completely agree with XB, the best riding is always the secondary and tertiary roads. In Ontario, Quebec and the northern US those roads are going to have heaves from frost, cracks from baking in the sun and broken/missing pavement. Some of it will be on the line you're travelling when leaned into a corner, it won't be fun with a poorly thought out bike.
 

·
Registered
Joined
·
754 Posts
this is a piece on top out springs, as fitted to the newer varieties for forks. don't mistake any of this for what you have done, which is put a very stiff spring in to reduce to fully extended length. a spring that short will have a very high rate and won't do a lot in terms of moving.

https://www.sportrider.com/how-do-top-out-springs-affect-sag

how much travel do the forks have? some long forks don't have that much travel, they're just long for the purpose of the motorcycle - ie, look like a chopper.

as i posted, cutting a spring is one way to make a spring harder, but you've not done any of the maths with regard to initial spring loads and modified spring loads.
 

·
Registered
Joined
·
38 Posts
Discussion Starter #69 (Edited)
Ok so here are the specs and facts:

Stock fork travel is quoted by honda at 5.5", that is for the full 21" spring. We can all agree that the travel(compression) amount is proportionate to the length of the spring. Also these springs are constant rate springs not progressive ones (aka there isn't more coils on one end). So cutting 3" off the spring would constitute a 14.28% change. So the argument then can be made that the new travel of the fork, given that its mounted in the same location as stock (which it is) would change by roughly the same amount. In other words the travel on the modified fork should be ~4.71". This is obviously very rough math, and we can agree that what honda quoted as the stock travel is actually probably slightly less. In any case the modified fork should have anywhere from 4" - 4.5" of travel. Now i agree spring constants do change when you cut the forks, and those are proportional as well. Which should result in a 14.28% easier to compress spring (if i am not mixing things up). So those 4"-4.5" inches should be quite a bit softer to compress. Yes of course that will result in a slightly softer ride, but its not like i cut the spring in half and made things a crazy amount more softer and less travel.

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 in conclusion to answer the question everyone is dying the hear the answer they already knew. I shortened the forks because aesthetically it is more pleasing. Now before you all start loosing your shit and take up your pichforks and torches and shoo this poser out of your performance oriented forum. I would like to add that i did that as an educated guess. I was aware i was changing the performance of the fork by shortening it, and the stance of the motorcycle. So ofcourse rake/trail were going to be effected. No i didn't do the math before hand, but i have now. And as you see above it shows that in regards to performance i've actually improved the bike. Making it slightly easier to steer (aka more nible), but still well within the sweet spot of a stable trail, ofcourse at the cost of some front end comfort.

So now that I have answered your questions, could someone enlighten me on what i have done wrong. If anything.
 

·
Registered
Joined
·
1,535 Posts
....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?

Rake Trail.jpg

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.
 

·
Registered
Joined
·
38 Posts
Discussion Starter #72
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.
 

·
Registered
Joined
·
38 Posts
Discussion Starter #74
uuuuu thats interesting, and he will deliver anywhere in GTA for $50 extra. i might actually take him up on that.
 

·
Registered
Joined
·
1,512 Posts
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
 

·
Registered
Joined
·
754 Posts
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.

fork spring.png

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/2013/12/marzocchi-43mm-fork-design-and-impact.html
https://bradthebikeboy.blogspot.com/2014/01/marzocchi-43mm-fork-design-and-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.
 

·
Registered
Joined
·
754 Posts
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.

180607 (1).JPG

then, instead of guessing, you know exactly where you are. just like doing the math.
 

·
Registered
Joined
·
5,609 Posts
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.
 

·
Registered
Joined
·
2,107 Posts
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....

Laser.jpg
 
61 - 80 of 137 Posts
Top