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Musings on EX250 Suspension upgrades.

Akatombo

New member
Joined
Jan 21, 2007
Location
Santa Cruz, CA
Moto(s)
EX250F
The Kawasaki EX250F is a nice little bike, but supplied with 20 year old suspension technology. For instance, it still uses a rod damper suspension for the front forks. With the stock springs extremely soft and undersprung, the 250 is uncomfortable and rather rough to ride around at low or high speeds.

First, what do Fork braces do? I've been looking at the Telefix Fork Braces from SpecII. There doesn't seem to be much written about the fork brace on the interwebs, so I wonder if it is effective or not. Comments are that the triple tee of the EX250 is made of poor quality steel, and is prone to flex and warpage. It is my assumption that the fork brace acts as an additional structural member, helping alleviate the shortfalls of the original triple tee of the 250. But what does a fork brace do, in terms of riding? Will I feel a difference? Or is the point of a fork brace NOT to feel anything - and prevent unusual sensations from ever happening?

It is also my understanding that the only other way to fix this problem of having a weak triple tee and fork clamps is to do a front-end swap with another bike, and changing out the front end entirely. This approach takes far more skill, experience, and money than I can commit. The front-end swap also has the danger of changing the handling dynamics of the bike quite dynamically, possibly towards a less safe direction. So that is out.

Who has used a fork brace? Do they work as I believe they do? Will they benefit the EX250, unlike the perennial "frame slider" suggestion? These are all questions I cannot answer alone.

Next, I must admit to hating my 250's front suspension. It is scary to brake, as the weight transfer to the front at low to moderate speeds is jerky, and rough. Especially after trying out a friend's Honda f4i, the suspension feels very uncomfortable. I really should learn better brake modulation, but I can't let the funky front suspension go scott free. The obvious solution is well documented, and I've decided in investing in race-tech Valve Emulators. I'm here seeking any criticism in my plans, or suggestions if you have them.

Race-tech's website and suspension calculator gives me a spring rate of 0.613 kg/mm for my body weight.
the stock springs are 0.440 kg/mm. My god, I'm quite shocked to see these numbers. Either race-tech and kawasaki engineers disagree in defining what constitues an acceptable suspension setup, or the EX250 is sprung to carry elves, midgets and fairies. I am not a very big guy at 130lb or so.
The EX500 Springs are .585 kg/mm, and the softest Race-tech spring is 0.800 kg/mm. I'm planning on using the EX500 springs, as the discrepancy from the suggested values are far less.
I think/hope the 0.020kg/mm shortfall is well within the adjustability of changing fork oil viscocity, and the adjustability of the GVEs. (Not to mention, the ex500 springs are cheaper, and availible on eBay)

I haven't had much issue with the shocks, probably because I am so preoccupied with the front end jerking down every time I brake. I also weigh little, and that must help with the undersprung rear shock. I do dream of upgrading to a shock with some adjustability in it, so that I may experiment with settings, or adjust for touring loads. Most likely canidate for replacing the stock shock is the Steeltracker from works suspension, as it has been mentioned by other ex250 as being fantastic investments. While I understand the suspension of a motorcycle should be tuned to work as a system, and not piece meal, cost and time are definate constraining factors. It is just that the front end issues are well defined, and more pronounced in my mind.

Let me know what you think. I'd love to hear the voice of experience and reason. :D
 
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ex250 springs are pretty soft, though it has been ten years since I owned one, so I don't remember how bad, I do remember my skinny 150 pound body overwhelmed the stock stuff.

Max McAllister suggests that the Works Performance shock is better for people with boats, so that once they put it on and discover it is a COMPLETE POS, it still will serve as a boat anchor.

I am sure the real 250 guys will have far better input.
 
This the only place you need to go for info:

http://faq.ninja250.org/wiki/Suspension

Not only can you learn about the front suspension, you can learn about the rear.

I haven't worked the front yet because I don't have the money, but I have stuck a ZX600C damper in the rear. Night and day, especially for 2+ riding.

As for frame sliders, the only ones out there are the ones you make yourself. I hope you're good at welding and fabrication.

BTW, if you're over 120 lbs. then the stock suspension isn't built for you.
 
IMO fork braces are difficult to align correctly, go out of alignment, stay tweaked and then induce stiction. Skinny 36mm forks twist no matter what and putting a stiff cross member in the middle of two pieces of spagetti is the answer to the question nobody asked.

I raced production EX500s at NHIS for a couple of years. The fastest guys did not use braces, and even went so far as to remove the fender and its metal under-shell so as to allow the forks to flex, deform and then unflex themselves as they saw fit. This practice reduced chatter to a considerable extent and laptimes dropped.

First gen EX500s and EX250s are not the same, but both suffer from flexy 36mm forks. Therefore, YMMV. AFM prod 250 guys should know more.
 
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Aftershocks does the lion's share of suspension for the 250prod racers and sponsors the class so, they know how to make 250's go fast (relatively speaking of course).

I never really noticed a difference between having a fork brace or not and they seem to love adding stiction so, I called it dead weight and tore it off.
 
What rick said, no fork brace and get your EX250's suspension setup by Phil at Aftershocks...Phil used to race a Ninja 250 (quickly!) and is quite the expert on that bike.
 
The best suspension mod on a ninjette is a different bike...
 
It is pretty telling that modern mountain bike front forks use 36mm diameter tubes. Ah, the advance of technology. In fact, my pedal-bike probably has more sophisticated suspension adjustments than the stock 250 suspension.

Thanks for the replies.

However, I still don't understand what a fork brace is intended to do. What problem is it supposed to address? When or how does this problem manifest itself?

Matt.Lai
Okay, so the fork tubes flex. I'll accept that, and move on. I'm not going to race this thing, ever. I'm hoping that normal riding is not going to expose this particular weakness of the 250's outdated design.

But curiosity does strike me. What exactly does a flexing tube do to the bike? It is my assumption that with a flexible front end, the torsional twist (and rebound) makes a nasty vibrational shudder that will shake the front wheel / handlebars back and forth.

When would this torsional stress occur though? In hard cornering? Rough terrain?

altomista:
I actually started with the ninja250 FAQ, to line up my plans as they exist now. There wasn't any mention of a forkbrace, especially regarding what a fork brace does, or what it is intended to do. ;)

> As for frame sliders, the only ones out there are the ones you make yourself.

Yes. I am aware of this. Not to mention the fact that the 250 doesn't have a perimeter frame to protect. I wasn't asking about the sliders, which don't offer any functionality.

> BTW, if you're over 120 lbs. then the stock suspension isn't built for you.

Which would be the reason to modify the suspension setup in the first place. And thus the reason I'm posting.

Since you've done some work on the suspension set up, I want to get some live data from you. You also seem to be knowlegeable about the suspension setup, so I hope you don't mind me asking.

How is the rear shock setup for you in detail? How much do you weigh? How often do you do 2 up riding? You say "better", but what aspect of the suspension is improved? Did you bottom out the rear shock often before replacement? How did you balance the suspension with the unmodified front end? How is the suspension balance with the ZX-600C shock? Any change in ride height that you've noticed? What pressure are you running in the shock?
 
Akatombo said:
Matt.Lai
Okay, so the fork tubes flex. I'll accept that, and move on. I'm not going to race this thing, ever. I'm hoping that normal riding is not going to expose this particular weakness of the 250's outdated design.

But curiosity does strike me. What exactly does a flexing tube do to the bike? It is my assumption that with a flexible front end, the torsional twist (and rebound) makes a nasty vibrational shudder that will shake the front wheel / handlebars back and forth.

When would this torsional stress occur though? In hard cornering? Rough terrain?

altomista:

Stand in front of your motorcycle while it is on its centerstand. Put the front wheel between your knees and grip tightly. Grab the handlebars and wiggle back and forth using a reasonable amount of force. You will be able to move your wheel back and forth a bit, independant of the steering direction. This sawing action that you can induce is what happens to the forks when various forces act on your tires and chasis. This occurs in regular day to day riding and is not confined to racing or track riding.

The frequency and degree to which it flexes is actually a dynamic part of your overall chasis and suspension. Changing it to be less flexible via a brace can have benefits, but only if the change in deflection is an improvement to the overall system. Often times forks that have such a change create a new and stiffer flex that is not optimal for the chassis and suspension. That has been my experience with EX500s.

Now, if you still insist and want to install such a device (cough, cough, snake oil), know that the normal flexing of your forks will, on a regular basis, cause the brace's clamping area to slip ever so slightly. As the forks unflex themselves, the brace clamps will stay in their new position and you now have a misaligned fork brace which holds the fork in a twisted state. This is STICTION and is very bad for your suspension action. So, if you must buy one of these things, be sure to loosen and re-align your brace on a very regular basis -- perhaps ever fill-up. A safer place for such a thing would be on your desk as a paperweight.

Good luck,
Matt
 
On the track this longitudinal and latitudinal flex usually presents as a rhythmic oscillation in the frame and forks. Kinda like a massage bed. It is usually present in rapid transition corners, or high speed corners where the front end is under lots of stress. The only " cure " is to give it more gas, not something we want do.
 
Akatombo said:
altomista:
I actually started with the ninja250 FAQ, to line up my plans as they exist now. There wasn't any mention of a forkbrace, especially regarding what a fork brace does, or what it is intended to do. ;)

> As for frame sliders, the only ones out there are the ones you make yourself.

Yes. I am aware of this. Not to mention the fact that the 250 doesn't have a perimeter frame to protect. I wasn't asking about the sliders, which don't offer any functionality.

> BTW, if you're over 120 lbs. then the stock suspension isn't built for you.

Which would be the reason to modify the suspension setup in the first place. And thus the reason I'm posting.

Since you've done some work on the suspension set up, I want to get some live data from you. You also seem to be knowlegeable about the suspension setup, so I hope you don't mind me asking.

How is the rear shock setup for you in detail? How much do you weigh? How often do you do 2 up riding? You say "better", but what aspect of the suspension is improved? Did you bottom out the rear shock often before replacement? How did you balance the suspension with the unmodified front end? How is the suspension balance with the ZX-600C shock? Any change in ride height that you've noticed? What pressure are you running in the shock?

For me, the ZX600C shock made a wonderful difference. The rebound rate is faster--my back doesn't hurt because of it as much. I weigh 160 lbs.

It also doesn't much sag once you mount the bike. I had taken measurements of before and after. With the ZX600C damper, unladen the seat height is lowered. When mounted, the seat height is the same. I also haven't bottomed it out since I've replaced it, 2+ or me alone.

I'm running the damper with no added air. I can't remember which rebound setting I have it. It might be #1

I haven't balanced the front end yet, which means it still plows. The bike will always plow until you work your forks.

It's best to work both ends at the same time, unless you're willing to keep the plowing. My back takes precedence for me.
 
I got few sets of new springs for ninja,.75,.80 and .85 if interested.
 
How much for the 0.75 kg/mm springs?
 
Matt.Lai said:
Stand in front of your motorcycle while it is on its centerstand. Put the front wheel between your knees and grip tightly. Grab the handlebars and wiggle back and forth using a reasonable amount of force. You will be able to move your wheel back and forth a bit, independant of the steering direction. This sawing action that you can induce is what happens to the forks when various forces act on your tires and chasis.
now move your knees very close to fork tubes and grip tightly and try again.
 
Matt.Lai:

I played with the front fork, and got a good idea of the fork twist concept.

So let me write outloud what I've learned. fork flex will manifest itself under any high front load condition, including a front mass transfer, or a rapid lateral load transfer. This will create a low frequency vibration in the front fork, affecting steering and stability, and possibly front end traction.
Strengthening the fork to reduce twist certainly isn't something a brace at one point is going to fix. It requires a more comprehensive design to balance torsional rigidity, mass, and steering geometry.


Combined with what I already know about mechanical engineering...

The chassis of the EX250 is old, outdated, and quite flexible anyways. Even if the front end were made stronger, the rest of the frame would still twist. This reduces the effectiveness of a modification, making it economically unrealistic. Suspension tuning itself is a complicated matter, as a single change will affect the rest of the suspension setup. It is best to adjust the system as a whole, instead of attempting to tune individual components one by one. (Complicating the whole suspension thing into an art, like everything else).

Okay. cool. I've copied this data into my moto diary / notebook/ data log.

Afm199, Altomista. Thanks so much for information. I've gained a good amount of information to mull over.

So work suspension parts aren't that great? A few other ex250 people who went to Aftershocks have been told that since the EX250 is limited in its capabilities, the works suspension components were really all they needed. (they don't race). I will go down there myself eventually, and talk to phil. I didn't know he raced the 250, but that really adds to the level of confidence.

I'll make sure to have my homework done before I go and visit though :D
 
Akatombo said:
Combined with what I already know about mechanical engineering...
The chassis of the EX250 is old, outdated, and quite flexible anyways. Even if the front end were made stronger, the rest of the frame would still twist. This reduces the effectiveness of a modification, making it economically unrealistic.

Just so we all don't go out and burn our EX250s for being the antichrist of motorcycles, please know there really is not much wrong with it and in capable hands it can be made to turn and burn with the best of today's machines. Sure, the spring rate up front needs help and maybe some folks will do better with an upgraded shock. Heck, the OEM tires can feel a bit wooden at times. Your plan to take it to phil is sound, he will offer a solution for your front end that will include springs and some modifications to the damper rod valving. Get some new tires, check the EX250 boards for the best sizes and brands. Replacement shock? Maybe, but I don't think that is what is holding this motorcycle back.

The biggest change you can make is tuning the wingnut behind the bars. Ride more. Buy a dirtbike. Go to the advanced MSF class. Do a trackday. Ride more. Rinse and repeat ad nauseum. Soon you'll find that your EX250 will suddenly start to handle much better, guaranteed.

Good luck, Matt
 
The biggest change you can make is tuning the wingnut behind the bars. Ride more. Buy a dirtbike. Go to the advanced MSF class. Do a trackday. Ride more. Rinse and repeat ad nauseum. Soon you'll find that your EX250 will suddenly start to handle much better, guaranteed.

Oh, definately. Its economically unrealistic to "fix" the bike and make it better. Especially to magically make the front fork not flex, the forks absorb every impact, and have super tires that remain glued to the ground. If money and engineering could solve everything, I'd have a motorcycle forged from raw unobtanium and powered on pixie dust and unicorn tears.

I ride a 250 for a reason. It is easy to maintain. It is a handful, but not deadly so. That, and I like the bike. Except for that scary soft front suspension :D

Tuning this particular wingnut is going to take some time. Especially a geeky one that isn't going to stop asking questions.
 
I admire people that take something and develop it beyond what it was designed for(look at me... I ride scooters)...

That people ride EX250's as fast as they do sometimes amazes me...

It's just that due to limitations in the design it will never handle as well as a properly set up well designed bike....
 
Well I agree with Matt. The 250 can go fast, go ride with Gary Jaehne sometime on the street. Or look up a Sears Point race results and note the 1:57 laps. A 1:57 lap at Sears Point is not exactly AMA standard, but on a 250 Production bike it is INSANELY fast.
 
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