Topic: Fork oil???

[DV Red Herring]

My question for today. How does different amounts and different viscosity of oil affect ride height, if at all. I am having an issue with the front fender contacting and binding on the exhaust while the forks compress while braking. NOT a very cool feeling going into a turn, looking and leaning, and the bike wants to go straight.

[Stev-o]

What exhaust?  There were reports of a certain aftermarket supplier on the West coast that made "crappy" exhaust that would hit upon compression.

[flatlander]

exactly. this is not a problem of fork oil but of components that don't fit.

[DV Red Herring]

I'm not sure of the exhaust manufacturer. It a 1-4 2-3 crossover design. These forks have shorter tubes than stock for sure.

[DV Red Herring]

It's enough to just catch the fender stay and barely hang up. I was wondering if heavier oil would lessen the compression or even up the ride height a little. I would rather not buy longer tubes or a new exhaust just yet if different oil can fix the situation.

[seanbarney41]

fork oil is probably not the best or most effective way to fix your problem.  Springs could be sagged...try adding preload spacers or better yet, replace your springs.  Tubes could be shorter than stock, from a 550, or slid up in the tree.  Also possible that someone internally lowered your forks by placing spacers under the bottoming springs.  A proper handling 750 needs every inch of it's fork travel. 

If you must, adding fork oil will somewhat stiffen your suspension.  But I think the ride will get very harsh before you solve your problem that way.

The situation you are in is extremely unsafe.  I would not ride the bike until this is fixed.

[przjohn]

I think you pretty much know what the problem is but don't want to address it. You state that your forks tubes were shortened. Sorry to say but that is your problem and as has been stated, a dangerous one at that. There is no Silver Bullet for this problem in the form of oil weight or height. Change the tubes.

[Yamahawk]

Take a center of axle to top of tube measurement and let's see what it is, to be sure that the fork tubes are shorter than stock. Unless you are sure they are shorter than stock CB750 tubes, because you installed them yourself. If they aren't shorter, then the exhaust is the problem.
Charlie

[DV Red Herring]

Charlie- They measure right on 27" from axle center to top of the tube.

Przjohn- Yeah, I know. I thought the notchy feeling I got every now and then was from the head bearings. So much that I just installed a tapered bearing set only to discover that it still has that slight catch in certain situations.

Seanbarney41- It looks to have progressive springs. I put the bike on the lift and drained the most horrific smelling fluid from the forks. With the top caps off I pulled the springs out and they are definitely tighter wound towards one end. With the forks completely unloaded, the springs come right to the top. Had to slightly compress the spring to start threading the caps back on.

In all, it makes sense. The exhaust protrudes more from cylinders 1 and 4 as they cross over 2 and 3. Sad to see this exhaust get hung on the wall... Thanks guys.

[ekpent]

 Can you post a couple pictures of the bike in question ?

[Yamahawk]

Ok... my forks measure 29" approximately, maybe a little more. The forks I have are from a 1978 CB750A, I believe. So, your forks may be 2" shorter than stock, and between that and the 'crossover' type exhaust header, it is a combination of the two. Does anyone else have a stock CB750K that they can measure for him?
Charlie

[DV Red Herring]

Here ya go...

[przjohn]

Have you checked to see if someone put spacers inside the forks to shorten them?

[DV Red Herring]

I pulled the springs out earlier and didn't see any spacers. I haven't had these forks apart any more than that, though.

[seanbarney41]

definitely lowered in both the front and back...I am surprised you have not scraped those pipes just doing normal everyday turns.  My first 750 had those same pipes and they would scrape ridiculously easy.  The bike got parked until I could afford new pipes.

Why don't you put the bike back to the correct ride height?  It really will do everything better.

You have to remove the damper rods to get lowering spacers out, or to see if that is how the front has been lowered.

Yeah, fork oil gets ta smellin like #$%*...that is one sign that your forks are long overdue for service.

[DV Red Herring]

While I am toying with the option of un-lowering this motorcycle, I have changed the fork oil to 20w. While bouncing it against the front brake I was unable to reproduce the contact of fender and exhaust. But that only allowed me to focus on the clunking sound that the forks are making by themselves as they compress. Now to tear these things apart and see what's in there...

Seanbarney41- I have a little over 7 inches of ground clearance on these. I get into corners pretty hard sometimes, but have yet to scrape. Although not now, as I have learned about the CS tires and fender contact when diving into a left corner.

[ekpent]

 You need to getcha some of these on the front      I need to break that out this spring again for some cruzin', been sittin' too long.
  PS - Yes that's a real ducktail seat it came with. 

[DV Red Herring]

Haha. Man I wish now that I kept the forks that I sold. I put up a pic of when I found it in the woodshed.

[Bankerdanny]

The springs control the ride height and how much force is required to compress the fork. The oil controls further effects the speed with which the compression will happen and also how fast they will rebound (no oil or too light oil means your front end might bounce like a pogo stick).

As you already experienced the thicker oil makes the fork less responsive to compression, which reduces rubbing, but if you hit a big enough bump the fork will still compress enough to cause the fender to contract the pipes. If you overfill the oil I would think that would also limit total travel.