60 MPH stops unsmooth, why? - Stromtrooper Forum : Suzuki V-Strom Motorcycle Forums
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post #1 of 31 Old 02-16-2015, 03:52 PM Thread Starter
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Question 60 MPH stops unsmooth, why?

From time to time, when freeway traffic is light enough that I'm certain there is and is going to be nobody coming off onto the same exit behind me, I practice maximum braking stops from 60+ MPH when exiting one of the freeways.

Nearly always, the initial 60-40 MPH speed reduction is accompanied by a strange, non-smooth motion. I'm not sure what it is. There is no sideways component to it. Sometimes I think there might be a tiny bit of fore/aft motion of the forks, which I sense through the handlebar. But that is hard to be sure of, what with much of my weight pushing forward on the grips at the same time. (And it's hard to distinguish a braking force variation from a tiny fore/aft motion.) I estimate its frequency at about 1.5 - 2 Hertz, (a little faster than once per second.)

My questions here are:

Is this normal for the V-Strom?
Or does it mean my bike is not setup right or has excess play?
Has anybody found a technique for making it not happen?

Last edited by Trepidator; 02-16-2015 at 03:55 PM.
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post #2 of 31 Old 02-16-2015, 05:13 PM
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Possibly brake rotors warped or bad bearings?

..Tom

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post #3 of 31 Old 02-16-2015, 05:18 PM
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Practice from 20mph. It's safer and uses the exact same reflexes and movements.

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post #4 of 31 Old 02-16-2015, 10:27 PM
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Quote:
Originally Posted by greywolf View Post
Practice from 20mph. It's safer and uses the exact same reflexes and movements.

No you want to practice panic stops at the speed you ride. You need to experience how hard you can brake and how long it takes to stop at speed. Also you need to overcome the fear of hard braking to scrub off speed, you aren't going to develop any of that at 20mph.

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post #5 of 31 Old 02-16-2015, 11:48 PM Thread Starter
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Post braking practice

Quote:
Originally Posted by greywolf View Post
Practice from 20mph. It's safer and uses the exact same reflexes and movements.
I do plenty of hard stops at 20 MPH and 35-40 MPH, mostly as practice. When I do, the strange, 1.5-2 times/second, braking force pulsation does not occur. It seems to occur only at the higher speeds, above 40 MPH.

As for a stop from slower speed using the same reflexes and movements, I respectfully disagree. At higher speeds, I am responding to things happening on a freeway, which virtually never involves something getting into my path at low velocity relative to mine. Events take longer to develop, and reflex is less important than mental engagement and a sense of trajectories and where the open escape routes will be. At arterial speeds, the quick, full stop is much more critical, and the reflex to get to full braking quickly is crucial. Before encountering this pulsating effect I opened this thread with, I would have agreed about "the exact same movements". But those pulsations are quite dramatic at high speed only when I approach as hard a stop as I can routinely do smoothly at lower speeds. So I end up modulating the brake pressure to keep that strange effect from being alarmingly severe, (a different movement). At lower speeds, I practice hard braking at the force which engages the ABS sometimes, and sometimes at a force just below that (traction) threshold. At the higher speeds, I have not engaged the ABS but once when necessity demanded. As I approach that same force during my 60+ MPH practice stops, that shimmying is enough to make me want to ease up.

My concern is really a safety concern. I want to be able to confidently approach either the ABS threshold or the "stopee" limit, from freeway speeds should that suddenly be necessary. I have slowly increased braking force during these practice maneuvers to be sure that I can recognize when the back wheel starts to lift, under conditions where there is nothing I'm about to hit in front of me. That way, I think I have a better chance of executing a maximum stop when it really counts.
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post #6 of 31 Old 02-16-2015, 11:53 PM Thread Starter
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Quote:
Originally Posted by V-Tom View Post
Possibly brake rotors warped or bad bearings?

..Tom
Do warped brake rotors do something at higher speed only? It seems like a reasonable hypothesis, but I do not see the pulsating effect slow down in proportion to speed. It just disappears when I have scrubbed speed down to about 40 MPH.

I checked bearing play about 6,000 miles ago, when they were fine. I'll have to do it again.

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post #7 of 31 Old 02-17-2015, 12:13 AM
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I was re-reading your original post and thought about the frequency. If it was two times a second at 60 mph that couldn't be wheel speed related. 60mph is 90 feet per second... Two hertz would mean a wheel circumference or about 45 feet so what I was thinking can't be right unless it was some strange harmonic.

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post #8 of 31 Old 02-17-2015, 01:06 AM
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Since the shimmy has no sideways component, and it is at such a slow rate, all the usual suspects are out the window. -Especially if the bearings are good.

The only thing I can think of is that it is either fore/aft fork flex, as you suggested, which you may be able to test by lowering the front a bit (raising the forks in the triple tree). That would make the front end stiffer, but is odd, since I have not read of anyone complaining of the same symptoms.

Spring preload set way low, with super low fork oil viscosity? I think that is unlikely.

This is a puzzler, for sure, with undamaged forks and proper sag settings and proper fork oil. I am going to watch this thread.

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Last edited by PcolaDennis; 02-17-2015 at 04:13 PM.
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post #9 of 31 Old 02-17-2015, 10:42 PM
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You have an ABS bike. In an emergency stop you just hold the brakes full on. I don't know what is the cause of your shimmy. There might be some flex of the upper fork tubes that no one else has reported. Or maybe something else, but we don't know what. Tire cupping and shimmy would be related to the tire rotational speed, not what you report. Brake disc judder (which is not warped brake discs) also would be related to rotational speed. Loose steering stem bearings probably would not shimmy, they'd just set in place. There is a possibility of one of the pressed brake disc buttons being tighter or looser than its mates, but this seems unlikely.

The semi-floating brake discs on a motorcycle are there to allow for both light weight and thermal expansion. These are the outer rotor attached to the inner section by the pressed buttons. The rotor should expand smoothly when it is warmed by the friction of the brake pads. If some buttons have loosened in service there might be a bit of unintended movement, but again, that should relate to the rotational speed.

Here's more about the myth of warped brake discs:
Tech Article: Warped Brake Discs

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post #10 of 31 Old 02-17-2015, 10:47 PM
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It may be the ABS system being poised for action.

If it were a rotor problem you would feel it slow down at the same speed you slow down

Last edited by FredBGG; 02-17-2015 at 11:21 PM.
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