modifying fork travel

[TheCafeKid]

That makes no sense at all, shorter springs aren't stiffer they just bottom out easier.

What the OP needs is a stiffer suspension to accomodate the weight. Heavier springs is the way to go; putting in spacers to preload the spring is fine for suspension tuning but not the way to fix a suspension thats bottoming out. The more preload spacer length you add the less fork travel you have, so if your forks have 6" of travel putting in a 3" spacer cuts the travel in half.

if they worked that way, but they dont.

[salty_monk]

Mr Killer2600 - You need to learn how springs work otherwise you make yourself look silly...

In a nutshell a spring is made stiffer in 2 ways (given that the diameter of the spring remains constant). Note I say Stiffer i.e. the spring rate changes not just the preload (which is initial compression of a spring that makes it more resistant to inital movement, once it starts moving the spring rate is the same as the spring without any preload).

1. Heavier / harder gauge steel is used to form the coils.
2. Less coils are formed over a given length.

In the case of number 2 you can either take as an example a 500mm spring at 1 coil per 10mm so 50 coils. You could make it much stiffer if you did 1 coil per 20mm ie. 25 coils. You could also cut the first 50 coil spring in half & replace the 25 coils you cut off with a 250mm solid spacer - the resulting spring rate would be the same the only difference being that the cut spring would reach the point where it binds (i.e. where all coils close up) at a lesser travel than the full length 25 coil spring.

That's very simple example but basically correct in the case of our forks. Cutting 3 or 4" off the stock spring & replacing with a solid spacer of the same length is a proven technique for stiffening up our stock fork springs. I have done it myself as have plenty of others on here.

Dan

[Killer2600]

if they worked that way, but they dont.

Actually they do work that way, if I take a spacer that it's length combined with the length of the fully compressed fork springs just fits inside the forks the forks will have ZERO travel. If your theory was right I could do the above mentioned and still get full travel, obviously in the above case one can not.

Mr Killer2600 - You need to learn how springs work otherwise you make yourself look silly...

In a nutshell a spring is made stiffer in 2 ways (given that the diameter of the spring remains constant). Note I say Stiffer i.e. the spring rate changes not just the preload (which is initial compression of a spring that makes it more resistant to inital movement, once it starts moving the spring rate is the same as the spring without any preload).

1. Heavier / harder gauge steel is used to form the coils.
2. Less coils are formed over a given length.

In the case of number 2 you can either take as an example a 500mm spring at 1 coil per 10mm so 50 coils. You could make it much stiffer if you did 1 coil per 20mm ie. 25 coils. You could also cut the first 50 coil spring in half & replace the 25 coils you cut off with a 250mm solid spacer - the resulting spring rate would be the same the only difference being that the cut spring would reach the point where it binds (i.e. where all coils close up) at a lesser travel than the full length 25 coil spring.

That's very simple example but basically correct in the case of our forks. Cutting 3 or 4" off the stock spring & replacing with a solid spacer of the same length is a proven technique for stiffening up our stock fork springs. I have done it myself as have plenty of others on here.

Dan

The spring is still no stiffer and the fork suffers, illusions are grand but no type of solution.

Maybe I'll try your theory in my truck though, I can always use more payload ability/spring stiffness and I'll be able to drop the truck at the same time while making it more heavy duty without having to spend a dime.

[TheCafeKid]

You also need to study how the front forks are designed...

[twiggy2cents]

i havent read this but i had an idea put a piece of rubber between the two tubes, around the piece that the bolt goes into, this will limit travel with out a sudden stop

[Killer2600]

You also need to study how the front forks are designed...

Thanx for the advice but I think you should study them more closely. Personally, I've had forks fully apart and know how they work. I may be no genious with a 200 IQ but I know how suspension systems work, more importantly I know the common misconceptions regarding motorcycle suspensions including ones regarding preload and preload spacers.

I'm just gonna put it this way for the OP regarding the matter of bottoming out the forks, you can try cutting the springs all you want and put in spacers but without changing the springs your still gonna bottom out. Whether it's bottoming out and hitting the wheel or bottoming out and the coils binding up and no longer having any give without hitting the wheel it's still bottomed out (at the bottom of travel).

[Steve]

Killer, sorry to say this, but shortening the springs does stiffen them up.

Picture it this way: The springs are supporting the weight by using torsion, or twisting, along the entire length. I don't know how long the wire is if you were to straighten it out, but let's say that it's six feet long. Anchor one end in a vise, clamp some vise-grips on the other end. Rotate the wire about 90 degrees, measure the force necessary. Now cut the wire in half, measure the force again, I think you will find it substantially stiffer.

OK, so we are not cutting the springs in half, but you should be able to see that a shorter spring is stiffer. The increase in stiffness will be somewhat proportional to the change in length of the spring that is left. When you cut off three inches of spring, adding three inches of spacer will maintain the same ride height of the bike. Now, when you try to compress the springs, they will resist harder, displaying their rate increase.

Ideally, yes, replacing the springs with ones of the correct rate is best, but for those of us on a tight budget, cutting the springs is almost free. Unless you get really carried away with how much you cut off, there should not be much problem with coil bind reducing overall travel, but the increased rate might just minimize the travel by itself.

.

[TheCafeKid]

Thanx for the advice but I think you should study them more closely. Personally, I've had forks fully apart and know how they work. I may be no genious with a 200 IQ but I know how suspension systems work, more importantly I know the common misconceptions regarding motorcycle suspensions including ones regarding preload and preload spacers.

I'm just gonna put it this way for the OP regarding the matter of bottoming out the forks, you can try cutting the springs all you want and put in spacers but without changing the springs your still gonna bottom out. Whether it's bottoming out and hitting the wheel or bottoming out and the coils binding up and no longer having any give without hitting the wheel it's still bottomed out (at the bottom of travel).

Maybe thats the problem?? Its quite apparent that you dont, nor do you understand how a spring works. Lets say that your spring is designed for 4inches of total travel. Now lets say the spring is designed to support 200 lbs per inch of travel. To bottom that spring out, how many pounds would need to be on it?? And thats just a normal spring...now understand that even the STOCK springs are progressive rate springs...IMO even if he cut the spring in HALF, the amount of force that needed to be applied to bottom it out so that the wheel hit the frame would likely BREAK something else first.

Put it this way. On my 1100 before i replaced the springs, they were completely sacked out. The total stroke on the 1100 forks is nearly 6 inches. With the static sag of the bikes weight and ME, it was taking up 2.5 inches of stroke. I STILL never bottomed that bike out. Ever... and i whipped that thing pretty hard.

[salty_monk]

Do a search on Google... plenty of people agreeing with me. It's basic physics. I have a physics specialism qualification from way back... actually two of them (School is different in the UK), this is one of the useful things I learnt there were a few others too!

Here's more detailed info:


Posted by: Papakeith---------------------

Has anyone here ever cut springs to increase the rate of the spring? I've heard of people doing this, but wouldn't mind getting a quick How-to if there's someone here that has done it in the past.


Posted by: Pete Payne---------------------

When done right - its works well ! when done wrong - its a mess!

order the lenght and rate you need from cannon racecraft.


Posted by: kawamaha---------------------

I wrote an article couple months ago, unfortunately the pictures are anywhere on my computer :-(

but keep in mind, first measure the spring for spring travel. if the spring travel is shorter than the fork travel you will reduce fork travel and maybe damage the cartridge!!!


Instruction - How to make fork springs stiffer

Basically the spring rate depends on material, diameter, wire thickness and number of coils of the spring.
The more coils, the softer it is.

1. chapter what you need
2. chapter how stiff can you make the springs
3. chapter how to cut your springs


1. what you need

you need an oxyacetylene torch, a grinder or angle grinder and you need to know how to get spacers (I have access to a lathe so I make them myself)

2. how stiff can you make the springs

every manufacturer has different springs (coils and wire thickness) so you have to find out the spring travel. The more spring travel you have compared to the fork travel, the stiffer you can make the springs.





- Measure the distance D1 between two coils at any point of the spring but not at the end of it.
- Count the coils of the spring. In my example it has 30 coils.
- take the number of coils minus the two outer coils (in my example 30 - 2 =28) and
multiply it with D1 (for example D1 = 11,8mm, then it's 28 x 11,8mm = 330,4mm)
- measure the distances D2 and D3 of the outer coils and add them to the result above (if D2 =11,5mm and D3 = 11,2mm then we have 330,4mm + 11,5mm + 11,2mm = 353,1mm in this example) - that's your spring travel.

Ok, you have a 0.47kg/mm spring for example and want to make it a 0.50kg/mm spring.
If 30 coils are 0.47 then 1 coil is 0.47 x 30 = 14.1kg/mm. Take the rate of 1 coil and divide it over the spring rate you want (14.1 / 0.50 = 28,2) that's the coils your spring must have. I prefer to calculate in 0.5 winding steps, so lets make 28 coils instead of 28.2 w.(then you will get 14.1 / 28 = 0.504kg/mm )

30 coils it has now and 28 coils you need, that means you have to cut 2 coils off of the spring (shown as a green mark in the picture)

Caution: The spring travel must be at least 5mm + spring preload more than the fork travel
Otherwise you will damage your cardridge!

What does that mean? Before you cut off your springs you have to calculate the new spring travel.
Take your original spring travel minus cw x D1. (cw means coils to cut). In our example it's 353,1mm - 2 x 11,8mm = 329,5mm
In this case it's no need to measure the fork travel. If we suppose a max. travel of 310mm of the fork and a spring preload of 5mm we need to have a spring travel of at least 320mm.
If the calculated spring travel is less than 320mm you have to find out your actual fork travel, maybe it's only 300mm then you can cut your springs down to 310mm spring travel

If you wonder if you can make a 0.47 CRF450 spring into a 0.50 spring - I did it a few weeks ago, the stock springs had enough spring travel.
Even a 0.52 spring rate is possible with these (2005 CRF450) springs.


3. how to cut your springs





It’s a good idea to write down your original spring length, then cut the springs with a grinder or angle grinder. The spring end now looks like pic a

Heat the spring up at the point shown in pic b and bend it carefully with a gripper. Repeat this ¼ turn further towards the end of the spring and repeat it once again, then your spring should look like shown in pic c.

Now grind the end of the spring until it stays almost 90° on the floor (pic d)

Heating the spring and bending it is a kind of art to get a perfect looking spring end – but it is not that important, the spring starts to move not until after the first coil.


Now put the spring into the fork leg, reassemble it and look how much free play you have. Add 5mm (or what you want to preload) and cut the spacer to this length.

If you use aluminium or plastic spacers, they will displace more oil than the cut off piece of the spring. Take care if you want to fill in the max. oil quantity. (for example max. oil for the CRF was 420cc, now that I cut off the springs it is 410cc).

Note: if you have the right tools it's an easy procedure - to create an article like that is much more complicated

[Killer2600]

Maybe thats the problem?? Its quite apparent that you dont, nor do you understand how a spring works. Lets say that your spring is designed for 4inches of total travel. Now lets say the spring is designed to support 200 lbs per inch of travel. To bottom that spring out, how many pounds would need to be on it?? And thats just a normal spring...now understand that even the STOCK springs are progressive rate springs...IMO even if he cut the spring in HALF, the amount of force that needed to be applied to bottom it out so that the wheel hit the frame would likely BREAK something else first.

Put it this way. On my 1100 before i replaced the springs, they were completely sacked out. The total stroke on the 1100 forks is nearly 6 inches. With the static sag of the bikes weight and ME, it was taking up 2.5 inches of stroke. I STILL never bottomed that bike out. Ever... and i whipped that thing pretty hard.

Here-in lies the misconception/miscommunication...I'm not debating whether or not the fork is still fuctional I'm just iterating that cutting a spring down and replacing the missing space with a spacer does not solve the problem of a bike loaded down with batteries bottoming out. It may solve a clearance problem but it doesn't "setup the suspension to handle the heavier weight".

If the issue was "my fat ass puts to much sag on the front and my wheel is rubbing some custom add-ons that are too close to the wheel without me on the bike" then spacers would work but that's not the issue nor is the issue "the ride is too bouncy" where you could cut the springs down and put in spacers to reduce the springs compressed to extended length; you could say the ride is made stiffer that way but you can't misconstrue the "stiffer" ride with stiffer springs that can now hold up a boat load of batteries AND not bottom out.

[salty_monk]

Man... you just don't get it!

By cutting the springs & replacing that length with a solid spacer you are STIFFENING the spring. INCREASING the spring RATE. Making it STRONGER, allowing it to support MORE WEIGHT! That is exactly what this guy wants to do....

It precisely does this "setup the suspension to handle the heavier weight".

You are fixating on preload & that is a completely different subject....

Don't hang around here spouting BS - if you don't understand it go get a physics lesson or read a book on it or something! I have already provided some info for you to read...

If you can understand this: http://optimumspring.com/products/co...n_springs.aspx

What we are talking about is reducing the number of active coils (over the same length because the length is made up by the solid spacer).

I don't know how else to try to explain this to you... if you don't want to study it then why not try it! You'll be surprised no doubt!

Dan

[Killer2600]

Man... you just don't get it!

By cutting the springs & replacing that length with a solid spacer you are STIFFENING the spring. INCREASING the spring RATE. Making it STRONGER, allowing it to support MORE WEIGHT! That is exactly what this guy wants to do....

It precisely does this "setup the suspension to handle the heavier weight".

You are fixating on preload & that is a completely different subject....

Don't hang around here spouting BS - if you don't understand it go get a physics lesson or read a book on it or something! I have already provided some info for you to read...

If you can understand this: http://optimumspring.com/products/co...n_springs.aspx

What we are talking about is reducing the number of active coils (over the same length because the length is made up by the solid spacer).

I don't know how else to try to explain this to you... if you don't want to study it then why not try it! You'll be surprised no doubt!

Dan

Hey if you like it then go with it but here's the issue with all the logic in this thread. Shorter springs are stiffer and stronger and hold more weight without bottoming out so then longer springs must be softer, weaker, and hold less weight. Which I suppose for the small scale of motorcycle fork application is fine. The problem is the physics of that idea, the physics of longer springs can get so long that they bottom out the instant a feather lands on them just doesn't work until the spring is unable to support it's own weight.

I'm not gonna debate physics with anyone but I'll just say if your idea/theory really works you got a gold mine on your hands. You know how many trucks you can convert from light or medium duty to heavy duty without having to spend money on those "heavier duty" springs that are apparently a sham.

[Dreef1999]

Take a broomstick and gently bend it on your knee by placing your hands as far apart as possible. You should be able feel it flex some.

Now move your hands inward so that the distance between your hands is 1/2. Try to bend the broomstick, the broom should feel 'stiffer'.

This is just a mechanical property of the material.

[Killer2600]

Take a broomstick and gently bend it on your knee by placing your hands as far apart as possible. You should be able feel it flex some.

Now move your hands inward so that the distance between your hands is 1/2. Try to bend the broomstick, the broom should feel 'stiffer'.

This is just a mechanical property of the material.

I understand that and I understand the goings on with spring rate. For sanity's sake I'll just leave it at that.

[first timer]

wow this really got off topic.

back to topic:

my feelings are you should not limit the travel of the fork or the rebound, they were designed with good purpose. Saying that, go with stiffer fork springs and redesign you battery tray to allow your forks to operate properly. Sounds like you are boxing yourself in by your battery location/configuration. Are there other options you can do? I saw posted on here a few weeks past with a Kawasaki similar to your bike refitted to run electric, he used stacked batteries, custom 72 tooth rear sprocket and some sort of small front sprocket. Might be worth looking for how he did his, it would save you some design grief. IF you can post some pics of this cross bar thing interfering with the front wheel that will help everyone understand what is going on.

Back to the springs, start cheap and start cutting at your stock springs in 1" increments, like said the shorter the spring the stiffer it gets, get a bunch of pvc and when you cut a inch of spring add a inch of pvc to the top of the spring. Keep in mind the move spring you remove the less travel you will have, this is bad but it will give you an idea of how stiff you need your springs. If you are in the 300lb weight ball park I would go 25 wt may 30 wt fork oil. Your springs are going to be so stiff you are going to need heavy damping to slow the rebound. Standard oil will not be heavy enough plus with all the force it is going to foam up any motor oil you have in there, fork oil won't. (I am 150lb and i run 15 wt)

Once you are able to set your sag (do a good search or i'll try to post the ohlins manual later) and all your testing is done and you are happy I would contact racetech or someone ask if you can send them your hacked spring you have and see if that can match a full length spring to the rate your hacked spring has. This way you will have the spring rate but will have the added travel you need to full absorb bumps.

But please don't try modifying how the forks work it was designed with purpose, design around the constraints of the bike don't redesign the bike.

linky electric ninja

http://www.youtube.com/watch?v=Rzi4t...layer_embedded