modifying fork travel

[ereckerdeet]

Hi, I've got an '82 GS750/EZ and need to modify the fork travel. I need it to bottom out after about 6 inches of travel from all the way out/fully extended (i.e. on center stand). The motor was pulled along with all combustion related components, and there's 300lbs of batteries sitting directly under and beside the tank. The battery mount has a crossmember which is about 7" above the front wheel and is causing interference.

The reason for needing to limit the travel is I have installed some custom hardware on the front end. The bike is being converted to electric. I have a blog up at www.battzilla.com if anyone wants to check it out!

What would be the best way to do this? Spring spacer?

What is the normal shock travel from all the way out to all the way in (bottomed out on the springs)?

Right now I've got the tubes filled to 35psi (when on center stand) and I can still bottom it out on the battery rack when I take it off the stand and lean on the tank. Air pressure seems to be holding, so does that mean that my seals are OK? I should still replace them when I take it apart, what part number would I need? I use the bike bandit for parts FYI.

The bike had been sitting for an unknown amount of time. The tires are from 1998 and are at about 50% tread (and need replaced obviously). It has 130,000 miles on it. When the motor

 

[reddirtrider]

I'm not sure that can be done without custom building another stanchion.

Why is this necessary?

 

[ereckerdeet]

Thanks for the reply, reddirtrider! I'd love to avoid additional cost of custom machining if possible. I'd also be open to any bolt-on parts possibly from a different model/year which would help clear up this problem. Longer stanchion, longer rake, whatever.

As for why is this necessary:
The battery mount has a crossmember which is about 7" above the front wheel and is causing interference.

 

[posplayr]

Sounds like you are suffering from much more weight than you had before (with an engine). You don't need more travel you need stiffer springs. Do you have any idea how much additional weight you added? If so you could probably go to Race tech and get an idea of how much additional spring stiffness that eight increase would warrant. Once you know that you need to set about increasing the spring rate to the desired stiffness.

You can go an buy race tech springs ($100) which go to about 1 kg/mm which are probably stiffer than a progressive ($75) and then you could either preload them to see if they are close enough or you can actually increase the spring rate by cutting length. In other words for a linear spring 25" spring, if you cut off 2.5" you would make it 10% stiffer.

You can always go over the top on preload, but you need to get the spring rate right.

To start you might start cutting the springs you have to see how much you have to remove to get the preload correct. Motor cycles are usually setup so that the front end sag with rider weight is withing a fairly narrow band. Ohlins has a setup guide.

Bottom line, if you want to keep using motor cycle parts, you are generally going to limit the total weight on the front end to be no more than the heaviest rider with normal engine/exhaust. Cutting the spring down works to an extent, but will limit the compression before the coils bind.

Just remember F=k*X
Force = spring rate times distance

Pos

P.S. I dont know what your question is about the cross bar other than it is in the way? You would need to move or replace with the equivalent stiffness.

P.S.2 You are likly to blow your seals with 35 psi in the forks. I would let that air out. It is not an airshock. It is intended for minor ride variation onlt no more than 10 psi.

P.S. 3 Charge O meter is cute

 

[ereckerdeet]

EXCELLENT post and great information posplayr.

The cross bar runs horizontal from left to right. It is about where the horn used to mount. I would estimate 2.5 inches down and 2.5 inches forward of the horn mounting holes.

The battery pack weighs 300lbs. Its center of gravity is approximately dead center on the bike left/right, below the gas fill port front/back, and at the middle of the head up/down (About where the spark plugs were).

This is balanced out by 50lbs of motor inside the engine compartment all the way back and down, and by 250lbs (me) on the seat.

I don't know how much the motor/starter/carb/coils/etc weighed so its hard to tell how much I've effectively changed the total weight and front/rear weight ratio. I don't have a big enough scale at home (its 250lbs max), but once I get it moving I have a heavy duty scale at work I can put it on.

The only thing about the cross bar is thats what is in the way. It's an explaination of WHY I need to shorten the travel.

I'll cut down the springs and do the spacers to start with. What else do I need to do while I'm in there? What weight oil should I refill with? What part number oil seal should I use? Is there anything else to change/do? What size(diameter) Schedule 40 PVC do I need?


P.S. I'll let the air out too it was just to reference how much it took to get the front wheel fender off of the cross bar.

P.S. 2 How do you set 'preload'? Is that just where instead of being able to easily screw the cap on, you have to compress the spring before you can screw it on?? *clueless*

 

[posplayr]

This is extracted from the Ohlins manuals when I was concerned about rear spring sag, but similar applies to the front. I might have misspoke in your circumstance. You seem to need to get your spring rate correct, and then set the preload (either spacer of top of your fork adjustment) so the sag is correct.

http://www.thegsresources.com/_forum/showpost.php?p=994640&postcount=7

This is the spring rate calculator. You can change the bikes from the site. You can see that the stock spring rate (for the earlier bikes) is only .450 kg/mm. Racetech have springs for this earlier bike up to .90 kg/mm. I know the 1100's go to 1.00 kg/mm which is what would be required for a 200 lbs rider.


I threw in a 300 lbs rider (an extra 100 lbs) and it came in at 1.068 kg/mm As an example of what you are dealing with. Of course the battery weight is further forward so that is more weight on the front forks. If you really want to go this way, and you are really adding that much weight I would look for an upgraded front end where you can put a wider tire on the front. The cheapest option would be to put a 1st Gen Gixxer fork on as that is 39mm vs. your 35mm and it will be much easier to get a stiff spring in the larger fork. Also the brakes will be better and you can get a wider tire.

If you don't want to do that, then I suggest removing some batteries and getting the weight distribution closer to the stock configuration. You might reduce your range, some (unless you are doing regen braking) and you can stay within the parameters of the stock suspension.

http://old.racetech.com/evalving/Sp...=77-80&ml=GS750&formuse=form1&SpringType=Fork


Just guessing the engine is about 200 lbs with another 75 in exhaust. Your batteries are 300lbs, but further forward it seems. The exhaust will mainly load the rear, so I would probably figure you are at least adding 100-150 extra lbs rider equivalent weight to the front forks.
At 350 lbs equivalent rider weight you need 1.15 kg/mm ; you would need to do a 15% length reduction on a 1 kg/mm spring. Looking in the 1100 service manual spring length is only 16.2" so 16.2*.15=2.5"

All these numbers are just examples to show you how to go through the calculations. Ideally you could put the bike on a scale (front wheel) in one case with the motor and the other with your batteries. You get on the bike with the batteries and see how much weight is on the front. Then put the motor back in and get on the bike. You will see less weight on the front, how many bags of cement (in your lap) do you have to add to get to the same front wheel weight?

The added weight is what you need to compare in the Racetech spring calculator to see what the spring rate should be. You can then figure what it will take to get there.

You could also do it by trial and error :-\\\

 

[reddirtrider]

Pos, it's a good thing you can read between the lines. I was wondering why he wanted to modify the fork travel length and never recieved an answer. It turns out that's not what he needed after all.

 

[ereckerdeet]

Is this '82 also .450kg/mm and 425mm long(16.75")? So if I put a 6" spacer on top of my stock spring, and chopped 6" off the spring, (16.75/10.75), I'd still only end up with .701kg/mm springs?

Also what seal do I want to change while I'm in there? I was looking on the bike bandit and there's a ton of parts in that shock, several say seal.

Can I use standard 15wt motor oil, or standard non-detergent 15wt motor oil, or do I need a specialty oil?


I'm scratching my head on this too - spring rate vs preload vs total travel.
Definately the preload needs to be higher - that way the front doesn't bottom on the welded in custom crossbar as soon as it sits down.
Definately the spring rate needs to be higher so I don't have to put rediculus air pressure in.
Maybe the total travel needs to be reduced so that the springs bind BEFORE the wheel hits the custom crossbar.

If anyone points me to the correct seals and oil and correct size PVC, I can cut up lengths of 4" to 8" PVC and try that for a starting point. Thanks all for your help and guidance so far!

 

[reddirtrider]

Is this '82 also .450kg/mm and 425mm long(16.75")? So if I put a 6" spacer on top of my stock spring, and chopped 6" off the spring, (16.75/10.75), I'd still only end up with .701kg/mm springs?

Also what seal do I want to change while I'm in there? I was looking on the bike bandit and there's a ton of parts in that shock, several say seal.

Can I use standard 15wt motor oil, or standard non-detergent 15wt motor oil, or do I need a specialty oil?

You'll need the oil seal and you should stick to fork oil. The viscosity of the fluid will change the damping and rebound action significantly. I use 2.5W, 5W, and 10W depending on what bike I'm servicing and what valve kit I have installed. Changing the spring rate significantly may alter the rebound requirements such that using the stock weight fork oil isn't cutting it.

All of that is tuning the suspension and you won't be able to do that until the bike is running. You really need to ride it to be able to adjust in detail.

 

[ereckerdeet]

Indeed, but I also need to have the front wheel able to spin - by getting it off of the battery rack cross bar - before I can ride. I'll stick to fork oil, I know the local shop as 15wt fork oil on the shelf. Is it 500ml on each side?

So I only need 'oil seal' 9: OIL SEAL 503470-001 $13.86 here: http://www.bikebandit.com/suzuki-motorcycle-gs750ez-1982/o/m6108 ? One for each side of course.

 

[rustybronco]

Can I use standard 15wt motor oil, or standard non-detergent 15wt motor oil, or do I need a specialty oil?

you can use a blend of detergent-not detetergent engine oil and a.t.f.(dexron-III, not type "F")
I have used a oil/atf blend for three years. use a figure of about 7-8 Wt for the atf.

Maybe the total travel needs to be reduced so that the springs bind BEFORE the wheel hits the custom crossbar.

Not a good idea to have coil bind at all.

 

[reddirtrider]

Indeed, but I also need to have the front wheel able to spin - by getting it off of the battery rack cross bar - before I can ride. I'll stick to fork oil, I know the local shop as 15wt fork oil on the shelf. Is it 500ml on each side?

So I only need 'oil seal' 9: OIL SEAL 503470-001 $13.86 here: http://www.bikebandit.com/suzuki-motorcycle-gs750ez-1982/o/m6108 ? One for each side of course.

That would be the seal you need.

You CAN use oils designed for different purposes - transmission fluid for example - I use fork oil, and will continue to do so unless stranded on a desert isle with none available.

 

[reddirtrider]

That would be the seal you need.

You CAN use oils designed for different purposes - transmission fluid for example - I use fork oil, and will continue to do so unless stranded on a desert isle with none available.

I should explain why I use fork oil, and only fork oil. It's not that other oils won't work, or that you can't mix up something that will do the trick. I use fork oil because I know what weight I'm getting. Then if I don't like the performance, I can switch to a lighter or heavier weight depending on what I'm aiming for on damping.

I have 2 dirt bikes that I have to tune the suspension to fit my riding style and weight. This means I have lot's of different weights of fork oil sitting on a shelf.

That said, use what works for you.

 

[salty_monk]

You need to limit the travel in the forks. Imagine you hit a bump & the wheel hits that crossbar? Instant stop maybe... maybe not if you're very lucky.
Will also bend any fender you have there.

Limiting travel by making the springs bind is bad practice, you will then send the full force of the bump through the spacers - I don't think that's good.

I would think the easiest way of limiting travel is by bolting a bump stop to the fork sliders somewhere below the lower triple tree, if it was rubber I would think it would be ok but someone here can confirm if they think the lower triple can take the shock when the bump stops are called for.... in theory you should never need them if you get the spring rate right but you never know. The other thing to do is to re-shape your crossbar to provide clearance for the full travel.

You might also be able to put a hard rubber bumpstop in the bottom of the fork leg so the bottom of the slider hits that & arrests travel short of what it is now...

One last idea - fit a 83750ES or 1150 front end - the smaller 16" wheel should help provide the clearance you need... The height should be similar to the original as those fork legs are longer. Just another idea.

Dan

 

[Killer2600]

You can go an buy race tech springs ($100) which go to about 1 kg/mm which are probably stiffer than a progressive ($75) and then you could either preload them to see if they are close enough or you can actually increase the spring rate by cutting length. In other words for a linear spring 25" spring, if you cut off 2.5" you would make it 10% stiffer.

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.

 

[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... :lol:

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... :lol:

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