Topic: Front End Bounces at 30-50 mph

[cjackel]

My 78 750K front end bounces as if I'm riding over a wavy section of road.

I recently replaced the fork oil and fork seals.

No bouncing at highway speeds and it takes bumps and potholes as if everything is normal.

Could this be due to air in the forks? I didn't bounce the forks prior to tightening the fork caps,
but recently learned this is what some mechanics do after changing the fork oil.

Steering and wheel balance appear to be ok.  I used 5.4 oz of Honda ATF as fork oil.  Tire wear is even and looks normal.
Thanks.

[cjackel]

My 78 750K front end bounces as if I'm riding over a wavy section of road.

I recently replaced the fork oil and fork seals.

No bouncing at highway speeds and it takes bumps and potholes as if everything is normal.

Could this be due to air in the forks? I didn't bounce the forks prior to tightening the fork caps,
but recently learned this is what some mechanics do after changing the fork oil.

Steering and wheel balance appear to be ok.  I used 5.4 oz of Honda ATF as fork oil.  Tire wear is even and looks normal.
Thanks.

[Mr. Grimm]

Try bouncing the forks with the front brake locked. Really slam the front end down and that should force any trapped air out. If the problem still persists, check that all parts are installed correctly and that the damper valve isn't plugged or damaged. Good luck.

[CHUNG]

  that's a tough one. Sometimes, Poor balance can only show itself at certain speeds. My 750 sidecar Outfits typically shake their head @ 'bout 25-35 MPH. This is related to a resonace of the wheel searching for center and depends on weight of the wheel and rake/trail combos. But it was always a few MPH different when I changed something.
If yours feels purely linier (up and down only) then I'd suspect tire/wheel balance. If you feel a wobble, again maybe tire/wheel but also steering head bearings. Most/all forks are not "perfectly strait" either. I install the axle and roll the forks in the trees till they both point the same way either forward or back. Depens on wheather I want to cheat the steering angle one way or the other. This may not help but most people don't consider that their"perfect" forks are not perfect. Also spin both wheels and see if they are indeed round and true. That's all I can think of

[Mark M]

This sounds like a damping problem, gentle changes are controled more by the damping than the spring ( the springs will be looking after things more on the pot holes etc). Sounds like you have little, or no damping effect going on, try bouncing the front as you may have an airlock keeping the oil away from the bottom of the leg where the damping rods need to be covered. If no joy then you need to strip them and check everything, you may have damaged/missing parts. 

[Jonesy]

My CB750 did the same thing. After getting the front wheel balanced it was much, much better. Also, although it's rare, is the tire out-of-round? I've seen it happen on a friend's bike with a cheapie tire he got from JC Whitney.

[CHUNG]

I agree with JONES, it may indeed be a "wheel" issue. (i guessed it first? LOL)
You can do a little home-boy dial indicator. Hold something close to the rim and give it a spin. See if it is indeed a fact that the rim isn't TRUE?

[Uncle Ernie]

When that happens to me, it turns out that the bead hasn't been set. Many mechanics are not used to tires with tubes and don't know what to look for or take enough time. You need to lube the bead really well, blow the tire up to 90 psi, bounce it some, wait a while, bounce it some more and then check around the rim to make sure the lines on the tire are even all the way around on both sides.

[Dennis]

Ernie. 90 PSI?
Do you like to live dangerously?
Seriously, be very careful with that. I wouldn't inflate the tire beyond the limit the manufacturer recommends for seating the bead.

[dpen]

An old trick to align the front end is, after checking steering head bearings, tighten the top clamp & leave everything else slightly loose (including axle nuts). Roll the bike forward & hit the front brake a few times. Then tighen everything up.
DO NOT PUT 90 PSI IN ANY TYRE!!!!!!
Mention this to any reputable mechanic or tyre shop & they will ask for your address so they can send a wreath

[Uncle Ernie]

Gee~ Did I forget to mention that I let the air back out (I run 28lb in front) before putting it on the bike?  If I'm doing that wrong, what is the recommended procedure for setting the bead on a new tire with a tube?

[dpen]

pump up the tyre until you hear two distinctive pops. That's the bead on both sides popping out. First time take it to about 40 psi. If they don't pop, let the air out & bash around the tread with a RUBBER mallet and squirt some dishwashing liquid around the tyre where it meets the rim. Then try again. Maximum I'd go to is 50 psi & that's with a new tyre & a good rim.
What tyres are you using? 28 psi seems a bit low which will affect handling. I run Bridgestones at 34 front & 36 rear which, from experience,  gives good wear & handling

[Uncle Ernie]

I work with a guy who used to rep Yokahama tires and did a lot of MC racing related stuff. This is how he showed me to mount tires... Well, I stopped getting tires mounted at a shop because I always have to bring them back again, and they always "lose" my old tires. Like I just got a new/smaller set for a bike but the old ones were still almost like new. Anyway, I seem to prefer that my tires not be quite as hard as some people like. I believe the CB400F owners manual recommends about 28psi for the front, though, too.