Monday, May 13, 2013

Rear steer

Ever since I got my Dyna I had been noticing that some times when accelerating out of a corner at speed it would seem as though the back end would shift sideways just a little bit. It was never enough to cause any control issues but always enough to ratchet the heart rate up a notch or two for a few seconds.
And so the hunt for the culprit began.
All nuts and bolts to spec – check.
Tires – replaced.
Tire pressures – check.
Wheel alignment – check.
Steering head adjustment – check.
Rear shocks – check.
Front forks – check.
Front engine mount – check.
Rear engine mount – check.
Swing arm bushings – check.
Everything was to spec but still, on occasion, the seat of my pants would tell me something was wrong. The closest I could remember to that sensation was many years ago when I test rode a Norton Commando on which the rear axle bolt hadn’t been properly tightened, allowing the alignment to change mid-corner causing the back end to swing out just enough to change the tracking. (I noticed this in the first corner after a straight road run of a few miles at 100+ mph!)
I wanted to get a better look at what was happening.
kennybrown
But decided instead just to let Google be my friend. It turns out that this is a not unknown problem with Harleys (and presumably other marques) that have the rubber-mounted engines. What happens is the engine mounts are designed to minimize the effects of the up-and-down vibration and so work best in the vertical plane. However when cornering hard the stresses are being put on those mounts in the horizontal plane. Combine that with some enthusiastic acceleration and the engine mounts can flex just enough to cause the rear wheel to track out ever so slightly. And the rider to say a hasty Hail Mary.
There’s even a term for it, rear steer, which is, as I think about it, a pretty accurate description of what is happening.
Which begs the question: Why doesn’t the MoCo address this issue in their design?
It turns out they did, on the Buell. But for whatever reason that enhancement didn’t find its way onto the Dyna. Perhaps it was too expensive, or they thought most riders riding under normal conditions would never notice (which may be true, but if they do they will REALLY notice). But the fact many Harley riders won’t push their bike that hard doesn’t change that it remains a design weakness, still being propagated as far as I know on current models.
Of course there are products on the market that purport to “solve” this problem through additional engine bracing, but they are not cheap. However since I’ve tried everything else and love this bike except for that one issue I will probably break down and do the handling upgrade. So now it’s a just matter of sorting through the options to find the best solution.

7 comments:

  1. Dave, great description, I immediately understood. It's the same spine quivering feeling I got on a tight curve after picking up a screw that caused a slow leak. I look forward to the report on the fix.

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    1. David - Very much like a tire with low air pressure. That's the first thing I thought of as well.

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  2. You're right, most of us don't ride our bikes at the edge of the envelope and may never experience this, But it sounds similar to the sensation one gets when the rear tire slips on a 'tar snake'... always kind of an 'uh oh' feeling when that fat rear tire slips off the tar patch and finds grip on the pavement.

    Never ride outside your machine's performance envelope.

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    1. B.D. - Tar snakes too. Anything that causes that rear tire to track off the line of the front tire will give you the same sensation.

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  3. Mine does a "shimmy". I had my Softail checked out because I felt a sensation of it shimmying *sp* in the back end (tire). Not sure if this is the same, but nothing could be found either. Happens to me on straight aways and cornering at high speeds.

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  4. KT Did - I've only ever experienced it when cornering. It seems to me you're dealing with a different issue, besides, I don't think the softails use rubber engine mounts but rely on counterbalancers to smooth out the vibrations. Not sure what to suggest in your case but if everything is to spec mechanically tires would be the usual suspects. Surprised the dealer couldn't pinpoint the problem. (Or maybe the dealer IS the problem.) That kind of back end wiggle can be really disconcerting as you never know if it will develop into something more serious real fast! So keep digging 'til you find it.

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  5. Not sure of all the details...

    But most Harleys run with the front and rear wheels offset from the common centre line.

    Me thinks going by what you have said that in a NON rubber mounted engine, the engine cases and less so the head mounts, would form a significant stiffening member within the frame.

    Given that it's a heavy bike, that frames do flex, and with the rubber mounts allow MORE flexing of the frame, and the chain of flexes "add up", and given that there are corrective kits available - I'd say that the idea of the back wheel tracking - in a rear wheel steering kind of a way, may in fact, actually be quite true.

    It may be of interest - that if you can ever get a bicycle to roll backwards down a slight incline or get a friend to push you backwards up to a jogging speed - and then let go, and then try to steer - back wheel steering on a bicycle or motorbike is very bad.

    And I think the back wheel steering - once underway, would tend to pull the flexes WIDE OPEN, leading to a sudden frightening effect...



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