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Bike unstable downhill at speed

Old 07-20-17, 08:08 AM
  #1  
europa
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Bike unstable downhill at speed

I recently bought a pre-loved 2013 KHS Flite 500.

Among all the servicing items you find yourself doing when buying a second hand bike (cables, bar tape, saddle, etc), I swapped out the wheels with their broken spokes for a good pair of wheels I already have - Ultegra hubs, Velocity DeepV wheels, 36 spokes (I was aiming for bomb proof when I built them).

I checked the head set and while I didn't touch anything, there doesn't appear to be any play in it, using the hi-tech method of giving it a good pull and push while the bike was in the work stand. It swings freely without any notchiness.

I've done a reasonable amount of riding, including some downhill runs at 50 km/hr. No problems.

Last night, I fitted brand new, 28mm tyres - watching the front one while riding, it appears to be sitting square on the rim with no wobble.

Today was my first commute. On a very steep dowhill at speeds between 60 - 70 km/hr, the bike did not want to track straight. It felt... unsteady. Hard to find the words for it but I've ridden this hill on my old eighties roadie and my Hillbrick which was based closely on that roadie and these both rode that hill like they were on rails. The KHS was a bit worrying.

As the shake started, I leaned forward to put more weight on the front wheel but it made no difference. I did not go onto the drops because at this stage, I haven't experimented with braking in the drops and at the bottom of a steep hill is not the time to start.

Before you tell me to simply ride more slowly, the safest way to ride this hill is to take the lane and ride with the traffic, hence the speed - the lane is narrow, the edge of the road very rough and the grade a bit over 10%.

So why's the bike unstable?

For commuting, I've fitted a rack and loaded a pannier to one side - that pannier held jeans, shirt, lunch and a few other things so it wasn't light. Was the weight at the back causing the problem? No problem with the other bikes but they have more relaxed geometry and a longer wheelbase than this more 'racey' bike. Could it be an airflow thing over the pannier?

I've rechecked the headset bearings (by the shake method) and can't find any play. If you want me to do something more rigorous, you'll need to tell me how.

The wheels feel solid with no side play and are both true. The new tyres look true when you spin the wheel.

Next door to no wind.

I hope to test the 'pannier theory' on the weekend by riding the hill without it and this might give more insight but in the meantime, I'm stuck with what I've got.

And maybe I'm just used to bikes with relatively stable handling and am a little freaked by a 'racey' bike, but I doubt that because this thing did not encourage me to push the speed any further which is surely what such a bike is designed for.
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Old 07-20-17, 08:08 AM
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Sorry about that, I didn't realise the post was so long. Maybe I should have just summarised it as 'my new bike is trying to kill me'
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Old 07-20-17, 08:39 AM
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Look up "death wobble" for bicycle. If it happens, touch your knee on the top tube should make it goes away. My old bike did that for down hill section right after they paved it.
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Old 07-20-17, 08:41 AM
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Take the rack off and do the same hill.
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Old 07-20-17, 09:03 AM
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". . . unsteady . . . shake . . . unstable . . . "

A better characterization of the symptom would be a good start, unless you're looking for a game of twenty questions.
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Old 07-20-17, 02:41 PM
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Take the rack off and try the hill. If it doesn't happen you can experiment with rack loading.

Beyond that, there aren't really good options. The cause is a combination of factors between bike geometry and rider position.
The main problem is that bike geometry and rider position are both fixed, so you can only make minor adjustments like new wheels.

I had a bike that did it, and just gave up on the bike, there wasn't anything I could. Techniques like knee on the top tube made it controllable but it was never stable.
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Old 07-20-17, 03:15 PM
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My bike when new did not want to track. There was a definate bias to head left. After pondering the situation for a while I checked the alignment of the front and back wheels. I did this by turning the bike upside down, then shimming under the handlebars until the back wheel was perfectly plumb using a carpenters construction level. I then ran a string alongside both wheels and turned the handlebar until both wheels were aligned with the string. At that point the front wheel should also be perfectly plumb but it wasn't.

What I found was that the front wheel was tilted. One of the dropouts in the front wheel was inserted incorrectly by just a tiny bit. The geometry is such that if the front axle is off by even 1/2 mm, the rim is off by 4 to 5 times that and the wheel will not track properly. I ended up filing the dropout the axle sits in with a round file, just a bit at a time until the wheel was plumb and also perfectly aligned with the string. Now the bike tracks well although I believe tracking would improve a bit with a new fork having a bit more trail. This is not urgent so I've not bought a new fork. Good luck tracking down your problem.
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Old 07-20-17, 03:38 PM
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Wheel alignment sounds like a good area to investigate, mainly because getting the front wheel to fit properly is a trial. It might be I'll just have to learn to be careful there.

I'll certainly experiment without the pannier - not willing to just pull the rack at this stage because it's a bear to get on (home made P clamps at the top that were made out of tin that's too heavy for easy working with).

And now it's off to work I go.
Will I return?
Will the killer bike get its revenge for fitting a rack?
These questions and more will be answered in the next episode of 'Bikes that p*** you off'.
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Old 07-20-17, 03:53 PM
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Do you have an extra pannier to toss on the other side?
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Old 07-20-17, 03:58 PM
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If you read the third edition of Bicycling Science, they state that stability analysis is very difficult to characterize mathematically. As an engineer, that sort of thing is up my alley, and a math analysis is how you "do" stability correctly, but they're right. On bikes, it's surprisingly hard.

Is the rack mounting super stiff? Oscillations (in bikes, anyway) by energy shifting between potential energy (bike or rack mount twisting) and unwanted velocity (the fork and wheel shifting back and forth.) If the rack is springy that could set up an oscillation.
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Old 07-20-17, 05:04 PM
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I, too, would remove the pannier and try the hill again.

Single-track stability is a complicated subject. You can learn much by looking around the net with Google. Most references will be for motorcycles but the elements are the same for bikes.

Some Notes:
• All spinning wheels tend to wobble when upset and they won’t stop wobbling until something dampens the wobble. Wheel wobble frequency varies with the individual wheel’s structure. Bikes (both kinds) depend upon the tires' contact patches for their damping forces. Those damping forces must be transferred to the chassis in a direct and predictable manner. Loose bearings, spokes and unfortunate vibration resonances can fail to dampen or even increase chassis wobble.

• Chassis geometry is critical: steering head angle, trail and wheelbase are among the more important factors. Large steering head angles and generous trail (within limits) are perhaps more important than the wheelbase. Touring bike designs with their ‘lazy’ steering angles, generous trail and long wheelbases tend to be more stable at speed, and --- speed is a critical factor.

• Everything must be ‘tight’ between the tire’s contact patch and the rest of the chassis. No looseness. Anywhere. Loose hub bearings, loose steering bearings, loose spokes, not enough air pressure, etcetera, all reduce the tires’ wobble damping effects and increase the likelihood of chassis wobble.

• When the chassis oscillates around the steering axis (because of rear wheel wobble), it referred to as weave. When the front wheel assembly oscillates it is called wobble. Weave frequencies are much lower than wobble frequencies, primarily because of the difference in the front/rear mass difference.

• Most bicycle and motorcycle 'wobbles' are actually weaves and can be dampened by ways mentioned earlier. A genuine front-end wobble is very violent, extremely rapid and usually results in loss of control.

• There’s more. Dented steering head bearings, too much weight too high-up at the rear and aerodynamics are, among others, possible wobble factors.

Wobbles are sometimes very dangerous and folks have died because of them. I would not ride a bike of either sort that had a weave and certainly not one that might wobble.

Joe

Last edited by Joe Minton; 07-20-17 at 05:22 PM.
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Old 07-20-17, 10:35 PM
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Originally Posted by europa View Post
And maybe I'm just used to bikes with relatively stable handling and am a little freaked by a 'racey' bike, but I doubt that because this thing did not encourage me to push the speed any further which is surely what such a bike is designed for.
Yes, that bike is quite "racy":
FLITE 500 - KHS BicyclesKHS Bicycles

I agree with the others: try the hill again without the pannier. A wobbly pannier on a "twitchy" bike is going to feel very different than a "stable" bike.

I would also try descending in the drops from the top of the hill. It sounds like you're on top of the bars or on the brake hoods.
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Old 07-20-17, 11:15 PM
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i agree with the other responses on checking wheel/frame alignment... and verify there is no twisting of the frame/fork relationship...

and also... is the FORK for sure the correct one, and is it UNBENT?

and look at the frame just behind the head tube... is there ANY hint of deformity, like wrinkles, or cracks in the paint?

"broken spokes"... scary... the bike was previously tortured... check EVERYTHING....

including the state of the lower steering bearing... it could be that while hanging in your stand, it is unloaded, and that bearing may actually be pitted/rusted some... even one bad ball, or some brinelling, can cause issues under load... that lower bearing sees no load when up in a work stand.... the lower bearing is usually the one that gets bad first.

yank the pannier off... go for a test ride... and see how the bike tracks while riding no-handed.... but be ready to grab the bars.

and make sure to check the ultegra wheels for proper centering, too... both with a gauge, and mounted in the bike... eliminate all possible problems... don't assume, find out.

fork is the most likely thing to cause headshake... but multiple issues may be found.
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Old 07-20-17, 11:29 PM
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here's the 411 on the 500... it has an aluminum steerer tube, btw... and a rather steep 72.5mm head angle... remove the fork, and CHECK that steerer tube for bending, with a GOOD straight edge... and cracks at the fork crown area....... remember those "broken spokes"? and use a magnifying glass to inspect the frame around the head tube, and top and down tubes, about 3 to 5 inches from the head tube...ok?

KHS Flite 500 review - BikeRadar USA
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Old 07-21-17, 04:37 AM
  #15  
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Thanks for all the replies. Lots to look at and sadly, I won't get to it this weekend. I'll keep you posted.
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Old 07-21-17, 06:30 AM
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A couple of my road bikes are a little small for me and they have a relatively short wheelbase compared to my other bikes. When I ride them over 50 kph downhill they are very squirrelly and twitchy. You mentioned that the bike in question also has a short wheelbase. My guess is that this is the probable cause of the weird handling.

I much prefer a less trendy bigger bike with a larger frame and a longer wheelbase.
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Old 07-21-17, 08:01 AM
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Originally Posted by Ball Bearing View Post
I much prefer a less trendy bigger bike with a larger frame and a longer wheelbase.
Unfortunately, my geared bike (my much beloved Europa - bought new in the early 80s) died (frame broke). I needed a geared stablemate for my fixed gear Hillbrick, but didn't have the money for a new frame. The KHS came up cheap, and people have been telling me for years I need to get away from my old school, steel bikes and buy a modern racer, and this seemed like a good chance to find out what they were yapping about. Well, it needs 28mm tyres to be almost as comfortable as my steel bikes and, as we're discovering, may have handling quirks when asked to work as I work my other bikes - of course, that may be due to a problem that can be fixed. I kinda like the bike, despite the frame being far too big when judged by racer standards and the 'fat' tyres make it comfortable enough to ignore. I just need to get it playing nicely down hills.

Buying second hand bikes is always a journey. I suspect everyone starts it hoping it'll be a match made in heaven from the first roll of the wheels but realistically, bikes are sold for a reason. The 'journey' is to sort out those reasons.
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Old 07-21-17, 08:03 AM
  #18  
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If you intend doing 60-70.km hr down hill you really need to think differently, that's fast on a commute bike, I would start with the front wheel complete check, spoke tension etc. tires etc.
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Old 07-21-17, 07:06 PM
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Originally Posted by europa View Post
I recently bought a pre-loved 2013 KHS Flite 500.

Among all the servicing items you find yourself doing when buying a second hand bike (cables, bar tape, saddle, etc), I swapped out the wheels with their broken spokes for a good pair of wheels I already have - Ultegra hubs, Velocity DeepV wheels, 36 spokes (I was aiming for bomb proof when I built them).

I checked the head set and while I didn't touch anything, there doesn't appear to be any play in it, using the hi-tech method of giving it a good pull and push while the bike was in the work stand. It swings freely without any notchiness.

I've done a reasonable amount of riding, including some downhill runs at 50 km/hr. No problems.

Last night, I fitted brand new, 28mm tyres - watching the front one while riding, it appears to be sitting square on the rim with no wobble.

Today was my first commute. On a very steep dowhill at speeds between 60 - 70 km/hr, the bike did not want to track straight. It felt... unsteady. Hard to find the words for it but I've ridden this hill on my old eighties roadie and my Hillbrick which was based closely on that roadie and these both rode that hill like they were on rails. The KHS was a bit worrying.

As the shake started, I leaned forward to put more weight on the front wheel but it made no difference. I did not go onto the drops because at this stage, I haven't experimented with braking in the drops and at the bottom of a steep hill is not the time to start.

Before you tell me to simply ride more slowly, the safest way to ride this hill is to take the lane and ride with the traffic, hence the speed - the lane is narrow, the edge of the road very rough and the grade a bit over 10%.

So why's the bike unstable?

For commuting, I've fitted a rack and loaded a pannier to one side - that pannier held jeans, shirt, lunch and a few other things so it wasn't light. Was the weight at the back causing the problem? No problem with the other bikes but they have more relaxed geometry and a longer wheelbase than this more 'racey' bike. Could it be an airflow thing over the pannier?

I've rechecked the headset bearings (by the shake method) and can't find any play. If you want me to do something more rigorous, you'll need to tell me how.

The wheels feel solid with no side play and are both true. The new tyres look true when you spin the wheel.

Next door to no wind.

I hope to test the 'pannier theory' on the weekend by riding the hill without it and this might give more insight but in the meantime, I'm stuck with what I've got.

And maybe I'm just used to bikes with relatively stable handling and am a little freaked by a 'racey' bike, but I doubt that because this thing did not encourage me to push the speed any further which is surely what such a bike is designed for.
Try a Silca wheel balancing weight.
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Old 07-21-17, 07:10 PM
  #20  
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Originally Posted by velocentrik View Post
Try a Silca wheel balancing weight.
The theory being that the new tyres threw a wheel out of balance? I'd have thought I'd be experiencing some sort of vibration if that was the case
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Old 07-22-17, 05:50 AM
  #21  
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A few years back I descended a fairly large hill and at about 45 mph my bike went into a speed wobble. My LBS discovered that the front quick release was slightly too loose.
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Old 07-22-17, 10:34 PM
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The usual emergency remedy is to get your weight off the saddle, putting it mostly on the pedals, move your body forward some, putting more weight on your hands, and at the same time, gripping the top tube very firmly between your knees. I've never experienced a full death wobble, but OTOH this is how I always descend, positioning myself like this before the bike picks up speed. The knee/top tube thing is more effective with a horizontal top tube. It's also a good idea not to grip the bars tightly, though that seems rather silly, as the tight grip will increase the vibration, not dampen it. I like to descend like a skier, back flat, weight forward, taking the bumps in my knees.
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Old 07-23-17, 04:05 AM
  #23  
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This wasn't an emergency situation and it wasn't the death wobble. I think Joe's post above was the closest when he talked about 'weave' and those who've suggested it's probably due to the weight of the pannier.
And it's going to be wet this week so I'm unlikely to be testing it.
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Old 08-12-17, 09:22 PM
  #24  
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Time to revisit this thread seeing the weather has finally improved enough for me to have some more answers ie, getting enough time to ride the thing while thinking about the steering.

I'm not dealing with the front end shaking, I'm talking about the bike veering to one side slightly such as from a slight steering input.

While not ignoring the possibility of some mechanical issue, I'm now inclined to think this is a case of the wrong bloke on the wrong bike.

I haven't had the head stem bearings checked out yet because my tame mechanic has been overseas and that's one area where I'm not completely confident in my own abilities to get absolutely right however, as you'll read in a moment, I think it's all me.

Riding without the panniers makes no difference, so it's not a weight or aero issue at the back end. Whew, I hate back packs and with the Aussie summer coming on, even more so.

Checking the bike over hasn't unearthed anything mechanical.

I am running new tyres that have a pronounced feather of rubber down the middle from the moulding process. There is a tiny thought that this might be affecting the steering though it's more grasping at straws. This will wear off fairly quickly (not so quickly on the front) and maybe I'll notice an improvement as that happens. Be funny if it did fix it. I could get aggressive and remove it but aren't that fussed to be honest.

Now, me, the human that rides this bike.
I actually do have a balance problem due to a medical condition. It's not major, it's not frightening, it doesn't make me unsafe on a bike, but it is enough to make me wary about riding one handed so signalling turns or pointing out potholes is actually an issue for me, which is why I don't ride in groups. With this bike so sensitive to steering input, I think I'm probably just making the thing jink around myself. Not badly, but enough to be unnerving when I first encountered it. My previous bikes have all had relatively stable steering which is probably why I haven't noticed it before.

So for the moment, I'm going with the thought that while it's nice having a very racy bike, it's not for me... and I'm glad I worked this out with a cheap, second hand bike rather than getting all glossy eyed with a new, full carbon job and finding it hard to ride. Needless to say, my next bike will be less aggressive.

Thanks for all your input.
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Old 08-13-17, 09:41 PM
  #25  
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So the first question I have was already sort of mentioned AnkleWork- What is the issue the OP is having? Is it a single veer off line, one movement without a repeated cycle of side to side movement? This is usually an alignment issue with frame/fork/wheels or body/load. Or is the problem a wobble? A cycle of rapid (often a few times a second) side to side swings of the front wheel, what we call speedman's wobble back in the day (shimmy is a more descriptive term).


While most of the advise given has merit WizardofBoz said it best in that shimmy is not understood by most and most bikes can suffer shimmy is certain situations. The best current view is that a harmonic flexing is happening, the front wheel's oscillation feeds on it's self and grows with increasing cycles. Breaking up the harmony is what one is doing by changing the load paths of the rider on the bike and/or changing the damping aspects of the rider/bike system. Changing the wheel weight, spoke tensions, load/rack stiffness or location, rider position, aero drag, speed, steering leverage (bar width) and more are all valid paths to try.


What's really hard to do though is to do this trial and error in a scientific method of only one change at a time, and the ability to revert back to the original set up exactly to try the next single change. just the act of riding down a hill then going back up to the top to re ride the hill (on the same day, same set up, same rider) will not really be the same conditions. Rider manor can have a big effect. (This is why it can be impossible for a shop or a friend to duplicate the conditions exactly and therefore duplicate the shimmy).


An example of this difficulty in duplicating conditions or how aspects that most won't consider do mater is I have bikes shimmy when I have been cold and been shivering. Only a few miles before or after if my shivering changed (as in I warmed up) the shimmy was gone. Now how does one ask others to duplicate that?


Both possible issues have been covered here in this forum (and many other forums) many times. I do suggest that those who are interested spend some time searching this forum's archives. One will find many threads and replies, some will even be helpful in gaining insight in what is not completely understood even by engineers and bike designers. Andy.
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