Chassis defeated over 90mph? - Ex-500.com - The home of the Kawasaki EX500 / Ninja 500R
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post #1 of 12 (permalink) Old 8-23-2019, 5:00 AM Thread Starter
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Chassis defeated over 90mph?

Hello ex community,

I took my rebuilt GPZ500S to a track day at Brands hatch yesterday (a track close to London in the UK) and it is a hilly/bumpy track. Did close to 1h30 on the track, was very good!

I noticed that when I am over 90mph if I lay too much when driving over a bumpy area in a curve the motorcycle feedbacks a kind of "dangling"/"whobble" or "ondulation" effect. It is not a vibration or a tremblor, more an ondulation "Chewing-gum". The effect is magnified under heavy breaking again with a slight angle and over bumps.

I had a look over the main bolts holding wheels, direction, etc. But to me it just feels like the frame and chassis is just defeated, it is an entry price, 35 yo bike design after all.

Do you think the same or do I have an issue with it

G
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post #2 of 12 (permalink) Old 8-23-2019, 7:47 AM
fog
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Yep that would tell a blind man he was riding an EX.
it cannot be tuned out, you must learn to ride it. a death grip on the bars is a no no. You must let the occultation happen or they just upset the chassis. on turns above 80 MPH you need to place all your weight on the outside peg and keep your nearside knee amid to the tank , while you let the bars float but still maintain control.
I started winning races when I learned these tricks.

FOG
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post #3 of 12 (permalink) Old 8-23-2019, 2:17 PM Thread Starter
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Ok FOG thank you for your answer buddy
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post #4 of 12 (permalink) Old 8-23-2019, 2:56 PM
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I also raced an EX for a while, and yes - you need to let it do its thing. It won't be rock solid at those speeds.

Stability can be much improved, especially over rough areas, if you get the suspension dialed-in.

Not sure what you've done to the suspension, but proper-rate fork springs and fork oil and a quality rear shock with the correct spring will make it much more manageable at speed.
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post #5 of 12 (permalink) Old 8-23-2019, 4:34 PM
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Yup, some of softness is in suspension and damping (it uses same rear-shock as 157kg Ninja 250 '86-87). Luckily, it's very easy to upgrade shock spring and fork springs.

Another mod that helps is to install solid motor-mounts to use engine as stressed-member to stiffen up entire structure.

Last edited by DannoXYZ; 8-24-2019 at 8:39 AM.
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post #6 of 12 (permalink) Old 8-23-2019, 5:26 PM
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The back straight at the old Bridgehampton course wasenít Straight at all but a series on left hand bends which were WFO . MY BIKE BECAME SO UNHINGED AT 100 MPH that folks that I passed declared that my rear wheel was coming completely off the pavement as the bike ocleated. At this point it actually became easier to ride.
All you can do is get a set of FOG BONES, a
Penske shock. Do the string thing. And be sure everything is tight and wired

Fog

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post #7 of 12 (permalink) Old 8-24-2019, 3:56 AM Thread Starter
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Thank you for your inputs guys so a couple of remarks.

The ondulation is not too much just surprising at the beginning but I guess I will have to play with the settings to reduce it again (if possible to reduce them even more). My set-up is a YSS shock on the rear, some +4cm tail up wishbones, and some Hyperpro springs on the front with a good 4cm spacer on top of it. My set-up is very hard for my weight which is 85kg but I guess I did not count all gear I wear on the track on me so I should increase my calculations to 100kg. (dear!)

To be honest without this set-up (with the OEM set-up), it was simply impossible to lean due to continuous bouncy feedback (even at low speeds) that would build up with more and more amplitude until it would actually eject you off the bike. (but that was the rear shock, not the frame itself if you know what I mean)

Back to what you said Danno I am not a fan of making a part of the bike stiffer e.g. reinforcements on engine mounts, stiffer parts, new fork or rear swingarm from another bike simply because you would transfer the stress to another part of the bike and exponentially increase it where there should not be that much exposing you to frame snaps or catastrophic failures. What do you guys think?
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post #8 of 12 (permalink) Old 8-24-2019, 5:44 AM
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Well, thereís fixed amount of stress. Have to analyse it in objective and quatitative manner. Where is this stress applied and how is it absorbed?

Currently, most of this stress is concentrated on rear backbone of frame above where removable piece bolts in. Due to leverage around that point (think wedges of pie with tip at that point), tiny 0.5 degree of flex becomes 3-4cm movement at contact patches. You can upgrade suspension all you want, but main problem is wet-noodle frame that connects everything.

What solidly mounting engine does is spread out forces thatís concentrated on one spot of frame onto multiple widely spaced points so that no one spot of frame is overloaded as it is now.

Example of this is try standing one-legged balanced on small 3cm rock. All your weight is concentrated on single spot. Thereís extreme pressure on that spot huh? Without shoes, you can tear skin apart! And imagine bloke comes along and shoves you. Youíre already unstable and he can shove you off easily.

Now, compare that with standing with both feet on ground widely-spaced apart... say 60-70cm. Load is distributed over wide surface area. Even with bare feet, you wonít feel much pressure since your weight isnít concentrated on any one spot. Now try to shove you over. Much, much more difficult as you have lot more support and stability.

Thatís what solidly mounting engine to frame does. It spreads out load from single spot onto 6 widely-spaced points. Force from tyres & wheels had less leverage and bends frame out-of-whack a lot less. More stable cornering at high speeds. Numerous people have done this and reported much improved performance.

Last edited by DannoXYZ; 8-24-2019 at 10:19 PM.
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post #9 of 12 (permalink) Old 8-24-2019, 5:53 AM
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Quote:
Originally Posted by fog View Post
The back straight at the old Bridgehampton course wasenít Straight at all but a series on left hand bends which were WFO . MY BIKE BECAME SO UNHINGED AT 100 MPH that folks that I passed declared that my rear wheel was coming completely off the pavement as the bike ocleated. At this point it actually became easier to ride.
Yes, you have to relax and accept oscillations. Learn its frequency and timing. That way, you can ensure that your tyres are in contact with ground at times when itís most critical.


Last edited by DannoXYZ; 8-24-2019 at 6:21 AM.
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post #10 of 12 (permalink) Old 8-24-2019, 8:30 AM
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Yikes, so it's true. I always thought folks were exaggerating


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post #11 of 12 (permalink) Old 8-27-2019, 2:01 PM
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So if I was interested in this solid mounting thing to make the motor a stressed member, how would I go about doing it? Is there a tutorial somewhere, or can someone explain it?

Paul

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post #12 of 12 (permalink) Old 8-27-2019, 7:02 PM
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I believe FOG makes/made them. He may still be...shoot him a message.

Otherwise BRGracing also makes them...
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