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I have a saddle somewhat similar to this one. One bolt holding saddle clamp to the seatpost.

enter image description here

Some half a year ago I was riding with no hands and leaned back a bit. Suddenly, I fell down the bike on my back yet later to realize that the bolt simply broke, its head unattached from the threaded shaft. Luckily, I didn't have any serious injury, and neither did bike.

So I replaced the bolt.

Today, same thing happened. But I wasn't even riding hands free, I had one hand on, and was talking on the mobile phone with other. This time I managed to stay on bike and discovered again that the head of the bolt unattached from its shaft.

Now, maybe I put the saddle to far back so I had extra large lever and too much pressure on the screw. However, the flat part of the metal construction is a lot shorter on my saddle than the one here in the picture and I really don't believe I can put it that far even if I want.

Maybe I over tightened the bolt, there was no indication on how much pressure to use, but I was careful, at least the second time.

Do you know what might be the cause for this? Is it possible to over tighten the bolt? Is there something else that might cause this?

PS. Not that it's your business but I weigh 90 kilograms (199 pounds)

PPS. Please save me from the comments "don't ride no hands" and "don't talk on phone while riding". I realize this is somewhat dangerous and I know it increases pressure on the bolt, I wanted to know is there some other thing that might have caused this that didn't occur to me.

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    Talking on the phone doesn't increase pressure on the screw. But it's still a stupid thing to do. Please don't.
    – David Richerby
    Jul 23, 2019 at 23:01
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    I'll give this a +1 even though you admitted to riding no hands and talking on the phone. :) When you replaced the screw, what did you use/where from. Fixings come with various tensile ratings, was it high tensile?
    – mattnz
    Jul 24, 2019 at 1:38
  • You likely have the seat set pretty far back, placing more strain than normal on the bolt. But a good quality bolt shouldn't snap. It's vaguely possible that the bolt is being stressed by being too loose or somehow bumping up against the surrounding bits. But try to find a better quality bolt.
    – Daniel R Hicks
    Jul 24, 2019 at 2:45
  • It is definitely possible to overtighten that bolt. I had a co-worker who broke 2 saddle bolts because his torque meter wasn't reading properly.
    – DavidW
    Jul 24, 2019 at 12:43
  • The same thing happened to me. I replaced it with a stainless steel bolt.
    – SurpriseDog
    Jul 27, 2019 at 2:39

1 Answer 1

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You need to use high tensile bolts for high tensile positions like this.

A class 8.8 bolt (metric) or grade 8 (US) or higher is appropriate. Cheese-grade steel from the local discount hardware store is not up to the task.

https://www.boltdepot.com/Images/charts/hex-head-marking-grade8.gif US Grade 8 head marking. These have a min yield strength of 130,000 PSI. (900 MPa)

https://www.boltdepot.com/Images/charts/hex-head-marking-class8-8.gif Metric Class 8.8 head marking, with a min yield strength of 640 MPa (93,000 PSI)

Your second option is to replace the seatpost with one that has two bolts for fixing, This will decrease the flex possible on one bolt.

https://cdn.road.cc/sites/default/files/styles/main_width/public/how-fit-and-set-your-saddle-18.jpg One style of two-bolt saddle clamp, but there are multiple other designs. They're all integrated with seatpost so its got to be exactly the same diameter as the seatpost you're replacing.

Underlying - your monthly maintenance check on your bike should have tested the saddle wobbliness and noticed any increase in play. At that time you'd retighten the bolt to torque. If it was over-torqued a low-grade bolt could have stretched and deformed, leading to eventual failure.

Finally - be a responsible road user and put the phone away. Ride with your hands on the bars. If not for your own safety then for others on the road with you.

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    Torque is the all-important word in @criggie 's answer. +1
    – Carel
    Jul 24, 2019 at 10:22
  • Putting the phone away is also pretty important! :)
    – DavidW
    Jul 24, 2019 at 12:45
  • 130,000 PSI works out to 201 pounds per square mm. Assuming the bolt is 8mm across that's over 10,000 pounds of tensile strength. Makes me wonder how weak the old bolt must have been!
    – SurpriseDog
    Jul 27, 2019 at 2:45
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    @Benjamin I'd suspect the replacement bolt was some 50c hardware store bolt that happened to have the right thread. Low-cost steel isn't known for high load-carrying numbers, hence "chinese cheese steel"
    – Criggie ♦
    Jul 27, 2019 at 5:18

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