Author Topic: strange bounce (Read 50400 times)

i-man · « **on:** October 29, 2014, 08:16:05 AM »

My 1990 k75 seems to have developed a strange bouncy feeling in the front end, or at least it feels like it's coming from the front. It's not always present, but it mostly is. particularly at low speeds. Kinda feels like I'm going over a very mild washboard road, or the tires have a flat spot.

The tires are inflated to their max pressure, but I've also ridden it with 10lbs. less then max. No difference. The tires look to be in good shape with lot's of life left in them. Though the crowns on both tires are a little flat.

I know this is pretty vague. I've ridden the bike about 100 miles, and didn't notice it in the first 95, so I don't think it's always been there.

Any thoughts?

Novafrk · « **Reply #1 on:** October 29, 2014, 09:33:55 AM »

It sounds like the tire/wheel needs to be re-balanced.

Motorhobo · « **Reply #2 on:** October 29, 2014, 11:24:05 AM »

Wheel bearings?

TimTyler · « **Reply #3 on:** October 29, 2014, 01:03:03 PM »

Does the bounce frequency get higher as speed increases?

Can you stand on the pegs and lean forward enough to see what the front end is doing while you're riding?

I've never seen it but I've heard tires can "break" on the inside.

i-man · « **Reply #4 on:** October 29, 2014, 01:22:15 PM »

Thanks for the replies. This really is a great site and I look forward to the day when I know enough to help out, instead of just asking for help.

I guess it could be out of balance, it does seem strange that that would just come on all the sudden. But maybe that's how it happens? I lack experience with it.

I get no side to side play when I check the wheel barrings in the way I know to check it. Is there a more advanced way to check, short of taking it to a professional?

The bounce, near as I can tell smooths out, at least somewhat, at higher speeds. I mostly notice it at medium speeds, say 30ish to 50ish. Not as bad (noticable perhaps? ) below 30ish and above 50ish. I'll try looking at the front tire while riding on the way home.

When it first happened, I found it so unnerving that I turn around and parked it. I did, unwisely perhaps, ride it all the way to work today. It doesn't feel like doom is right around the corner, but it is quite annoying.

Dennis de Vries · « **Reply #5 on:** October 29, 2014, 02:00:19 PM »

I ride a similar aged k75, running Michelins, and mine does it too. And i've heard about more k75's doing it, albeit most people notice it after a tire change. If yours does it out of the bleu on the same tires that's odd. You could lift the front end and spin the wheel feeling for bumps in the tire, which would indicate a broken carcass. If that's the case i would replace it, if not i honoustly wouldn't know where to look (but i'm not to worried about my front wheel having a little bounce, so i run this tire untill it's done for...)

i-man · « **Reply #6 on:** October 29, 2014, 02:32:46 PM »

it's a little vexing. I can't swear it wasn't doing it before, but I did turn the bike around and park it on a beautiful fall Sunday morning when I REALLY want to ride it, so I'm 99.9% sure that it just started. But the time before was the first real ride of any distance, so maybe the bike being unfamiliar masked it somewhat.

I'd appreciate it if others who have more experience and an opinion about whats been offer up so far -tire out of balance, barrings, ect., could give there take on those possible causes.

Thanks again!

i-man · « **Reply #7 on:** October 29, 2014, 05:23:56 PM »

Seemed considerably worse on the ride home. I did glance at the front tire a couple of times and it seemed wobbly. Could have been an optical illusion, as the front wheel was definitely jumping around.

Just seemed annoying this morning, but after this afternoon's ride, i won't ride it again until i have it figured out. Not quite an white knuckle affair, but no fun either.

rbm · « **Reply #8 on:** October 29, 2014, 05:51:05 PM »

My K-bike developed a bounce at 60 kph consistently. the pogoing would start at around 55 and continue through about 65; never higher or lower. I thought it was tires and I did change them but the pogoing continued, albeit less. I adjusted the steering bearings and refilled the fork oil. It pretty much disappeared.

ReneZ · « **Reply #9 on:** October 29, 2014, 07:50:15 PM »

First make sure the tyre is at the right pressure. Too many don't check their tyre pressure often enough. If you ride for too long on deflated tyres they will result in a kind of knobbly ride.

subforry · « **Reply #10 on:** October 29, 2014, 08:17:13 PM »

rim could be bent. As it has been said the fork oil level could be off. If the tire looks fine then change the fork oil - easy and cheap.

i-man · « **Reply #11 on:** October 29, 2014, 09:21:17 PM »

i think i'll start with the fork oil. Probably needs to be done anyway.

I took the wheel off in preparation of taking it to the shop for balancing. i was looking it over in a good light, flipping it around when I noticed the direction arrow. I had the wheel off earlier (before getting it on the road). I remember thinking to myself 'self, this wheel only goes in one direction, be sure to put it back on in the correct direction.' But I'm getting older and my brain doesn't work as well as it used to. Still, I'm pretty sure I put it on it the correct direction. But, if I'm wrong and had it on backwards, would it cause the symptoms I describe?

Dennis de Vries · « **Reply #12 on:** October 29, 2014, 10:41:00 PM »

Yes that could be the cause.
As you ride the carcass of the tire sets (a bit) to the direction the forces work in.
Now flip the tire and have the forces working the other way, it's a bit like going against the grain.
That could cause a tire to behave strangely, although i've never heard of it being as bad as you describe...

Grim · « **Reply #13 on:** November 01, 2014, 12:42:18 PM »

Take the wheel off and spin the bearings by hand and see how it feels. It's likely your bearings are shot.

"Sealed bearings" are a misleading term. It means you don't need to add grease, it doesn't mean you can't inadvertently add water with a blast of a hose especially high pressure at a car wash. Resist trying to blow the crud out of the bearing area in the future. Leave it and Let it dry after a wash then take a towel and wipe the area clean.

i-man · « **Reply #14 on:** November 02, 2014, 08:40:27 AM »

i'll try that.

I changed the fork oil and put the wheel back on (in the correct direction, though I'm 99.9% sure it was in the correct direction to begin with) and the bounce was still there. so the wheel will come back off and i'll spin those bearings.

i-man · « **Reply #15 on:** November 02, 2014, 04:15:42 PM »

i'm not sure how freely the bearings should spin. i can turn them, they're not frozen, but I can't say they spin freely.

Is this normal?

Novafrk · « **Reply #16 on:** November 03, 2014, 01:06:43 PM »

They won't free wheel because of the grease inside. Things I look for:

Does the inner race turn smoothly or does it feel "crunchy"
Is the inner race loose and wobbly?
Are there signs of overheating (discoloration of the bearing)

If any of those conditions exist I replace them.

i-man · « **Reply #17 on:** November 03, 2014, 05:17:55 PM »

the inner races spin without any crunch or wobble. they feel solid without any side to side play.

Novafrk · « **Reply #18 on:** November 04, 2014, 06:33:14 AM »

Then I'd say they are probably good.

johnny · « **Reply #19 on:** November 04, 2014, 07:12:36 AM »

f-ed up rear shock and / or f-ed up rear tior can cause front end whack...

j o

mjydrafter · « **Reply #20 on:** November 04, 2014, 07:59:43 AM »

One thing that can happen is, if the tread is blocky, the blocks will develop a "saw tooth profile", which could cause some "whack".

It's more noticeable on a knobby off-road style tire, but I could see it in a Metzeler my bike came with. You might be able to feel it rubbing the tread with your hand, rubbing the top of the front tire forward will be easier than rubbing it backward. The Metzeler that I had, you could see it if you got down and looked at a nice profile view of the tire. Also if the tire is dusty, rubbing it will show you any high/low spots or irregularities that you might not feel or see. As you will wipe the dust off the high spots.

Just a few other easy to look at things.

Novafrk · « **Reply #21 on:** November 04, 2014, 10:22:58 AM »

Quote from: johnny on November 04, 2014, 07:12:36 AM

f-ed up rear shock and / or f-ed up rear tior can cause front end whack...

j o

I'm following this thread keenly. I haven't had my K75 very long but have a definite porpoising motion going on between 35-40. I know my rear shock is blown. I'm saving the scratch to buy one. If it fixes my problem I'll let you know.

Novafrk · « **Reply #22 on:** November 04, 2014, 10:25:30 AM »

Do you know if the shock is the original? Good possibility it's worn out. Mr. Johnny seems adamant that the shock could cause the issue.

I tend to defer to the experienced knowledge base on most forums. :riding:

i-man · « **Reply #23 on:** November 04, 2014, 12:05:22 PM »

One of the troubling things about the bike is I know nothing of it's history. It was ridden from California, blewup the clutch in NC (where I am)and was abandoned when the guy saw the repair bill. I've never even met that person, or spoken with him. I got it from a guy who bought it to part out. I usually pass on bikes like this, but the price was good (not great) so I decided to take a chance.

That's a long way of say I don't know if the shock is original or not.

Is there a way of testing the shock?

I guess I'll try adjusting the steering head bearings (anyone got a link to a good tutorial? ) and try some Ride-on / dyna beads for balancing. I'll certainly post the results.

Novafrk · « **Reply #24 on:** November 04, 2014, 12:08:59 PM »

Hold the bike up, try to bounce the rear end like you would a basket ball. There will be little to no resistance to your push if it's bad.

News:

Author Topic: strange bounce (Read 50400 times)