Stability

silva

Active Member
Location
Belgium
My current bicycle frames geometry is in some way different from my previous.
When I first tried it, barebone (the bike not me), after acquirement, it felt very "nervous", it was hard to ride straight / control it. To the point that it felt unsafe.
I first thought the tire (27.5", 62-584) pressure was at 2.5 bar too low (it was, for my weight) but an increase to 3 bar and beyond didn't make it better.
I spend a week riding it like that, and the last day felt as uncomfortable as the first.
Then, I started to fit the bike to my needs, first double bags hanging over the rear rack. Again a week later, a basket on top of the rear rack. Shortly after, I had no free space avail in the bags, and I put about 3 kilo luggage in the basket.
Big surprise: the "nervous" feeling was all of sudden completely gone from that very moment (leaving a market).
Flashforward 2 years to present day, couple days I discovered several bolts of the rear rack being loosened / lost. I had to completely dismount my luggage gear in order to reach some bolt locations. In meantime, before putting it all back, I made a ride, didn't think of it, but it reminded me: that "nervous" bike response was back in all its glory.
And gain gone after refitting.

Clearly, the bikes geometry must have a flaw somewhere. But it's hard to pinpoint.
Since filled bags under the rear rack didn't make a difference, but filled basket above does, it seems like vertical weight center related.
Ex a boat with a high sitting weight center, rolls easier, and is more stable with a lower weight center. But this bikes case is the opposite.
I always have a couple backpacks, a couple waterproof bags and an aluminium mountain trek frame mounted above my rear rack, so this is an everyday solution to the problem.
But what can be wrong with the geometry to cause this?
 
Maybe not geometry but tighter/looser headset bearings ?
 
OP
silva

silva

Active Member
Location
Belgium
Maybe not geometry but tighter/looser headset bearings ?
But then the hard control should be regardless luggage on top of the rear rack, no?
Can you go to your local bike shop, pretending to buy a bike and take a test ride? Could be that you are so used to having a loaded bike, that makes a light bike very sensitive.
I still possess my 2 previously used bikes, and when I ride those loaded or unloaded, I don't feel a control responsivity difference, it's just easier to achieve speed that's all.
A pic of the offending bike would be useful plus your height.
It's the bike in my signature pic, I'm 1.93m.
The bike is a "travel bike", a brand named "santos", a model named "travelmaster3+".

A difference between older and new bike is a much bigger tire width, which I inititially also suspected as a cause for that harder controlling, until I put some luggage ON TOP of the rack, and it was gone just like that.
It really feels unsafe without that weight there, and I have then a hard time evading the crap that government here littered the roads with. That first time I stopped several times to check what was wrong with the bike, even thought a wheel nut had loosened.
I found the solution, but I'd like to know the specific cause.

What Sharky said (used to have a loaded bike) is what I also saw as most plausible answer, a weight high mounted on the bike gives more inertia when pushing it left/right, it like "dampens" the effects of pushing. But a previously used bike without any luggage on top, does not give that feeling. And in the past, with those older bicycles, I didn't have that standard outfit on top of the rack, I just had that double bag, just only rarely something mounted there.

To add another difference between previous bikes (4) and current bike: the frame size of the current is bigger. Which was a major reason to buy another bike - tired of knees hitting handlebars, feet hitting front wheels fender when turning, heels hitting bags and the makeshift solutions that posed other problems on themselves. Also, the height of the current bikes horizontal frame tube is quite high - it's harder to get leg over it, which is to take into account when you have a heavily loaded bike that you have to hold up when putting leg over. The size of the frame could have been chosen bigger if that tube had been designed somewhat lower.
 
Last edited:

Cycleops

Guru
Location
Accra, Ghana
From what I can see of your bike there is no reason for it to be at all unstable. Looks like a long wheelbase and slack frame angles so there's nothing in its geometry that would would make it inherently unstable. So it must be something else like @DaveReading suggested. A short stem might adding to the feeling.
Hold on the front brake and push the bars back and forth. Any looseness?
 

Shadow121

Well-Known Member
The bike is designed to carry a load, take that load off and it
will be twitchy if it is designed to be light handling with the load on, which it is.
Also, a load or some of that load hung below the axle will cancel
out the bikes ability to remain easy to handle, twitch will return.

You have the sweet spot, problem solved.
 
OP
silva

silva

Active Member
Location
Belgium
The bike is designed to carry a load, take that load off and it
will be twitchy if it is designed to be light handling with the load on, which it is.
Also, a load or some of that load hung below the axle will cancel
out the bikes ability to remain easy to handle, twitch will return.

You have the sweet spot, problem solved.
My previous bikes were not designed to carry a load (no travel bikes, just common hybrids costing a quarter of the price) and there I didn't have any control difference load or no load on top of the rack.
 

Shadow121

Well-Known Member
My previous bikes were not designed to carry a load (no travel bikes, just common hybrids costing a quarter of the price) and there I didn't have any control difference load or no load on top of the rack.
That would be right, they were not designed like your travel bike, and would
not react the same, they would get no twitcher. If you design something
for a specific job, then it will often need to be used in a certain way, like
you found with your traveling bike.

Put a level load of whatever in a car trailer, you won’t have a problem,
remove the back half of the load, this shifts much more weight to the drawbar,
and causes the car to become very twitchy, same with putting weight below the
axle on a bike designed to be stable with weight above the axle, above the axle is the only
way to get more on your bike, that’s where the available space is, is it too much to understand
that given this a designer would design the bike to handle better in this configuration, now imagine
if the designer did not optimize for this high up load, well you would be fighting your bike
big time when loaded up in the vertical plain.

When you turn the front wheel loaded you are not fighting your bike, what else would you expect.
 

Tigerbiten

Veteran
If it was a new bike then I'd say a twitchy and almost unrideable bike which should be stable maybe a sign that the forks could be fitted backwards.
This shortens the wheelbase and changes the bikes trail causing it to be much more twitchy.
But you're had it two years and would have seen if this was the case.

I do know about some touring bikes which do handle better when fully loaded vs unloaded.

Also you do have to relearn how to ride with a twitchy bike if you've never ridden one.
The trouble is you get into a negative feedback loop where because because everything reacts/happens faster than you expect you feel unsafe and tense up. The more you tense up then the more you fight your own muscle and the slower you can react. The slower you react the more unsafe you feel which causes you to tense up more ...........
That effect is also very common when learning to ride trikes and recumbent bikes when things happen which you don't expect.

Luck ......... ^_^
 
OP
silva

silva

Active Member
Location
Belgium
That would be right, they were not designed like your travel bike, and would
not react the same, they would get no twitcher. If you design something
for a specific job, then it will often need to be used in a certain way, like
you found with your traveling bike.

Put a level load of whatever in a car trailer, you won’t have a problem,
remove the back half of the load, this shifts much more weight to the drawbar,
and causes the car to become very twitchy, same with putting weight below the
axle on a bike designed to be stable with weight above the axle, above the axle is the only
way to get more on your bike, that’s where the available space is, is it too much to understand
that given this a designer would design the bike to handle better in this configuration, now imagine
if the designer did not optimize for this high up load, well you would be fighting your bike
big time when loaded up in the vertical plain.

When you turn the front wheel loaded you are not fighting your bike, what else would you expect.
I define a good design as one that does a good trade-off between benefits and drawbacks.
Having to put some kilo's on top of a rear rack in order to ride safely, I don't exactly consider that a good design.
Since I got the bike, I discovered (the hard way) quite some flaws in the entirety of the design.
Which have had small upto big consequences.
Some examples:
- the rear tire clearance is over 20 mm but there's no room for an 48T chainring without having to shift the rear hubs flanges 5 mm offcenter (causing need to adjust the wheels spokes to correct the tires line), and on top of that 5 mm spacers, a total of 1 cm. Sacrifying a mere 5 mm tire clearance would allow to fit nearly any common chainring size.

- the rear rack, nicked "XXL" supported my double bags even less than on my older bike, causing same problems - they touch the tire and get damaged, can't place them far enough away from my heels, rear light sits in a pothole, and so on.
when asking why named "XXL" if it's that small the answer was that the XXL referred to the strength not the size...
I ended up having to attach dedicated triangle shaped frames to make the rack longer / hold the bags away from the wheel, and also had to move the rear light too, mounted on a equally makeshift solution.
And even now, 2 years post purchase, I keep discovering the hard way design flaws: that rear rack is attached to the seat tube along two arms mounted to the rack side along two teflon nuts (adjustable distance), with the nuts on the INSIDE facing eachother, impossible to reach without having to completely disattach the entire rack but then you don't know the position to fix them, apparently a problem that the assembler stuck into too, since the inability to adjust and tension together, prevented a tight fix, resulting in loose bolts, that ended up somewhere on the roads I've passed, being the reason for my attention and subsequent discovery.
Last week I spent after work an entire evening upto 12 o'clock night, to disassemble my rear rack extensions in order to find out why the arms to the seat tube hung loose, I replaced the nuts with butterfly ones (no other solution since no room for a tool, even not for one click/step of a precision ratchet and even not with a long extender from the other side) and locked them with silicone. I replaced the two hexagon head bolts (one lost) with new ones, and since the blocks that lock them behind the L profile of the rack turnt out to be aluminium (travelrack XXL is advertised as steel for strength - but apparently not entirely...) I also chosed non stainless, to prevent galvanic corrosion, and sticked some water protection around them

- rear light has no switch, meaning that when I park it at a shop it draws attention to my bike for like 10 minutes without any benefit, probably just to save the cost of a switch despite the small thing costing 50 bucks.

- took a month riding to discover bottom bracket axle wiggling causing chainring to scratch the frame, reason turning out as the mount requiring Loctite to not dismount itself (which is a sign of a bad... design).

- they choosed stainless steel bolts (304 grade) to mount in aluminium, causing galvanic corrosion, after having to re-tension the bikes stand numerous times, I decided to unmount to see what was going on: full with white aluminium powder - I needed to replace the ss bolts with not stainless steel bolts to avoid further damage to stand and frame thread.

- the horizontal tube sits way too high, apparently they received enough complaints to design a new frame with a lower one.

- the brake fluid line between handlebar and rear brake cilinders was mounted under that high horizontal tube, requiring a sharp turn under the handlebars, causing it to break and leak. I had to let it change to the tube towards bottom-bracket then back up - since then no problem anymore (I also mounted a protective tube through which the fluid line was directed that entire path)

- even the pedals had a flaw: to give grip, they drilled holes and screwed protruding headless bolts (so inside the threaded cilinder, with an allen hexagon insert), allowing to adjust. But the hexagons sit on the wrong side and get worn off by the shoes, preventing any adjustment, resulting in shoe soles ruined.

And this was just a part of the troubles, I didn't mention the most important (for me) drivetrain related ones.
So, when I'm asking here with this topic about that instability with an empty rear rack, I do have some reasons to suspect a design flaw, see it would just be a next in the list.
And why? Well, to solve them ofcourse.
See above... ALL solved. To hunt a solution, one has to hunt the problem cause first.

SO, you say here that the instability without a mass on the rear rack may be an intentionally accepted trade-off in the design. Can you be more specific, what parts of the frame, their dimensions (length ex) are determinating this?
If the frames sizes were equal then I could make some measurements on my previous bikes and compare these with measurements on this bike.
But I don't even know what frame parts get longer with bigger frames.

.
 
Last edited:
OP
silva

silva

Active Member
Location
Belgium
If it was a new bike then I'd say a twitchy and almost unrideable bike which should be stable maybe a sign that the forks could be fitted backwards.
This shortens the wheelbase and changes the bikes trail causing it to be much more twitchy.
But you're had it two years and would have seen if this was the case.
Okay but the bikes trail doesn't change by laying a bag potatoes on the rear rack eh ;) ?

I do know about some touring bikes which do handle better when fully loaded vs unloaded.

Also you do have to relearn how to ride with a twitchy bike if you've never ridden one.
The trouble is you get into a negative feedback loop where because because everything reacts/happens faster than you expect you feel unsafe and tense up. The more you tense up then the more you fight your own muscle and the slower you can react. The slower you react the more unsafe you feel which causes you to tense up more ...........
That effect is also very common when learning to ride trikes and recumbent bikes when things happen which you don't expect.

Luck ......... ^_^
I rather prefer to solve causes than trying to live with consequences.
Also, in this case the consequence comes without and goes with a bag potatoes, and last time I checked I didn't see a switch on a muscle for it.
Fact is that the design of my other older now first spare bicycle must have been a better trade off, since I did the same things with it as with the current newer one, for years and twitchy behaviour ment a broken rack, spoke, brake or stick between the spokes slamming the fork, not a chosed design.
Luck.... is for sheep that hope.
The world isn't built by hoping ppl.
There, some psycho talk on sunday. :biggrin:
 

Shadow121

Well-Known Member
I define a good design as one that does a good trade-off between benefits and drawbacks.
Having to put some kilo's on top of a rear rack in order to ride safely, I don't exactly consider that a good design.
Since I got the bike, I discovered (the hard way) quite some flaws in the entirety of the design.
Which have had small upto big consequences.
Some examples:
- the rear tire clearance is over 20 mm but there's no room for an 48T chainring without having to shift the rear hubs flanges 5 mm offcenter (causing need to adjust the wheels spokes to correct the tires line), and on top of that 5 mm spacers, a total of 1 cm. Sacrifying a mere 5 mm tire clearance would allow to fit nearly any common chainring size.

- the rear rack, nicked "XXL" supported my double bags even less than on my older bike, causing same problems - they touch the tire and get damaged, can't place them far enough away from my heels, rear light sits in a pothole, and so on.
when asking why named "XXL" if it's that small the answer was that the XXL referred to the strength not the size...
I ended up having to attach dedicated triangle shaped frames to make the rack longer / hold the bags away from the wheel, and also had to move the rear light too, mounted on a equally makeshift solution.
And even now, 2 years post purchase, I keep discovering the hard way design flaws: that rear rack is attached to the seat tube along two arms mounted to the rack side along two teflon nuts (adjustable distance), with the nuts on the INSIDE facing eachother, impossible to reach without having to completely disattach the entire rack but then you don't know the position to fix them, apparently a problem that the assembler stuck into too, since the inability to adjust and tension together, prevented a tight fix, resulting in loose bolts, that ended up somewhere on the roads I've passed, being the reason for my attention and subsequent discovery.
Last week I spent after work an entire evening upto 12 o'clock night, to disassemble my rear rack extensions in order to find out why the arms to the seat tube hung loose, I replaced the nuts with butterfly ones (no other solution since no room for a tool, even not for one click/step of a precision ratchet and even not with a long extender from the other side) and locked them with silicone. I replaced the two hexagon head bolts (one lost) with new ones, and since the blocks that lock them behind the L profile of the rack turnt out to be aluminium (travelrack XXL is advertised as steel for strength - but apparently not entirely...) I also chosed non stainless, to prevent galvanic corrosion, and sticked some water protection around them

- rear light has no switch, meaning that when I park it at a shop it draws attention to my bike for like 10 minutes without any benefit, probably just to save the cost of a switch despite the small thing costing 50 bucks.

- took a month riding to discover bottom bracket axle wiggling causing chainring to scratch the frame, reason turning out as the mount requiring Loctite to not dismount itself (which is a sign of a bad... design).

- they choosed stainless steel bolts (304 grade) to mount in aluminium, causing galvanic corrosion, after having to re-tension the bikes stand numerous times, I decided to unmount to see what was going on: full with white aluminium powder - I needed to replace the ss bolts with not stainless steel bolts to avoid further damage to stand and frame thread.

- the horizontal tube sits way too high, apparently they received enough complaints to design a new frame with a lower one.

- the brake fluid line between handlebar and rear brake cilinders was mounted under that high horizontal tube, requiring a sharp turn under the handlebars, causing it to break and leak. I had to let it change to the tube towards bottom-bracket then back up - since then no problem anymore (I also mounted a protective tube through which the fluid line was directed that entire path)

- even the pedals had a flaw: to give grip, they drilled holes and screwed protruding headless bolts (so inside the threaded cilinder, with an allen hexagon insert), allowing to adjust. But the hexagons sit on the wrong side and get worn off by the shoes, preventing any adjustment, resulting in shoe soles ruined.

And this was just a part of the troubles, I didn't mention the most important (for me) drivetrain related ones.
So, when I'm asking here with this topic about that instability with an empty rear rack, I do have some reasons to suspect a design flaw, see it would just be a next in the list.
And why? Well, to solve them ofcourse.
See above... ALL solved. To hunt a solution, one has to hunt the problem cause first.

SO, you say here that the instability without a mass on the rear rack may be an intentionally accepted trade-off in the design. Can you be more specific, what parts of the frame, their dimensions (length ex) are determinating this?
If the frames sizes were equal then I could make some measurements on my previous bikes and compare these with measurements on this bike.
But I don't even know what frame parts get longer with bigger frames.

.
I agree about getting a good balance, not every designer will design a bike to cope
well with the kilos piled high on the back, but you can load higher if the need arises and still manage the bike.
As being more specific, no, it is what it is, If I wanted to compare the bike with another based on measurements,
I would need a dam good reason, there’s also too many variables regarding load dispersion and rider position
and ability for a normal person to invest in trying to paint a crystal clear picture.
 

Tigerbiten

Veteran
Okay but the bikes trail doesn't change by laying a bag potatoes on the rear rack eh ;) ?
But it does change the bikes moment of inertia.

It's like trying to balance a pencil on your hand.
It's difficult with a plain pencil because it moves fast and twitchy and once started over it's hard to correct.
Now attach a weight to the top of the pencil and it become easy because it's feels a lot more stable.

That what has happened on the bike.
The sack of potatoes acts in part as a mass damper and has slowed any fast side to side oscillations into a range you find easy to control.
The higher the weight is sited on the bike then the more the moment of inertia changes and the more you feel the effect.
That's why it works well on top of the rack and not so much in side panniers.

How twitchy a bike is an interaction between it wheelbase, headset angle, trail and even the bottom bracket height.
As you've two bikes, one stable and one twitchy, then measure all the distances and angles on both.
The draw a scale diagram of both from the same starting point and you'll easily see how and where they differ.
 
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