[avocet]

I think I've learned something about the twitchy behavior of my bike. I mentioned in my original post that the tires were correctly inflated at 42 lbs each. I had checked them using a BMW tire gauge that I've had for years and believe is accurate.

I hadn't really paid attention to the readings of the tire pressure monitor built into the bike, since I don't really know how accurate it is. I've thought of it more as a way to detect a leaking tire than as an accurate indication of actual tire pressure. I did notice, however, that it was reading around 3 or 4 lbs lower than my tire gauge, perhaps because its been quite hot here recently and the TPM is temperature compensated and my external gauge is not. Anyway, I decided to pump up the tires until the TPM displayed the proper 42 lbs pressure for both tires.

I only had time for a short ride after pumping up the tires, so I'm not sure if the problem is totally cured, but it is certainly greatly improved. Apparently much, or possibly all of what I've been feeling is flexing of the sidewalls of the tires, allowing the bike to move side to side with respect to the contact patch. I'll take a longer ride tonight to check it out some more.

Assuming this has been the problem, I wonder what will happen when the weather gets colder. The temperature compensated TPM will continue to read the same, but the actual air pressure in the tires will be reduced, perhaps as low as it actually had been before I pumped the tires up. Wouldn't that mean the problem would return even though the TPM is still reading the correct pressure?

 

[KOCook]

TPM is outside temperature compensated. If you are using your handheld pressure gauge, you should be adjusting your tire pressure for temperature. Base o/s temp is 68F for the 42 PSI. Add/subtract 1PSI for every 10F increase/decrease.

As I need accurate measurement for the track, I got a high quality gauge with certified accuracy range. One of the biggest issues I have found with the cheap tire gauges, like that offered by BMW, is their inconsistency in their range of inaccuracy. Sometimes they are off by a pound or two--other days, by more or less.

PS...set your pressure before you ride (cold) and never bleed off air from a tire that was just ridden (hot).

 

[mneblett]

I decided to pump up the tires until the TPM displayed the proper 42 lbs pressure for both tires.

I only had time for a short ride after pumping up the tires, so I'm not sure if the problem is totally cured, but it is certainly greatly improved. Apparently much, or possibly all of what I've been feeling is flexing of the sidewalls of the tires, allowing the bike to move side to side with respect to the contact patch. I'll take a longer ride tonight to check it out some more.

Interesting -- I tried varying the air pressure, as well as running at 42/42 on the TPM display -- no difference.

The small but noticeable play in the upper ball joint on my bike remains the primary suspect. I'll be going nutz waiting to get the bike back from the ball joint replacement to find out whether this really is the issue!

 

[avocet]

KOCook, Thanks for that information. I'll keep that in mind. Using the method you describe, or using the temperature compensated TPM on the bike should yield similar results, but wouldn't either method result in the tires being softer in cold weather than in warm weather?

Both methods assure that there is the same quantity of air (same number of air molecules?) in the tires as there wouldd be if you measure tire pressure with an external gauge when the air temperature is 68 degrees, regardless of what the ambient air temperature actually is. In other words, those methods can enable me to put the same ammount of air in the tires on a 40 degree day as I would put in on a 68 degree day. The problem with that would seem to be that the tires will be too soft on a 40 degree day unless I put in more air than I would use on a 68 degree day, since the air contracts when it is cooler. My recent experience with instability due to under inflated tires (tires that are too soft) would seem to indicate that this won't be a good situation, and that I would actually want to add more air to the tire as ambient temperatures drop. If I do that, however, the TPM will read higher than the recommended 42 lbs.

I guess the issue here is whether the goal is to have a particular number of air molecules in the tire, or to have a particular pressure, which is required to support the tire and to give it the necessary stiffness to provide stable handling. The TPM, due to its temperature compensation, would seem to do the first of those choices, but an external tire gauge would seem to do the second. I would have thought that the second result would actually be the desired goal.

 

[avocet]

Mneblett, I'm also very curious to see if the replacement of the ball joint corrects your problem. This discussion seems to imply something about the Paralever and Telelever front suspensions used by BMW that I never considered before.

The Telelever only has a single ball joint, but the Paralever has two. The spacing between the two ball joints on the Paralever are, I believe, closer together than the spacing between the ball joint and supports at the tops of the forks on a Telever bike. This would imply that play in a ball joint of a Paralever bike would result in more side to side wheel motion than the same play in a Telever bike. I wonder if there is any history that shows that Paralever bikes need the ball joints replaced more often than Telever bikes.

Is BMW covering your ball joint replacement on warranty?

 

[mneblett]

I guess the issue here is whether the goal is to have a particular number of air molecules in the tire, or to have a particular pressure, which is required to support the tire and to give it the necessary stiffness to provide stable handling.

The latter -- which is why I'd rather that the TPM report actually pressure. A side benefit would be that we could see whether the tire pressure is actually increasing the desired 2-3F from cold to hot (an indication that the cold pressure is the right pressure for that tire and load combination) -- with the current temperature compensation, it's hard to tell whether any change in pressure display from cold to hot is due to tire over-/under-inflation.

 

[sbassin]

KOCook, Thanks for that information. I'll keep that in mind. Using the method you describe, or using the temperature compensated TPM on the bike should yield similar results, but wouldn't either method result in the tires being softer in cold weather than in warm weather?

Both methods assure that there is the same quantity of air (same number of air molecules?) in the tires as there wouldd be if you measure tire pressure with an external gauge when the air temperature is 68 degrees, regardless of what the ambient air temperature actually is. In other words, those methods can enable me to put the same ammount of air in the tires on a 40 degree day as I would put in on a 68 degree day. The problem with that would seem to be that the tires will be too soft on a 40 degree day unless I put in more air than I would use on a 68 degree day, since the air contracts when it is cooler. My recent experience with instability due to under inflated tires (tires that are too soft) would seem to indicate that this won't be a good situation, and that I would actually want to add more air to the tire as ambient temperatures drop. If I do that, however, the TPM will read higher than the recommended 42 lbs.

I guess the issue here is whether the goal is to have a particular number of air molecules in the tire, or to have a particular pressure, which is required to support the tire and to give it the necessary stiffness to provide stable handling. The TPM, due to its temperature compensation, would seem to do the first of those choices, but an external tire gauge would seem to do the second. I would have thought that the second result would actually be the desired goal.

I assumed that the temperature compensation in the bike's TPM is compensating for internal tire temperature vs. ambient air temperature, not ambient temperature vs. some fixed reference temperature. I could be wrong, but if that's the case, you would need to put more air in the tires when it is cold outside and less air in the tires when it's hot outside (the way it should be). In other words, I believe the TPM compensates for temperature when the tires heat up due to riding, not due to seasonal changes.

 

[mneblett]

(Quick side note: the Duolever front end is on the K bikes -- the Paralever is the parallel link rear swingarm design)

Play in a Duolever would result in greater side-to-side motion than in a Telelever, due to the greater separation of the pivot points in the Telelever (i.e., the same lateral distance shift in mm would result in a smaller angular displacement of the wheel contact patch in a system with a longer distance between the pivots).

Warranty coverage hasn't even been discussed yet, as we are all too deeply involved in trying to figure out what the real techical issue is. If this is a damaged/defective ball joint issue, I have no doubt BMW will cover it.

As far as ball joint wear -- from what I've heard the number of replacements has been low -- like cars, they go a long way before wearing to the point of needing replacement.

 

[mneblett]

I assumed that the temperature compensation in the bike's TPM is compensating for internal tire temperature vs. ambient air temperature, not ambient temperature vs. some fixed reference temperature.

The Rider's Manual states that it is corrected to 68F (or was it 70F? can't remember).

you would need to put more air in the tires when it is cold outside and less air in the tires when it's hot outside (the way it should be). In other words, I believe the TPM compensates for temperature when the tires heat up due to riding, not due to seasonal changes.

Both correct, as far as I've been able to figure out.

 

[behrler]

The latter -- which is why I'd rather that the TPM report actually pressure. A side benefit would be that we could see whether the tire pressure is actually increasing the desired 2-3F from cold to hot (an indication that the cold pressure is the right pressure for that tire and load combination) -- with the current temperature compensation, it's hard to tell whether any change in pressure display from cold to hot is due to tire over-/under-inflation.

The frustrating thing to me is that when I check the pressure before I ride and set the pressure at 42lbs at 70 degrees that pressure is all over the place on the front and rear tires during the ride. The indicator can show as much as 3 lbs. difference from front to rear and even be lower than when I started by 3 lbs even though the air temp. has increased 20 degrees. I seriously question the accuracy of the onboard TPMS system.

 

[avocet]

Sbassin, compensating for internal temperature changes as opposed to outside ambient, as you suggest, would make much more sense. That's probably what the TPM is doing. That puts my mind at ease since I was concerned that the TPM would be misguiding me to provide tire pressure that is too low when it gets cold outside. Thanks.

On the other hand, if that is how it works, shouldn't I expect that if I check the tire pressure with an external gauge before riding (when tires have cooled to ambient temperature) that the TPM should at least initially provide the same reading when I first ride off, assuming both the TPM and external gauge are accurate? After the tires heat up from riding the TPM should read the same, but the external gauge should read higher, since the internal pressure would have increased. As things stand right now, when I check my tires with my external gauge (when the bike hasn't been used for several hours) the external gauge reads around 3 lbs higher than the TPM. I suppose that could be due to inaccuracy in either the TPM or the external gauge.

 

[sbassin]

Sbassin, compensating for internal temperature changes as opposed to outside ambient, as you suggest, would make much more sense. That's probably what the TPM is doing. That puts my mind at ease since I was concerned that the TPM would be misguiding me to provide tire pressure that is too low when it gets cold outside. Thanks.

On the other hand, if that is how it works, shouldn't I expect that if I check the tire pressure with an external gauge before riding (when tires have cooled to ambient temperature) that the TPM should at least initially provide the same reading when I first ride off, assuming both the TPM and external gauge are accurate? After the tires heat up from riding the TPM should read the same, but the external gauge should read higher, since the internal pressure would have increased. As things stand right now, when I check my tires with my external gauge (when the bike hasn't been used for several hours) the external gauge reads around 3 lbs higher than the TPM. I suppose that could be due to inaccuracy in either the TPM or the external gauge.

I'm a little out of my depth here, but suspect that there's really no such thing as "accurate" pressure readings, much like the speedometers aren't really accurate. And, as someone else alluded, the temperature compensation between tire temp and ambient temp may be more accurate at 68 degrees than at 58 degrees or 78 degrees. A lot of variables. I personally don't worry about it if I'm within +/- three or four pounds according to the GTL's TPM. If I start to see abnormal tire wear, maybe I'll adjust accordingly, but I'm not really worried about relatively small differences from a safety standpoint.

Scott

 

[avocet]

Sbassin, normally I would agree with you that 3 or 4 pounds of pressure wouldn't be that big of a deal, but it seems that, at least for my K1600GT, it matters. I also have a GS (have owned several of them) and never felt too much difference in how they handle if the tires were a bit low, but if my bike is typical, the K16GT seems to be more sensitive than other bikes. I started this thread to talk about funny behavior at highway speed, and at the moment I believe that it was caused by the tire pressure being low. I plan to give it a more thorough test this evening, unless we have the severe thunderstorms that are predicted for my area.

 

[Wildrider]

Sensitive Bike

I don't think I have noticed the problem you mentioned. I have
driven with no hands and cruise control with and without the top
box with no stability or pulling issues. It does take minor corrections
to stay straight but I would say the bike is completely neutral. I think
the bike is so sensitive to input that small bumps or user inputs cause
the bike to react. To do what you want you simple need to command it!
I have noticed that bumps can cause unwanted input into the throttle as well
as steering. It is a good thing that the bike is so responsive. You just
need to stay on top of it.

 

[avocet]

Last evening I had the opportunity to put some more miles on the bike now that the tires are pumped up to the point where the TPM reads 42 lbs in each tire. The wandering, twitching feeling that I described in the first post of this thread seems to be pretty much eliminated. When I initially reported the issue the TPM was reading 39 lbs in the front wheel and 40 in the rear. It seems that 2 or 3 pounds difference in tire pressure is enough to make a large difference in how this bike behaves. It is quite a bit more sensitive to tire pressure than any other bike I have owned, so I'll have to keep a close eye on it.

Even with the pressure at 42 lbs, this bike is more sensitive to road imperfections than the 12GS I've been riding for the last four years. I can feel road grooves and tar strips much more distinctly on the K16 than on the GS, but perhaps that is normal for this bike. Whatever the reason, last night's drive was the first time I was able to ride without being constantly distracted by the bike spontaneously twitching from one side to the other for no apparent reason.

 

[sbassin]

Last evening I had the opportunity to put some more miles on the bike now that the tires are pumped up to the point where the TPM reads 42 lbs in each tire. The wandering, twitching feeling that I described in the first post of this thread seems to be pretty much eliminated. When I initially reported the issue the TPM was reading 39 lbs in the front wheel and 40 in the rear. It seems that 2 or 3 pounds difference in tire pressure is enough to make a large difference in how this bike behaves. It is quite a bit more sensitive to tire pressure than any other bike I have owned, so I'll have to keep a close eye on it.

Even with the pressure at 42 lbs, this bike is more sensitive to road imperfections than the 12GS I've been riding for the last four years. I can feel road grooves and tar strips much more distinctly on the K16 than on the GS, but perhaps that is normal for this bike. Whatever the reason, last night's drive was the first time I was able to ride without being constantly distracted by the bike spontaneously twitching from one side to the other for no apparent reason.

avocet - What tires do you have? Mine came with the Bridgestones. I was on the bike for about an hour on Sunday and my TPM read 39/42, but I didn't notice any change in handling.

 

[Capt. Meteor]

I've been thinking about your concern regarding your bike's weaving sensation while I've been riding my L. I have about 1800 miles on it so far in a little under a month and I've felt what you've described at times. I don't think it's the tires causing it. What it may be though is the asymetric fairing shape causing ari pressure changes at speed. I'm not an engineer but it does make sense to me. The wandering I've felt on my bike is not enough to cause me concern.

I liked my GS too ...but the wind always played havoc with me because of its non-aerdynamic shape and height. I still wish I was affluent enough to own both bikes.

The GS was an easy bike to ride, I felt that from day one and put 16K miles on it in seven months. I also owned an LT and put 66K miles on it during the time I owned it. It was also an easy bike to ride. I'm still learning how to ride the L.

I'm in a love affair with it ...but its developing slowly.


Scott

 

[mneblett]

What it may be though is the asymetric fairing shape causing ari pressure changes at speed.

I'm not following -- what asymmetry?

 

[avocet]

Sbassin, I have Metzler Z8 tires.

 

[Capt. Meteor]

I'm not following -- what asymmetry?

Look at your bike from the front, down low. It's easiest when on a lift. You'll see the asymetry of the faring. Its due to it being wider on the bike's left side to accomodate the cooling system.