Straight Line Stability

[JamesW]

I probably could have picked a better title but I guess my creativity lacks today.  Anyway, I was recently told that if a bike tends to wander slightly side to side in the lane of travel with constant rider input required to keep going in a straight line it may be caused by the front wheel having a smaller diameter than the rear wheel.  Could there be any truth to this?  I was also told that the GS bikes don't exhibit this wander lust because the front wheel diameter is greater than that of the rear wheel hence they are less fatiguing to ride on paved roads.  I've no experience with the GS so I have no clue.  I'm just curious.

[Hosstage]

Never heard of this. Usually due to worn tires, road design, misaligned wheels or chassis, low tire pressure. Even windshield or mirrors blowing the front end around.

If the bike has saddlebags, it could happen with one side being overloaded, or loaded much heavier than the other.

[JamesW]

Yeah, I've never heard of this either.  The internet should be renamed WWBS.

[Scott9999]

1 hour ago, Hosstage said:

Never heard of this. Usually due to worn tires, road design, misaligned wheels, low tire pressure.

... and/or worn wheel bushings, swing arm bushings, or any similar component that could cause the wheels to shimmy.  Check 'em all out for excess play.

[szurszewski]

I don't know anything about relative wheel diameter's impact on this, but I can ride my 1200GSA with no hands in most straight paved conditions and through light curves easily - it's much harder to do that on other bikes I've had, like my current R1100S. 

[Skywagon]

I think this is a common problem on the RT's.  My oil head and wet head will not hold a straight line if you take your hands off the bars.  DR and some other had a bit of a remedy on the oil heads if I remember correctly; I'm probably not remembering correctly but it seems like it was adding a washer or something to shim the rear wheel a certain way.  

 

It doesn't matter on my wethead if the tires have 1 mile or 10,000 miles, it does not track straight...same with the 1150. It isn't a big deal as I can weight shift enough if necessary to give my arms a break.  The last BMW I had that tracked without input was a R100RT.  Never owned the GS family.

[JamesW]

I'd like to know if the GS has a larger front wheel diameter than the rear wheel.  Also, I think the GS has much more suspension travel which, I would think, would result in a smoother more plush ride on pavement but that's getting off subject.  

[MikeB60]

Yes, the GS has a larger diameter front tire. Don't notice any difference in stability in comparison to a RT or GT.  

[9Mary7]

My experience with R11XX series bikes is that they don't track straight. The R1200's do much better and I don't notice it anymore..... I think it has something to do with the longitudinal crankshaft orientation.......YMMV

[AviP]

On 1/23/2022 at 10:35 PM, szurszewski said:

I don't know anything about relative wheel diameter's impact on this, but I can ride my 1200GSA with no hands in most straight paved conditions and through light curves easily - it's much harder to do that on other bikes I've had, like my current R1100S. 

Ditto on my R1100RT. And when it hydroplanes, the steering oscillates from full left to full right and straightens up automatically if I release it. Weird but true!

[Red]

I don't know if it's true, but I have read where the larger wheel generates more centrifugal force and tends to keep the front end more stable.  One of the reasons why chopper riders have relatively large dia wheels on those extended front tends.

[Beech]

My chopper was only for style, 21" and lucky it never slid out. I think not all brands of tires run well on any motorcycle. You have to zero in. Maybe that is why in Germany the TUV recommends specific tires and you can not run any other on said bike.

 

[Lowndes]

On 1/23/2022 at 7:59 PM, Hosstage said:

 

If the bike has saddlebags, it could happen with one side being overloaded, or loaded much heavier than the other.

 

I'll ride with just one saddlebag for day rides to the mountains.  All my junk fits in one and it's so much easier getting on and off the bike with one.  A rainsuit, extra warm or cold weather gear, tire pump and patch kit, plus a few spare items and tool kits, maybe 25 lbs total.  I've not noticed any handling or tread-wear differences at all doing this on several bikes.

 

My experience is that the tires, the brand, the size, air pressure, and the tread pattern all can have a huge difference in handling/tracking/steering.  

 

My ST1300 has a well known and common habbit named "the Pan Weave" at speeds over about 60mph.  I've modded the front suspension with air and dampening with no effect on the weave.  My conclusion there is aerodynamics of the front tupperware.  

 

Try a Michelin r5 on the front next time.  Check the wheelbearings and steeringhead bearings for play and check the calipers for pad clearances.

[dirtrider]

On 1/23/2022 at 6:45 PM, JamesW said:

I probably could have picked a better title but I guess my creativity lacks today.  Anyway, I was recently told that if a bike tends to wander slightly side to side in the lane of travel with constant rider input required to keep going in a straight line it may be caused by the front wheel having a smaller diameter than the rear wheel.  Could there be any truth to this?  I was also told that the GS bikes don't exhibit this wander lust because the front wheel diameter is greater than that of the rear wheel hence they are less fatiguing to ride on paved roads.  I've no experience with the GS so I have no clue.  I'm just curious.

Morning James

 

 

This is a very complex question to answer. There is no easy one word answer like "yes" or "no" as single track vehicles are strange animals. 

 

Typically different  motorcycle  tire diameters also includes different tire widths and/or a different tire crown radius. The contact width & length as well as crown radius have a BIG effect on turn in & stability. 

 

Also, larger diameter tires spin slower at the same travel speed so that effects  gyroscopic precession.

 

When it comes to the GS vs the R or RT or RS  then you need to keep in mind that each motorcycle is designed to use a specific tire diameter & tire width/profile . So the designed-in rake & trail reflect that.  

 

In any case, it is mostly the front tire that changes things as the rear tire is basically along for the ride & doesn't effect the cornering or straight line stability like the front tire does. 

 

If a motorcycle is well balanced then it should track straight all by itself with little input from the rider.  But things like sail area allow side winds (or angle winds)   to effect the rider/motorcycle & even effect the spinning front tire. Then if the front fairing or windshield is attached to the forks or steering that can REALLY allow wind currents to effect straight line stability.  

 

When it comes to the BMW 1100/1150 bikes that is whole difference ball game as those darn bikes are not not well balanced over the front & rear tires as most have a PTTR (Pull To The Right) tendency due to weight offset from the vehicle center line. This alone has a big impact on the motorcycle holding a straight line without continuous rider input. 

 

Then, to make things even worse BMW doesn't put the rear tire directly behind the front tire so most ride along with a slight front/rear tracking off-set. 

 

Even if all the above were accounted for you still have asymmetrical tire wear, road surface imperfections, road camber variances, inherent oscillation conditions, other vehicle wind currents. Some even require a steering stabilizer to help control oscillation. 

 

So bottom line-- "the front wheel having a smaller diameter than the rear wheel"  has little effect, but front tire/wheel being a different diameter, or different width, or different crown profile than the motorcycle was designed for can have a big effect.