Fork spring tech needed!

[El Camexican]

Pre-loading a spring increases the initial rate of resistance when momentum begins, but has no effect on total resistance. i.e you can crank up the pre-load on a spring to the max, but it will still bottom out with the same amount of force as it did with no pre-load. I agree that a reduced length of spring will develop resistance at a faster rate than a longer one, but total resistance just before binding occurs would be slightly less than that of the full length spring. However, shortening the springs has eliminated the risk of binding and oil levels/viscosity are what will determine if and when the suspension bottoms out.

In other words the rate of resistance of the shortened spring will increase more rapidly than that of the longer spring, but if you have a reduced travel distance isn’t that what you’d want it to do?

I will retract my initial comment of “proper way” and suggest that it is the "simplest way" for a do it yourselfer that is unable to raise their fork tubes up in the tree.

Now on to more important things ,like wind tunnel testing trikes. Seriously? I’d be more apt to believe that Yamaha was wind tunnel testing pianos AND yes it looks VERY MUCH like the previous years Honda racer, only green… with envy… well, except for the brown fuel tank.

[sick85crownvic]

A lot of useful info here, I am aware of spring rates are measured and listed, not due to my suspension experience(or lackthereof) but from my experience setting up top-ends on performance oriented cars. I don't plan on cutting any springs, but I will be investigating all possible avenues before I make any decisions. Lots of good info coming in guys, thanks again.

Barnett, I'm starting to see where your brand loyalty lies! lol I'm sure the XR600 motor and dry sump set-up will give me plenty of nose weight...I'd like it to be fairly balanced....I don't plan on big air...but a small chance of mx riding exists....I'm primarily looking at "fast trail" riding. I just blew $90 on head stem bearings/races.....this goofy size being used as a conversion bearing is apparently somewhat common overseas, but not so much in the U.S. market...apparently the same size as some Ducati's. I think I'll just be doing a basic rebuild of the forks initially and then I can ride it and work out the kinks later....of which I'm sure there will be plenty.

I think I'll be pulling the Big 6 motor soon....I'd like to have some weight in this frame so I can start figuring out suspension and weight bias.

Thanks again!
-Don

[barnett468]

Barnett, I'm starting to see where your brand loyalty lies

Hello sic85crownvic;

As far as brand loyalty goes I actually have none. I also love Honda’s and have had some Suzuki’s, Yammie’s, KTM’s, and Harley’s. etc..

I was a racer so all I wanted was the best bike I could get. I could care less who made it. I just like to razz the Honda guys because the Kawi guys are vastly outnumbered here, lol.

…green… with envy…

Green with envy?


Below are even MORE comments of HIGH PRAISE for the 86 Tecate [the fastest production three wheeler in the world], from a 3 wheeler magazine test.

...this year the Kawasaki techs included the Kawasaki Integrated Power-valve System (KJPS) they use on their motocross motorcycles. The power is greatly improved... Is it fast? In a word, awesome! The '86 comes on from a mild low end to a hard-hitting growl in the mid-range, and if the top end ever peaks, it happens somewhere in the outer reaches of space. The power comes on smoothly… …the Tecate will rev forever. When other 250s have peaked and gone flat, the Kawasaki will continue to accelerate and pull away. This engine is fantastic!

Other new items include a redesigned frame that has the Tecate sitting even lower than last year's model and allows the '86 to corner better than an African springboc being chased by a lion. The 86 Green Machine is a turning fool. Sitting on the Kawasaki is when you'll first notice just how low this beast sits. The twist throttle (bravo, Kawasaki!) and general feel of the bike (even while parked) leave no doubt that this is a serious competition machine.

The rear shock is now fitted with a temperature-compensating damping control that helps reduce premature fade…

…the whole tail end has been lightened by the use of aluminum for the Uni-Trak swingarm.



Look at this MONSTROUS wall of sand thrown up by the Teacte [the fastest production 3 wheeler in the world], with STOCK TIRES, it’s simply LUDICROUS!

Notice that it’s also wheeling out of the corner with the rider LEANING OVER THE HANDLE BARS in a USELESS attempt to keep the front end down IN THE SAND. That’s right, this is SAND with NO PADDLE TIRES.



OMG Barn…Please stop, you’re killin me.

[barnett468]

A lot of useful info here,

Hello sic85crownv.

Are you referring to the piano wind tunnel testing?

I actually did see a Yamaha piano at the NASA lab when I was there. It was next to the Steinway they also wanted to test.

I am aware of spring rates are measured and listed, not due to my suspension experience(or lackthereof) but from my experience setting up top-ends on performance oriented cars.

Hey, you can start a car thread in the open forum. I’m a car a holic. I do a lot of restos on 60’s and early 70’s muscle cars, mainly Fords, and also do medium performance engine building and strokers etc.. I leave the 700 plus hp builds to my drag race engine builder friends.

Since you set up heads, of course you know that preload affects total resistance as I showed in my previous post. It's simple addition. Preload is simply added to the stroke, or in your case the lift. As you know, if you put 20 lbs [amount of preload] on a scale, then add another 180 lbs [additional resistance created by compressing the spring the length of the suspension stroke] it adds up to...well 200 lbs.

I ran the cylinder head department for an auto motive machine shop and setting closed and open spring pressures is obviously one of the things you do with your nifty little spring tester.


SPRING RATE AND OIL VISCOSITY

In the case where the same newly shortened higher rate spring is being used in a tube that is 2" shorter than my previous example of 10", the total compressed resistance will be 8” travel x 22 lbs per inch spring = 176 lbs plus the 2" preload x 22 lbs per inch spring = 44 lbs = 210 lbs of resistance which is actually 30 lbs less than the uncut spring with 2 inches of preload in a 10” inch travel tube.

This is a reduction of around 12 1/2% in spring resistance in the shortened travel tube, plus as mentioned, the initial part of the stroke will be stiffer, which will be by around 10 percent. This means the initial has become stiffer as mentioned, and the resistance increases more quickly as mentioned, which I agree is what one would want or be required in this instance, however, at full compression, the shorter travel tube still has around 12 1/2% LESS spring resistance when fully compressed. Please see the formulas in previous post. This means that if it was set up ideally before with the longer 10” suspension, it will now bottom out like a pig dropped off a roof [almost] over large jumps.

If one were to keep the same springs when shortening the travel but cut the 2” off to compensate for the 2” reduction in travel, they would need to compensate for the bottoming by increasing the oil viscosity. Increasing the level if possible will help slightly but will be insufficient.

Ok, now here’s another fun part. The thicker oil will now obviously make the ride stiffer, so here’s a bad and incomplete explanation of approximately what happens. You already have a stiffer initial ride due to the approximate 9% increase in preload spring resistance, but you have now added more damping resistance by increasing the oil viscosity from 7.5 to 10 for example. By doing this, you add at least 20% in damping resistance at all fork travel speeds, which in technical suspension lingo means that the initial ride/travel has become “harsh”. This also obviously increases the firmness of the suspension throughout its entire stroke.

Fluid is interesting, if you put a spring 4 times heavier in the tube with no oil and dropped a bowling ball on it from 30 feet, the fork will still bottom out, however, if you remove the spring and install the 7.5 oil only, and extend the tube and drop the ball, it might still bottom, however, if you increase the oil viscosity by 4 times and drop the ball the forks will not bottom. This means that increasing the oil viscosity will increase compression resistance more than increasing the spring rate by the same factor which in this example is 4, therefore, for the shortened forks to perform at their optimum under the same riding conditions, the spring rate must be increased and the preload must be reduced and the oil viscosity must be increased.

This is why short travel suspension will never, never, ever, ever, ever [sounds like a Taylor Swift song], perform/feel the same as a longer travel one. The physics simply don’t permit it. This being said, there is little difference in feel between a properly set up 9’ suspension and a properly set up 9 1/2” one, but there is a huge difference between a 9” and 11” one.

I don't plan on cutting any springs, but I will be investigating all possible avenues before I make any decisions. I think I'll just be doing a basic rebuild of the forks initially and then I can ride it and work out the kinks later....of which I'm sure there will be plenty.

I also think it’s obviously best to leave the suspension as is until you ride it as you plan to. The front suspension is actually pretty easy to dial in. Our lengthy posts could likely be reduced to around 1 short line, but where’s the fun in that?

“If your forks are too soft, install stiffer springs and thicker oil.”

When we make long bloviating, pontificating, posts, we at least hope to sound like we know what we are talking about, especially when we occasionally toss in a fancy high tech word or two like “telemetry” or “prefabulated amulite” and so on.


SOME OTHER POSSIBILITIES

PROGRESSIVE DAMPING

In the old school, non valve forks, one can modify them to have progressive damping so to speak. This is done by changing the location, number and or size of the holes in the rod. I did a bit of this as early as 1974 but it’s not often used or really advantageous in most cases, however, it is cool to see the affects of it and how it works.


BOTTOMING CONES

These things are really cool and extremely effective and pathetically simple in how they work and all forks have them. We tested many different shapes of these at Kawi. They basically increase the damping rate at the end of the travel, typically beginning at around 1 1/4” before the end of the stroke.

If you have a set of forks that work well except they bottom a little too hard, you can reduce the hard feel and the bottoming by increasing the diameter and/or length of the cone.

I'd like to have some weight in this frame so I can start figuring out suspension and weight bias. I'm sure the XR600 motor and dry sump set-up will give me plenty of nose weight...I'd like it to be fairly balanced....I don't plan on big air...but a small chance of mx riding exists.

As far as weight bias goes, it’s a tough decision because it’s obviously just a wee bit hard to change, lol. I can tell you that a 1 percent change is noticeable. In your case you might want it slightly nose heavy likely 50 or 50.5 percent front bias. I would not go more than 51.5 up front and you may already know, the longer the bike is the more slowly it turns per se and the greater the turning radius.

For trail riding, I would not exceed the stock Tecate or Honda overall length. These Honda’s with plus 3” or more swing arms wouldn’t stand a chance on tight trails against the same bike with a stock swing arm. As I mentioned, the Honda is a slightly better trail bike than a Tecate imo. That’s not a bad thing.



PS - Below is a photo of a group of Honda riders walking back to their vehicles after ALL of their CDI units simultaneously FAILED.



Oh God…there goes my spleen.




Hi, my name is Timmy and I approve this post.

[El Camexican]

there is a huge difference between a 9” and 11” one.

I'll take your word for it and remind you that this is a family site.

[DohcBikes]

^LMFAO noones even trying this week are they

The local fun police are bound to be amused

[barnett468]

there is a huge difference between a 9” and 11” one.

A lot of useful info here,

..........

[sick85crownvic]

Things are quickly taking a left turn here! lol Those stinkin' bearings I ordered won't be here until late NEXT week...so no setting that up this weekend...maybe the Big 6 gets freed from it's two-wheeled home this weekend...depends on how motivated I am...It's hard to pull one running machine apart to build another when the one running machine is all you have to ride! I guess that should be motivation enough right there.

[barnett468]

Things are quickly taking a left turn here! lol

You left us alone too long. We need supervision.

[DohcBikes]

Nope leave the running bike together till you're ready!

Patience is the most important tool in the box.

Is there a time limit on this project? Hopefully not one of those self imposed imaginary timelines...project like this takes as long as it takes ya know.........

[sick85crownvic]

Nope leave the running bike together till you're ready!

Patience is the most important tool in the box.

Is there a time limit on this project? Hopefully not one of those self imposed imaginary timelines...project like this takes as long as it takes ya know.........

Yeah, there is a self imposed imaginary timeline...lol. I need to NOT get over-zealous and start welding everything together and tearing stuff apart...still a lot of little things to do...like make the LTZ400 axle carrier play nice with the LT250R swingarm...and pick up some frame tubing...there are just a lot of things I can't really do until I have the motor in the way....like set up the rear shock upper mounting location...decide between a shortened RM250 back-bone or do a split rectangular tube back-bone...need to take things one at a time I suppose.

-Don