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65 votes
Accepted

What is the purpose of the top tube on bicycles?

Same basic purpose as the roof on a unibody car. To prevent the back end of the bike folding towards the front end when vertical forces like your brother are applied to the cranks or seat. Very ...
Warren Burton's user avatar
18 votes

What is the purpose of the top tube on bicycles?

I don't have an engineering background, but I believe the top tube makes the bike stiffer laterally and torsionally (i.e. resisting twisting movements). You could build a bike without a top tube. Many ...
Weiwen Ng's user avatar
  • 32.7k
11 votes

Rear light with failure warning

The problem is that you have to start considering the failure modes of the failure detection system. You're quite likely to end up with a less robust system or a lot of false alarms if you have a ...
Chris H's user avatar
  • 59.9k
10 votes

Rear derailleur got caught in the spokes, what could be a root cause

If the low limiter screw on the rear derailleur was very badly adjusted, it would be possible to move the derailleur far too far past the end of the cassette by shifting into the lowest gear (i.e. ...
Diado's user avatar
  • 691
9 votes

Can a road bike stand constant 40km/h for long distances?

Stopping distance is definitely something to be aware of. As speed increases, the distance needed to stop increases non-linearly. For illustration, in cars, with 1 sec reaction time: 20 km/h = 8m 30 ...
IKY.LI's user avatar
  • 421
8 votes

How to tell when to change steel front fork?

Forks are not a 'wear component' that need to be replaced periodically. Bicycle frame makes are clever enough to make their products sufficiently durable and it's not something riders generally need ...
Argenti Apparatus's user avatar
8 votes

Rear derailleur got caught in the spokes, what could be a root cause

(in addition to Diado's answer) I would suspect a bent derailleur hanger. Usually the derailleur cage should be perpendicular to the wheel's axis. When the derailleur hanger is bent (it's said that ...
pseyfert's user avatar
  • 1,023
6 votes
Accepted

Seat post bolt sheared off - twice

Any idea why the bolt fails? Bottom line: I believe the bolt fails because the seat post clamping mechanism design causes fatigue in the bolt. ASTM E1823-10a, Terminology Relating to Fatigue and ...
David D's user avatar
  • 21.9k
6 votes
Accepted

Which kind of test has bike parts to pass to be sold?

There are quite a number of standards that should be adhered to. The list below comes from the references section of the Bicycle page on Wikipedia The TC149 ISO bicycle committee, including the ...
Andy P's user avatar
  • 18.3k
5 votes

Fork Failure on Vintage Steel Tandem Bikes - Any insights?

I'm not saying I don't believe them, but surely they have legal and financial incentives to recommend I err on the safest side. Yes, but now that you've asked them and they've instructed you to buy ...
Adam Rice's user avatar
  • 30.1k
4 votes

Coaster brake brakes itself

Are you sure that the friction comes from the hub, not from tyre rubbing against frame? Axle mounting point looks as it is not tightened properly. Second thing to check is to assure that hub is not ...
krzyski's user avatar
  • 2,721
4 votes

Can a road bike stand constant 40km/h for long distances?

Yes - the bike can stand higher speeds fine. Generally. wear is caused by distance travelled, not the speed it travels. You might suffer increased wear through increased vibration but this would be ...
Criggie's user avatar
  • 126k
4 votes

Seat post bolt sheared off - twice

It looks like the way you positioned the seat and the seatpost clamp bolt are both working hard against the bolt's metal fatigue rating. You selected the farthest back hole in the seatpost and also ...
karasugoi's user avatar
4 votes

Can pumps fail due to age?

Can a pump that is not being actively used fail? On the one hand - nothing lasts forever. On the other hand - you don't need your cache of pumps to last forever. You only need your cache to last: ...
David D's user avatar
  • 21.9k
3 votes

What Causes Tubeless Road Tires to Blister?

Manufacturing defect. The bond between the casing and tread has broken down, air is leaking though the casing and stuck under the outer lay. I would look at taking back to place of purchase and see if ...
mattnz's user avatar
  • 51k
3 votes

Seat post bolt sheared off - twice

I have had several seat clamp bolts break on my BMC Roadmachine. The clamp does not bed well on the seatpost when the bolts are tightened to spec. They use 5mm bolts which are not strong enough. TO ...
d kurtz's user avatar
  • 31
3 votes

Rear derailleur got caught in the spokes, what could be a root cause

As for repairs... Axle - you're up for a new axle. Check the condition of the cone nut and their bearing surfaces - they may be okay but any dents or pits and they're better-off being replaced. ...
Criggie's user avatar
  • 126k
3 votes

Rear light with failure warning

Most of the Busch & Müller IQ Dynamo lights seem to have this feature. Their lights have a well designed beam pattern, si it's hard to tell whether they're on or off during the daytime. So they ...
Nuі's user avatar
  • 1,213
3 votes
Accepted

Why did my rear derailleur fail in the middle sprocket?

From the information available I would advise you to make sure: Chain is of correct length Derailleur is not running too close to the cassette (B-screw adjustment) In the workstand pay close ...
Olof Sivertsson's user avatar
3 votes

Which kind of test has bike parts to pass to be sold?

There is a growing neurosis over carbon fiber bikes it seems. This article describes this trend and the developing legal battles over liability. Source is long-published magazine, Outside, and there'...
Jeff's user avatar
  • 10.8k
3 votes

Failure modes for suspension forks, air and coil

The air spring operates on the inside of the stanchion tubes, so is not affected by the scratches you can see on the out side surfaces. However the damage to the outside of the stanchions will allow ...
mattnz's user avatar
  • 51k
3 votes

Failure modes for suspension forks, air and coil

Basically the air and oil seals get worn and leaky. You'll get less air spring pressure, droop and increasingly poor damping. Of your options, 2 and 3 are not realistic. If the air spring pressure ...
Argenti Apparatus's user avatar
2 votes
Accepted

How to tell when to change steel front fork?

My experience is: Any bike part can, and will fail under heavy use. This includes frames, forks, stems, seat-posts, handle bars, cranks, you name it. I have broken at least one of all these in my life ...
cmaster - reinstate monica's user avatar
2 votes
Accepted

Fork Failure on Vintage Steel Tandem Bikes - Any insights?

Based on my (limited) understanding of metallurgy, steel does not weaken over time. The 2 factors which contribute to failure of steel are the environment and fatigue. To determine the extent of ...
sam's user avatar
  • 759
2 votes
Accepted

What Causes Tubeless Road Tires to Blister?

This problem was not uncommon when tubeless was new, but this is the first case i've seen in a modern tyre. In the early days this was thought to be due to Stans sealant containing ammonia and some ...
Andy P's user avatar
  • 18.3k
2 votes
Accepted

Shimano 105 front derailleur pivot broke : what am I doing wrong?

In general, it is possible that you did nothing wrong and the derailleur broke. I bet that bike equipment in general has been getting more reliable over time, as processes and materials have improved. ...
Weiwen Ng's user avatar
  • 32.7k
2 votes

Shimano 105 front derailleur pivot broke : what am I doing wrong?

I've also never seen that, but you mention "steep climb" Is it possible that you shifted the front derailleur while pushing hard on the pedals ? Essentially shifting under load so the chain ...
Criggie's user avatar
  • 126k
1 vote

Shimano 105 front derailleur pivot broke : what am I doing wrong?

I’ve never seen or heard of this happening. But only yesterday I was looking at my bike (while sitting in the train) and marveling at how thin and fragile those front derailleur linkages look ;) I can’...
Michael's user avatar
  • 28k
1 vote

Can a road bike stand constant 40km/h for long distances?

As you may know kinetic energy is ½mv² where m is mass, v is velocity. You have more mass on the bike with the addition of motor and batteries. A bigger concern is that energy increases with velocity ...
Argenti Apparatus's user avatar
1 vote

Rear light with failure warning

There is an expensive but modern approach to this problem: Smart lights that pair with GPS head units - connected devices usually report the battery status to the computer and you get a warning when ...
DoNuT's user avatar
  • 4,496

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