Smoother ride on 91 K75S?

I just rode my 91 K75S for about 30 miles tonight. It runs great,and handles much nicer than my R1100RT. But...I feel every ripple and slight bump in the rode. It's a bit wearing to say the least. Here are the details: new Dunlop tires, with correct pressure; new fork oil, BMW 7.5 wt; rear shock set to the lowest of three settings.
Any suggestions to get a better ride? Should I set the rear shock stiffer? Change the fork oil to 5wt? Lower the tire pressure? Or are S models just set up for a stiff ride? I hope not. Thanks.

jconway607 said:

I just rode my 91 K75S for about 30 miles tonight. It runs great,and handles much nicer than my R1100RT. But...I feel every ripple and slight bump in the rode. It's a bit wearing to say the least. Here are the details: new Dunlop tires, with correct pressure; new fork oil, BMW 7.5 wt; rear shock set to the lowest of three settings.
Any suggestions to get a better ride? Should I set the rear shock stiffer? Change the fork oil to 5wt? Lower the tire pressure? Or are S models just set up for a stiff ride? I hope not. Thanks.

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I have a 93 K75s, I picked it up last summer and spent this winter going through it mechanically to get it running again after a 12 year slumber. I put on new Metzler’s and set them in the factory specified pressures. So far I have put a thousand miles on it and have been enjoying it. Mine has a Koni shock, I’m not sure about whether it has aftermarket springs or not. I have found mine not to be terribly harsh and nice and firm at a sporting pace. I did find this regarding suspension when I was doing research and putting it back on the road. A little less travel and a firmer suspension for the S.

jconway607 said:

I just rode my 91 K75S for about 30 miles tonight. It runs great,and handles much nicer than my R1100RT. But...I feel every ripple and slight bump in the rode. It's a bit wearing to say the least. Here are the details: new Dunlop tires, with correct pressure; new fork oil, BMW 7.5 wt; rear shock set to the lowest of three settings.
Any suggestions to get a better ride? Should I set the rear shock stiffer? Change the fork oil to 5wt? Lower the tire pressure? Or are S models just set up for a stiff ride? I hope not. Thanks.

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John,


Are you feeling the harshness in the front (through your hands) or in the rear (through your butt)?

What model Dunlops?

What are your tire pressures?

How much fluid did you put in each fork?

What brand and model rear shock is on the bike?

Your approximate weight?

to add to the above..

what brand, model and AGE of the rear shock? and is the rider light, med or heavy?

rear shock could be dead, well,it IS dead if original. if rider heavy, the low preload setting could be allowing the shock to just bottom out.

but...this is a K75S. S is for sport. it is not, nor will it ever be, an RT. the ride should be firm, that is why it handles so well. it should never be "cushy".

98lee said:

John,


Are you feeling the harshness in the front (through your hands) or in the rear (through your butt)?

What model Dunlops?

What are your tire pressures?

How much fluid did you put in each fork?

What brand and model rear shock is on the bike?

Your approximate weight?

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Lee, Here is the information you needed:
Both front and rear tires are Dunlop D404.
I checked and am running 36 psi in the front and 38 psi in the rear.
The shock doesn't seem to have a brand. Black springs, chrome top. It's probably the original. It is older, but only has about 20,000 miles on it. I have it set to the lowest notch (softest I would presume)
I just switched the fork oil to 5wt from the 7.5. I haven't tried the bike with that in it yet. I put 280cc in each leg.
I'm about 180 pounds weight.

I was thinking of dropping the air pressure in the tires, maybe 5psi. Any ideas on that? Maybe the Dunlops can run lower pressure than the Metzlers.

As for the ride, it just seems rougher than I would like, stiffer I suppose. Interestingly, I ran into a guy the other day riding a well worn 87 K75 (C I think, it had the small fairing). Well over 300,000 miles on it. He bought it new. It was pretty beat. But he said he also had a KLR 650 , and he said it rode a lot smoother than the K75. I found that educational.

jconway607 said:

Lee, Here is the information you needed:
Both front and rear tires are Dunlop D404.
I checked and am running 36 psi in the front and 38 psi in the rear.
The shock doesn't seem to have a brand. Black springs, chrome top. It's probably the original. It is older, but only has about 20,000 miles on it. I have it set to the lowest notch (softest I would presume)
I just switched the fork oil to 5wt from the 7.5. I haven't tried the bike with that in it yet. I put 280cc in each leg.
I'm about 180 pounds weight.

I was thinking of dropping the air pressure in the tires, maybe 5psi. Any ideas on that? Maybe the Dunlops can run lower pressure than the Metzlers.

As for the ride, it just seems rougher than I would like, stiffer I suppose. Interestingly, I ran into a guy the other day riding a well worn 87 K75 (C I think, it had the small fairing). Well over 300,000 miles on it. He bought it new. It was pretty beat. But he said he also had a KLR 650 , and he said it rode a lot smoother than the K75. I found that educational.

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I have run 30 40 in our K75s for years. Or 28 36 in cold weather. I doubt tire pressure is causing the harshness. Front end harshness can come from too much oil in the fork legs but this doesn't seem to be your problem. So in my mind it comes back to the rear shock.

tire pressure not the issue. i run 38/42.

your description sounds like a dead original rear shock. mileage is not relevant at this point, age is. it is shot.

what you are adjusting is not shock stiffness, it is rear preload. increasing preload may keep the shock from bottoming out at your weight. dial it up.....while you are waiting for a new shock to arrive. suggest progressive 465.

John,

I agree with Paul and Marshal. The rear shock is probably toast.

See what change bumping up to max preload does. It won’t fix it, but it could tell you if that is where the problem lies.

I dropped the air pressure in both tires by about 5psi, but as others say, I don't think that did much. Changing to 5wt oil in the forks seemed to help some. They still seem pretty stiff. I can push down on them when stationary, but not easily, and they don't move much. I also moved the rear shock up one notch, but it really seems more of a front wheel problem. Of course at the moment the bike isn't running again, so the ride isn't my main concern. I posted another thread detailing that situation.

It might be a case of slightly misaligned front forks. The first thing to do is to loosen the axle clamp bolts on the right side of the bike. Vigorously press/bounce the front end compressing and extending the forks. If the front doesn't seem to move freely you can remove the front axle and grease it a little and try again. If the forks still seem to be sticking loosen the fork brace bolts - no need to remove. Then repeat the process and once the forks are not sticking carefully re-tighten the brace bolts.

What was the result of setting the preeload on full hard?

K75S fork swap for a better ride?

it appears from my reading that I am not alone in wishing my 91 K75S had a slightly softer front fork. With mine, as I ride the fork slider seems to rarely move much. On my oilhead, it was constantly shifting with small bumps. At a standstill, when I lock the brake, I can only move the forks about two inches, and that's with applying some force. It may be a sporty ride, but it's tiring. I've put 5wt oil in them,instead of 7.5, and in the exact amount. No change.

The parts diagram for these "Sport" forks seem utterly complicated with three different springs in each fork, washers, spacers, etc. Yet if I'm correct, only one side does all the work. A bit baffling. I gather that with the S forks, you can't do much to alter them. With the other K75 forks, simple spring changes can be made.

I came across a post on Motobricks from a guy who had the same issue. He said someone on Germany had told him to just switch out both forks for some non "S" types. He said it completely solved the problem. There are plenty of fork sets for sale on Ebay. My questions are, is this feasible, and will all the S fenders, brake lines, etc work with non S forks? Do they need to be from the C model, or base model, etc? I would think the RT forks might be just as stiff, but I could be wrong about that, since it is a " touring" model. I'm thinking any forks other than the S type will be an improvement in ride comfort.

John,

Properly set up Ss are not harsh or overly stiff.

Are you SURE that you have the Sport forks? Ss went to Showa forks in 8/91. Sport forks have the bulbous rubber dust covers over the seals at the top of the sliders. Showas do not.

How much oil did you put in the forks?

Are you sure a previous owner did not change the springs or the preload spacers?

You might have bent, binding, or otherwise damaged forks.



The C forks have 2” more travel and poorer damping than the Sport forks, so they dive much more during braking. BMW went to the Sport forks on the RT and later K100s because they handled so much better. Then in late 91, went to the Showas with similar tuning.

John,

Just noted that you have another thread running about your harsh riding condition.

I am going to combine this thread with your other one.

98lee said:

John,
The C forks have 2” more travel and poorer damping than the Sport forks, so they dive much more during braking. BMW went to the Sport forks on the RT and later K100s because they handled so much better. Then in late 91, went to the Showas with similar tuning

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Lee,
I believe the bike was made in 9/90, if that helps. It definitely has the sport forks, with the "S" stamped on the top and the bulbous covers on the sliders. It's impossible to say if someone has tampered with the forks. Like most of these older machines, it's probably had many owners. I put 280cc of 5wt oil in each of the forks. Is that correct?

Are all the fork sliders the same for k75s? If so, all the mounting hardware should match. Would you say the Showas would be an improvement in ride comfort over the S forks? If so, it sounds like all K75s including the S models, will have Showas after 1991. So if I decide to switch, I should get a set of Showas from a 91 and up bike? Are the fork tubes the same diameter? If not a change is out, as I'm not going to change the triple tree. Or, are the Showas almost identical to the S forks, in which case it isn't worth all the work to change them?

Two inches seems like a lot of travel of give up on the S model from the Cs. They would have to be a lot stiffer to avoid bottoming out. Perhaps going back to a C type would give me a better ride. I understand that the K75S models were, I suppose, sold back in the day to people wanting a "sporting" suspension for the "twisties". But really, anyone like myself who is riding a 30 year old motorcycle is probably more interested in a comfortable ride that dragging the pegs all day. Comfort is my goal these days. I would like just a bit more on this bike.

For perspective, I had a 1986 K75T - a K75C with a windshield but not the C fairing, a double bucket seat, and bags. It had the long travel forks. I did not like the cushy, deep dive forks. In 1987 when the K75S appeared I ordered all of the "S" fork innerds and converted the OEM forks to the "S" forks. The difference was amazing, and to my liking.

jconway607 said:

Lee,
I understand that the K75S models were, I suppose, sold back in the day to people wanting a "sporting" suspension for the "twisties". But really, anyone like myself who is riding a 30 year old motorcycle is probably more interested in a comfortable ride that dragging the pegs all day. Comfort is my goal these days. I would like just a bit more on this bike.

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The K75S forks should not be harsh. That spring set has a soft spring for slight bumps and a stiffer spring for real shocks. It cuts the dive but still cushions the lesser bumps. If you have harshness that implies stiction.

A careful alignment of the forks is in order. The usual culprit is at the axle, but an issues is possible at the fork brace. I do it with the springs removed and slide the sliders up and down by hand to ensure the brace is correct and the axle is correct.

Lee,
I'm seeing a lot of forks on Ebay that claim they are from a C model. But, many of them have the large, balloon stile dust cover at the top of the slider. I thought that only the S forks had that style cover. Can the early (say pre-89 bikes?) with the C forks have that large style of dust cover? I'm not sure that people who are selling these know there is a difference in forks, and if I decide to change mine, I don't want to just put another set of S forks on. Thanks.

John,

The early pre-Showa (pre 9/91) forks were all made by Fichtel-Sachs (both C and Sport). They use the same 41.4 tubes and the same sliders. All F-S forks have the bulbous dust boots. They are the same on the outside, just completely different internals. The C forks will work with your triple clamps.


HOWEVER, I still don't think that is your problem. I believe that you have damaged or binding forks. My bikes move a couple inches when pushing on the bars at a stop with the front brakes locked. The S forks SHOULD not be harsh. Just for the better handling and less dive at a stop, I would recommend finding the problem and fixing it rather than swapping forks. There are several internal parts that could be worn or damaged or the forks could be bent or misaligned.

What did you ever do about the rear shock?

Lee,
Thank you for the quick reply to my question. Sorry to keep asking for answers, but I trust your knowledge base.
I've taken a few shorter rides on the bike at this point, about 15 miles each. I'd have to say that the fork action is really not harsh. In fact when I do the same thing you suggest, locking the front brake and rocking the front, mine does go up and down a few inches also. And coming to a stop, the front doesn't dive much, just slightly. Maybe it's working like it should. Wish I knew someone around here with a k75 to compare it with. I wouldn't mind a bit more movement over slight bumps, but like someone said, it's not going to perform like my oilhead did. Maybe I'm being too sensitive.

Due to the movement of the sliders when I rock it, I didn't think the forks were bent. But I could be wrong. I would like to test them, but I assume that to do that I would need to separate the wheel, fenders, etc. and then take all the parts out of the tube so the sliders could mover freely. Is that right or is there an easier way to do the slider test?

I did reset the rear shock to the firmest setting before taking it out. It was a firmer ride. I don't know if I noticed a difference with the front's performance, but overall it seemed good.

Thanks again for the help, Lee.