R1100RT Pull To The Right

[Boxerdad]

I know the R1100RT PTTR has been beaten to death here. After 70K of a tired left arm I'm looking to sell my 1100 and get on an 1150RT.

 

Do 1150RTs also pull to the right?

[snod]

Every BMW boxer twin that I have ever ridden pulls to the right to some degree. Some worse that others, but from my view point TADT.

[MT Wallet]

I must be living right. I ride down the road with my left hand on my thigh and the right on the handlebars and the bike goes where I point it. It doesn't fight me and I don't even think about it. I have a 2000 R1100RT with metzlers and run 36 psi in the front. I weigh about 208# on a good day and rarely ever ride 2 up. I don't hot dog and ride conservatively(kinda). I guess I've been lucky.

[roger 04 rt]

Mine seems dead true. Why would a motorcycle pull to the right?

[snod]

Look at the FAQ I am not enough of an engineer to tell you why. I can only report that based on my observations, TADT.

[MT Wallet]

I read the article. I'm not saying it doesn't happen. I am saying it's not happened to me. So having said that and having a 2000 the question is: Is this confined to a particular series of model years? Say 96 and 97 but not 2000? I've also seen the complaint that one side of the front tire wears but my tires don't.

[RAMBLIN RED]

My 1996 r1100rt pulls right hands off but that may be the crown of the road. with hands on it is not noticable. I have never understood pttr and left side tire wear. go figure

[nrp]

My 2000 R1100RT does not PTTR or PTTL.

 

Check the alignment of the front wheel with the back wheel with some longitudinally stretched strings, to see if they are running in the same plane. I don't know how critical it is but I'd guess that they should be within 1/8 inch.

 

This assumes that the steering head axis and the rear swing arm axis are truly perpendicular.

[wheeliepup]

Here's a theory, and only just a theory. Over the life span of a front tire, on average, 50% of your turns will be to the right, naturally leaving the other 50% to the left turns. Assuming that you are driving on the right hand side of the road (USA), you will actually spend more time completing a left turn than you would in a right turn. Think of it this way,...you are at an intersection expecting a left turn, you will be into that turn for about 200 to 300 feet befoer your strait. In a right turn, you are into the turn for maybe 50 feet if that! Thus more wear on left side!

 

Of course, the tire wear would be on the right side for those folks who drive on the left side of the road!! It would be interesting if some of our British friends chime in on this to confirm this theory.

[tallman]

This topic has been bandied about several times.

MCN did an article on tire wear too.

 

Short version.

Which side of road you ride on doesn't do it.

 

Length of turns, maybe.

 

Suspension vagaries, can.

 

Owner neglect ert tire air pressure can.

 

There's more but I'll let someone else...

[dirtrider]

Afternoon John

 

On a smooth level road most 1100/1150's pull to the right just a bit.

 

Most rides don't notice it as they ride with a hand or two on the bars all the time.

 

Some 1100 as well as 1150's are pretty bad while others are not bad at all.

 

Having a 1100/1150 with a radio helps as the radio weight helps even the balance out somewhat.

 

How a person sits on the seat, how they position their legs, how they hold the bars all effect the actual & perceived PTTR.

 

If you look at how the trans weight is positioned in the bike (internal shafts & gears) there is more weight right of center so the bike has an inherent PTTR all else being equal.

 

Then mix in the very wide BMW spec for rear wheel offset that can make it better or worse. The BMW spec on the 1150RT is plus or minus 9 mm based on a +3.5mm. That is a LOT of possible variance & still be within spec.

 

[rich t]

My 78 airhead pulled to the right.

My 99 r1100rt pulls to the right.

With the throttle lock on on either side of the crown, the bike has a slight pull. I removed the spacer on my rear wheel, which helps, and also try to put a little extra weight on the left side to offset the pull a bit.

Doesn't matter if the bikes have new tires or not.

I'm not going to get involved in a debate as to why, it's not enogh of an issue for me to get stressed about. I think that has been discussed well enough in the FAQ.

Just wanted to cast my vote as further evidence of this condition existing.

[Selden]

...With the throttle lock on on either side of the crown, the bike has a slight pull. I removed the spacer on my rear wheel, which helps, and also try to put a little extra weight on the left side to offset the pull a bit.

Doesn't matter if the bikes have new tires or not.

My experience, exactly. No spacer, pack heavier luggage items on the left side.

[dstrat1]

Just had my complete rear assembly off, and it seems to me if you could adjust the swing arm a little left and right with out too much work........

[Long Range 308]

It's no theory. It's exactly what is happening.....more time and lean angle in left hand turns. Add in crowned roads and a tendancy to favor oustside on left turns due to oncoming traffic and that is where the left side tire wear comes from.

 

I spent about an hour with a mechanical engineer colleague who is really sharp and that is precisely what we came up with. I have yet to hear a credible explanation otherwise.

 

So I have slowed down a bit carving corners, cut apexes more efficiently and accept that the tires are going to wear a bit more on the left side.

[szurszewski]

It's no theory. It's exactly what is happening.....more time and lean angle in left hand turns. Add in crowned roads and a tendancy to favor oustside on left turns due to oncoming traffic and that is where the left side tire wear comes from.

 

I spent about an hour with a mechanical engineer colleague who is really sharp and that is precisely what we came up with. I have yet to hear a credible explanation otherwise.

 

So I have slowed down a bit carving corners, cut apexes more efficiently and accept that the tires are going to wear a bit more on the left side.

 

The logic makes sense to me as well, but I've seen several posts, including on this forum, from people that ride in "left lane" countries but experience the same wear pattern.

[Boffin]

It's no theory. It's exactly what is happening.....more time and lean angle in left hand turns. Add in crowned roads and a tendancy to favor oustside on left turns due to oncoming traffic and that is where the left side tire wear comes from.

 

I spent about an hour with a mechanical engineer colleague who is really sharp and that is precisely what we came up with. I have yet to hear a credible explanation otherwise.

 

So I have slowed down a bit carving corners, cut apexes more efficiently and accept that the tires are going to wear a bit more on the left side.

 

The logic makes sense to me as well, but I've seen several posts, including on this forum, from people that ride in "left lane" countries but experience the same wear pattern.

 

Count me among them. My tyres wear more on the left than the right too. I do not have any theory but know that it is not: A, road crown, or B, favouring LH turns because of traffic, or C, more time in LH turns.

 

Andy

[dirtrider]

Pretty easy to figure out what you are doing to wear the tires in a certain spot.

 

Motorcycles are a single track vehicle so they have no tire scrub from opposing tires on the same axle.

 

That basically means the part of the tire that is worn must be touching the road to have worn in that area.

 

So, just get some help to lean the motorcycle over until the part of the tire that shows the most wear is touching the ground. That is how the bike was leaned over when that part of the tire wore so all you have to do is figure out what you were doing to lean the bike to that angle while riding & how much you ride it in that leaned over condition.

 

If you lean the bike over 15° to match the tire wear spot & you think is was road crown then you would have to be riding on roads with close to a 15° road crown (unless you ride on almost level roads with the bike leaned over 15°). That would be some VERY extreme road crown wouldn't it?

 

[Trullion]

My '96 1100RS has always pulled to the right a little. Despite this there has never been any evidence of more tyre wear on either side of the front tyre, nor has there been any "cupping". Recently I've removed the spacer between the back wheel and FD as an experiment. This seems to have helped a little, as now I no longer seem to have to push forward on the left bar all the time, which over a distance has at times produced a pain between the shoulder blades. Subjectively the bike seems to steer a little better too, though I cannot say that removing the spacer has had a dramatic effect.

 

Ironically, one of the reasons cited in favour of shaft drive is that wheel misalignment is a thing of the past.....At least with chain drive the owner can easily adjust misalignment.

[Joe Durt]

A couple of theories…crowned roads…heavier stuff in right bag….you are right handed….your wallet is in your right pocket or you have a couple if Jed Clampett rolls in it…or…it has something to do with the earths rotation.

 

Honestly…dont know where you live but crowned roads is the most likely answer. Same thing for me and I notice up here in Ohio there are not many hills..My bike tends to drift to right and the left side of my rear tire wears faster than the right. A lot of Interstate miles for me. 17 to 20K a year.

Unless of course you have something miss aligned or bent.

 

Loitering with intent..

 

Joe Durt

[Boffin]

A couple of theories…crowned roads…heavier stuff in right bag….you are right handed….your wallet is in your right pocket or you have a couple if Jed Clampett rolls in it…or…it has something to do with the earths rotation.

 

Honestly…dont know where you live but crowned roads is the most likely answer. Same thing for me and I notice up here in Ohio there are not many hills..My bike tends to drift to right and the left side of my rear tire wears faster than the right. A lot of Interstate miles for me. 17 to 20K a year.

Unless of course you have something miss aligned or bent.

 

Loitering with intent..

 

Joe Durt

 

So why do my tyres that live in the UK with me where we drive on the correct (left) side of the road wear on the left?

 

Andy

[beemerman2k]

The RT drifts to the right because it does. Mine has always done it, regardless of the roads, regardless of the tires (new, old, Michelin, Metzler), regardless. I've ridden GS's and RS's from my bike's era and none of them exhibited the same behavior.

 

It's in the R1100RT's genes for some reason.

 

Oh, and it's no big deal, too, by the way. I can't ride for more than a 1/2 minute with no hands on the bars, but having said that, the pull is so slight that I have never felt any phisical effects from the phenomenon. I find it easy to believe, however, that it's worse on some bikes than it is on others.

[SKYGZR]

Just ride the damn thing for craps sake

[philbytx]

Well, looking at the planet from about 400 miles above, and if you are using the middle of the Atlantic as the midpoint. The curvature of the earth on your side Andy clearly shows a right hand camber, which would wear the left hand side of your tyres .

 

[roger 04 rt]

A couple of theories…crowned roads…heavier stuff in right bag….you are right handed….your wallet is in your right pocket or you have a couple if Jed Clampett rolls in it…or…it has something to do with the earths rotation.

 

Honestly…dont know where you live but crowned roads is the most likely answer. Same thing for me and I notice up here in Ohio there are not many hills..My bike tends to drift to right and the left side of my rear tire wears faster than the right. A lot of Interstate miles for me. 17 to 20K a year.

Unless of course you have something miss aligned or bent.

 

Loitering with intent..

 

Joe Durt

 

So why do my tyres that live in the UK with me where we drive on the correct (left) side of the road wear on the left?

 

Andy

 

Good question Andy.

 

Here are some back of the napkin calculations and estimates. They're not exact but are in the right ballpark.

 

Assuming all else being equal (e.g. no built in compensation in the frame or tracking), here are three forces that all want to make the motorcycle turn to the right:

 

1) At 65 mph, in 6th gear the engine torque which must be resisted by the motorcycle is about 20 lb-ft about 19" off the ground. If you accelerate there is more torque. If you decelerate the torque tips the bike in the other direction.

 

2) With a full tank there is about 20 lbs weight at 28" off the ground and about 9" right of center.

 

3) 10 lbs of unbalanced driveshaft on the right versus exhaust on the left that's 15" off the ground and about 6" right of center.

 

I estimate that the motorcycle and rider are 800 lbs. and about 28" center of gravity off the ground.

 

To counteract #1, the bike must be leaned 0.4 degrees left.

To counteract #2, lean 0.45 degrees left

To counteract #3, lean 0.15 degrees left

To counteract all of the above, lean roughly 1 degree to the left.

 

Of course a 1 degree lean is something that we can all do simply and unconsciously but without that slight effort the motorcycle will turn right.

 

Bottom line from leaning to the left almost all the time, the left side of the tire should wear more whether you're in the UK or the US, Northern or Southern Hemisphere.

 

RB

[philbytx]

Roger,

 

Nice job, but I like my explanation better

[roger 04 rt]

Philbytx,

So do I ... and it took you a lot less time than it took me.

[dirtrider]

Evening Roger

 

On your #1--at a constant engine RPM the torque is reacted from the wheel to the ground in the fore aft direction, there is no lateral torque to contend with.

 

That changes upon engine acceleration or deceleration as then you get that old action/reaction thing due to rotational mass resistance but at steady throttle no reaction at crankshaft.

 

[roger 04 rt]

Evening DR,

 

I know i often end up eating crow when I disagree with you but at 45 mph there's about 4 lb-ft of static torque which results in about 0.1 degree left shift, tiny. The acceleration torque is higher (at 3000 rpm, suddenly to WOT, perhaps 50 ft-lbs.) and easier to feel than the static torque. Still though the static torque can't be resisted with lateral road friction it needs the weight of the bike rotated on the lateral axis. So I think I've still got it right.

 

Or think of it another way, to rotate the crankshaft clockwise (viewed from the front) the motorcycle wants to rotate counterclockwise (Newton). To stop the counterclockwise rotation of the bike you lean it left and let gravity and the bike's mass resist the force.

 

When I ran the numbers I was surprised that the largest force was the weight of fuel on the right. Why wouldn't BMW have shifted the center of mass of everything a centimeter toward the left side? That would have solved the pull and left side tire wear.

 

RB

[dirtrider]

Morning Roger

 

What are the reactionary components that give that static torque reaction of 4 lb-ft? (that's not very much). Are you figuring something like bearing resistance, or air friction on the flywheel, or something like constant firing pulses on the crank pins.

 

Don't forget the rotational mass of the overdriven alternator rotor way high up. Shouldn't matter at constant RPM but has effect on acceleration/deceleration in a different position from roll center. The later 1150's have a free wheeling over-running clutch on that rotor so decel is independent of crankshaft decel.

 

Something else you might want to look at is the trans layout from a rear view. The input shaft is on crankshaft center but the intermediate & output shaft (both of those have most of the heavy gears) are right of center. Starter hanging on left side off-sets this a bit though.

 

Also don't forget the rear wheel lateral position possible per BMW alignment specs. There is a possible +/- 9mm shift in bike mass centerline at rear wheel so probably about 5-6 mm at engine trans area. (personally I have always though this explains the different PTTR between an 1100/1150 that has extreme PTTR & one that only has slight PTTR.

 

One last thought--years ago when some of us were chasing the PTTR in the 1100's we would run the bike up to 80mph then kick the trans into neutral & kill the engine & coast back down & they still had about the same PTTR as with the engine running & driving the bike.

 

[tallman]

A couple of theories…crowned roads…heavier stuff in right bag….you are right handed….your wallet is in your right pocket or you have a couple if Jed Clampett rolls in it…or…it has something to do with the earths rotation.

 

Honestly…dont know where you live but crowned roads is the most likely answer. Same thing for me and I notice up here in Ohio there are not many hills..My bike tends to drift to right and the left side of my rear tire wears faster than the right. A lot of Interstate miles for me. 17 to 20K a year.

Unless of course you have something miss aligned or bent.

 

Loitering with intent..

 

Joe Durt

 

So why do my tyres that live in the UK with me where we drive on the correct (left) side of the road wear on the left?

 

Andy

 

 

Andy,

This is one of those topics that will pop up now and again.

No amount of information will suffice to change POV's as a cursory

explanation suffices (ie road crown) for most.

 

I have seen thousands of BMW bikes with various degree of tire wear.

The overwhelming majority show L side.

There are some with no dif and once in a leap year blue moon a R sider.

 

I think tire pressue/riding habits/suspension vagaries/alignment contribute

but I don't know the why/how.

 

But, I do know that around the world it is a constant on beemers no matter

what side we ride on.

Best wishes.

[roger 04 rt]

Good Morning DR,

 

This reminds me of weight and balance calculations from my flying days.

 

When I used the word static torque it was short for net-output-torque from the motor which rotates clockwise when viewed from the front. At 45 mph it takes about 5 HP, in 5th gear which means about 13 ft-lbs engine torque. The engine centershaft is 19" above the road so referenced to the tire contact line I get about 9 ft-lbs (not the 4 ft-lbs I used kentucky-windage to arrive at last night). That takes about 0.25 degrees lefthand lean to overcome on an 800 lb. RT with rider.

 

Although real, and would cause the bike to pull right, it is smaller than the 20 lbs. (est. may be more, total fuel weight is about 35 lbs.) of fuel that hangs right-of-centerline. I get almost 14 ft-lbs torque to be resisted at the tire contact point (and I'm saying torqure here, not lateral friction).

 

The transmission, exhaust, final drive and starter motor seem to me to produce about a 5 ft-lb. torque on the bike using the point of tire contact as the frame of reference.

 

So even if there were no contribution from the motor's driving torque you still have nearly 20 ft-lbs from the fuel and (est.) net frame imbalance. BMW could have overcome part of that by rigging the frame so that the mass of the bike and rider was shifted 0.5 to 1 centimeter to the left. Not sure if they did (don't think so since as you point out a coasting bike pulls) or why they didn't.

 

The net torque effect of the transmission intermediate and output shafts, looks to me to be zero since the reversals cancel, and since it is the motor force which is attached to the frame. Since the driveshaft rotates in the same direction as the motor I didn't think about it too much.

 

The internal frictional losses in the engine, transmission and drivetrain are included in my horsepower at 45 and 65 mph.

 

I don't have a good model for steering geometry and front rear wheel tracking which it seems could/should contribute.

 

I went out and tried riding no hands yesterday afternoon--under power with the throttle lock, and coasting--and the bike indeed pulls to the right. I had to shift my body quite a lot to overcome it. It's a lot easier to put a tiny force on the steering and get that 1 degree lean. To be honest, I don't even notice it except no hands.

 

Forgetting the wheel alignment issues, I calculate (and might try today) that 50lbs. in the left hand case should correct the pull. If I get a minute I'll carefully "overload" the case with weights and see.

 

RB

[dirtrider]

----When I used the word static torque it was short for net-output-torque from the motor which rotates clockwise when viewed from the front. At 45 mph it takes about 5 HP, in 5th gear which means about 13 ft-lbs engine torque. The engine centershaft is 19" above the road so referenced to the tire contact line I get about 9 ft-lbs (not the 4 ft-lbs I used kentucky-windage to arrive at last night). That takes about 0.25 degrees lefthand lean to overcome on an 800 lb. RT with rider.

----

 

Morning Roger

 

I can see the engine torque reaction thing in an airplane as the engine mounted prop is reacting against the air. In the BMW boxer bike the first place the torque sees any reaction to external is at the ring & pinion as the pinion gear tries to climb the ring gear. (all above assuming steady engine RPM)

 

I guess I still don't understand the engine reaction torque that you are talking about. We must be looking at different aspects.

 

I can see some minor torque reaction due to bearing friction or maybe a bit to firing pulses but the actual engine driving torque enters the chassis at the ring & pinion & not the engine. For all practical purposes the engine, transmission, frame, & final drive are one big (Fully integrated) power plant. The engine is solid frame mounted as well as the final drive (in the drive shaft rotational direction) so the first place the engine torque reacts to anything external is at the rear wheel.

 

Take your airplane & turn the engine 90° with a right angle gear box driving the prop. The torque would still be at the prop to air not the engine to air frame.

 

[roger 04 rt]

Imagine if you detached the engine from the frame. But left the crankshaft connected to the drivetrain. It would spin around the crankshaft. That's the force to be resisted.

 

The reason that force is obvious during say WOT is that its about 5-10 times larger than during cruising.

[roger 04 rt]

Thinking about this some more, it is the rotation of the transmission output shaft and torque at that point which creates the torque to be resisted. The direction is clockwise (same as the engine) but the torque is greater. In 6th gear 1.3 times greater for the 65 mph numbers and in 5th gear 1.7 times greater for the 45 mph numbers. In both cases that adds to the required left lean by a bit.

 

At the rear wheel, the torque to be resisted about the wheel axis, or not, is what allows the front wheel to be lifted. In that axis it is the forward CG and mass which produces the resisting force. In this case a lot easier to see.

[roger 04 rt]

Just loaded some 8 and 12 lb. barbells into the left hand case. No other asymmetric loads, and about 2/3 tank of fuel.

 

It took a couple tries but 25 lbs, in the left case and I was able to ride a gently curving road at 45 mph, throttle lock on, no hands (and no traffic).

 

It seems that the effect of the forces was correct but they are all somewhat less than estimated, about 2/3 to 3/4 of my armchair calculations. With that load, no pull and I bet no left side tire wear. Why didn't BMW do this by shifting the mass a bit?

[dirtrider]

Morning Roger

 

Roger__Imagine if you detached the engine from the frame---

 

Yes, THAT changes everything as now the reaction point is between the engine & the chassis (or air, or whatever). With the engine, transmission, drive shaft, final drive all (effectively) bolted solid to the frame that ENTIRE unit is the drive unit. The place that you get the torque reaction to anything outside the chassis is the final drive OUTPUT & that output is at 90° to engine rotation.

 

 

Roger__Thinking about this some more, it is the rotation of the transmission output shaft and torque at that point which creates the torque to be resisted. The direction is clockwise (same as the engine) but the torque is greater. In 6th gear 1.3 times greater for the 65 mph numbers and in 5th gear 1.7 times greater for the 45 mph numbers.---

 

Think about this some more. Imagine you are sitting on the bike with a torque wrench in hand. You reach down & tighten an engine bolt while seated on the bike. The chassis will move slightly as you ACCELERATE the torquring moment & shift your body mass but once the pull is steady the chassis won't move a bit. Same with the engine torque reacting to the final drive. Now switch that torque to the drive shaft & you get the same non-chassis movement. The ONLY place that the torque you apply will be reacted is between the rear wheel & the ground.

 

Or, lets put it another way--

 

You make a 6'x6' platform, then attach a rope pulley in the middle of that platform, then you run a rope through that pulley & attach it to a tree limb 10' above the platform.

 

Lets say you + the platform weight 200lbs.

 

Now you sit on that platform & pull the rope towards you with a force of 205 lbs. Using your analogy above the platform would move sideways when you pull on that rope. Using my analogy the platform would only go up, not sideways.

 

[roger 04 rt]

Hi DR, this is fun (okay I need meds),

 

Here's an even easier way to illustrate the rotational torque at the engine although opposite the the engine reaction. And to demonstrate that it is a rotational force.

 

Imagine that you could perfectly balance the bike. Put a wrench on the bolt on the HES, no matter what angle you start the wrench at (3, 6, 9, 12 o'clock) the bike tips over. This is the rotational force although as I mentioned the engine reaction is opposite crank rotation.

 

Any how, did you see the 25 lb. in the left case ride report?

[dirtrider]

 

Morning Roger

 

Yes I did see you weight in the case report. In that past we have been placing a brick in the radio compartment.

 

Your wrench on a bolt is a good example. Do that sitting on the bike & the bike doesn't tip over.

 

Do that standing beside the bike & it tips over.

 

Now rig a pulley system (attached to the bike) to turn that wrench by pulling from the rear of the bike & again it doesn't tip over.

 

 

[roger 04 rt]

Hi DR,

Nice drawing what tool do you use. I labored over the Motronic block diagrams that I made in PowerPoint.

 

If the pulley were on the crank of a perfectly balanced bike and you pulled on the rope, the bike would tip over. I can see the rotational forces even in this diagram.

 

If you were sitting on a perfectly balanced bike and applied a rotational force to the crank, same thing, the bike would tip over.

 

I think the difficulty here is the size of the force relative to other torques and inertias.

 

In the motorcycle case, there is a torque with rear-wheel axis and a torque with motorcycle longitudinal axis. The rear wheel torque is larger but both exist.

[dirtrider]

Afternoon Roger

 

I was in a hurry so just used Photo Shop.

 

Roger__I can see the rotational forces even in this diagram.

 

 

So, you are saying that by pulling on the rope (while sitting on the platform) the unit will go sideways as well as up? It should at first apply of force as the arm mass is moving from a standstill to motion (accelerating) but once the pull is steady the platform should only go up.

 

 

About all I can say is to think it over very carefully & see if you still feel the same after researching it & thinking it over some more. You need to distinguish accelerating/decelerating crankshaft torque from constant (fixed RPM) torque.

 

[nrp]

The net roll torque that is reacted thru lateral forces at the tire patches is the sum of the net rate of change of angular momentum of all the rotating elements in the M/C drive train.

 

The drive line converts most of the engine torque into a longitudinal thrust at the rear tire patch. Only the component of engine torque used to accelerate the flywheel and other driveline elements will show as a roll moment to the M/C chassis. This torque will be much less than the rated torque of the engine.

 

I really think PTTR and PTTL are reflections of wheel alignment issues and possibly chassis manufacturing issues. There is a small component of pull caused by lateral net CG placement, but it seems trivial compared to the unbalances I sometimes deliberately have in my bags.

[dirtrider]

Afternoon NRP

 

I agree with you on the rear wheel off-set possibly being part of the issue but not on the tracking part being the cause.

 

Having owned & worked on Harleys for years, they have no pull to either side (we ride them miles & miles on end with no hands on bars)

 

Due to the wide drive belt on the older Harley dressers there is a significant rear wheel off-set. They might be a single track vehicle but leave 2 distinct tire tracks on the road.

 

When aligning them we have two ways to align the rear wheel. One is a straight off-set with the rear tire tracking straight ahead but off-set from the front. The other is to align the rear wheel to track slightly angled so the rear tire foot print follows the front tire tracks almost exactly (that also means the chassis goes down the road at a slight angle to path of travel.

 

I have run both alignment conditions on many Harleys & never get even a hint or pull to either side.

 

[roger 04 rt]

Okay DR,

 

I've got crow warming in the oven. My points 2) and 3) from before seem sound but regarding my point 1)I'm starting to understand your argument that the only unconstrained torque is along the axis of the rear wheel. Also the only longitudinal torque is the from the transient change in the engine's rotational inertia. I'll think about it some more but while I do I'll get my knife and fork out.

 

In the meantime, I'm going to start marketing 25 lb. weights to bolt onto the frame. Bet I can make a killing.

[Trullion]

And here was me thinking that if you ask three economists a question you'll get four opinions.....and the assumption that there is a tin opener.

 

As a non-engineer I believe that the real answer is that oilhead owners are a self-selecting group whose right arms are slightly longer than their left ones.

 

Darwinian? Who knows (or cares)?

[roger 04 rt]

Until it was mentioned, I hadn't noticed. My first ride was with 40 lbs. in the left case which produced a PTTL equal to the current PTTR. It was very noticeable and felt weird, almost unstable. So either I've adapted to PTTR as normal or BMW knew that PTTR is preferable. They certainly knew they designed an unbalanced bike.

 

It's kind of obvious now that with a full tank there's about 20-25 lbs. of fuel on the right that's not counterbalanced by fuel on the left. The starter and exhaust seem to roughly balance the swing-arm and FD. If one cares, 22 lbs of weight in the left case (which is the case limit) seems to do the trick.

[snod]

I am glad that someone with some engineering expertise agrees with my long standing opinion that BMW boxer twins are heavy on the right side, and therefore pull to the right. I believe that the /2/5/6/7 Airheads had the same issue, just to a lesser extent. It also seems to me that the RT gas tank with the huge right compartment adds even more weight to the right side. I keep all of my tools and spare parts in the left saddle bag, and when I take both hands of the bars my arse automatically shifts left. I don't have a problem with the PTTR, but for me it has always been there. Over the last 500k miles and 35 years, or there abouts, I have learned to live with it.

[dirtrider]

Morning Roger

 

Engineering wise I just hate to add weight to correct a problem (just doesn't seem directionally correct).

 

Moving the rear wheel to the right seems the best approach.

 

If there is a shim between the rear wheel & final drive axle (& tire clearance allows) remove that shim then move the rear brake caliper over a like amount (or with worn rear brake pads just let it go as is). If the wheel speed sensor is then out of spec, gap wise, correct that.

 

Then shim the (fixed) right rear swing arm to final drive pivot bolt to the right as far as reasonably possible. Then turn the L/H (adjustable) pivot in to compensate.

 

It doesn't seem to take much wheel movement to the right to move more chassis/rider weight to the left of center.

 

[roger 04 rt]

Hi DR, I understand that point of view and wouldn't add the weight myself. I found the handling characteristics interesting with the 20lb. imbalance moved to the left-side by adding 40 lbs. of weight--try it sometime.

 

If the weight imbalance is at the root of the problem, and since weight varies with quantity of fuel, it would seem that there aren't any good solutions to the problem. I'd like to check the bias sometime with an empty tank.

 

I'm still not clear why BMW designers would have ended up with a clearly imbalanced design. They must have had a reason. Maybe the instability I felt with left side weight bias is a hint. Of the several bikes I've owned I haven't ever had one with asymmetric bias and all could be ridden hands free. So I guess I'm confused.

[JamesW]

Hi Roger,

You just had to bring up the subject of BMW engineering. There are or is a multitude of examples of BMW design questions. In this post i was going to go on and on but then decided why bother when all you have to do is spend a few hours reading thread after thread, on and on.......