I’ve never had a bike like a Brompton before, I’ve noticed that when you close the clamp on the seatpost, part of the frame bends to hold the seatpost in place. Would this Not eventually fatigue that part of the frame?
Hopefully this isn’t a stupid question!
Thanks
by Tidings_Bearer
9 Comments
Not a stupid question. This is very unlikely to fail. Below a certain stress threshold steel can take basically infinite stress cycles, and I imagine Brompton did the math. My Brompton is 12 years old with no issues, and I’ve never heard of a Brompton failing there.
See [Fatigue Limit:](https://en.wikipedia.org/wiki/Fatigue_limit)
“The fatigue limit or endurance limit is the stress level below which an infinite number of loading cycles can be applied to a material without causing fatigue failure”
Fatigue can be a future possibility, if the clamp adjustment is made very tight, including tightening the clamp instead of replacing a worn plastic sleeve. For small adjustments of tightening the seat post clamp, this data sheet web down load has details about the seat post clamp. Brompton recommends tightening the seat post clamp nut, when the clamp lever is locked down, 1/8 to 1/6 turn, to reduce risk of over tightening. (Or torqued within the range 4-7Nm.)
https://www.vlerickfietsen.be/download-asset/1c72a742-090b-11ea-8046-005056985c04
In general, steel alloys, such as the Brompton frame tubing, have a fatigue strength, which if never exceeded, the steel will last forever. But if steel is flexed past its fatigue strength, then the structural piece will have a limited life.
In the case of the Brompton seat tube, avoiding over flexing of the split seat tube, via the clamping action, will give good longevity of the frame’s seat tube.
If the seat post is slipping :
Brompton Steel Seat Post
The seat post sliding (slipping) can often be a result of the post and sleeve having residual manufacturing release chemicals, such as mold release compound, human hand oils, or other small amounts of oily chemical residues, including hand oils from grabbing the seat post. Wipe the entire seat post and the black plastic sleeve inside the frame with 91% isopropyl alcohol soaked paper towel or clean rag. For complete access to the frame sleeve, the seat post must be removed by removing the Pentaclip seat clamp from the seat post so that the seat post can be lowered/removed from the sleeve. (Keep the alcohol from wetting or getting on the frame paint, because the paint can dissolve or become damaged from the alcohol.)
https://trade.brompton.com/Uploads/QPart/QPart-Datasheets/ds-pentaclip.pdf
I have had my acoustic for 12 years. The frame is unchanged.
Pro bike mechanic here. I’ve seen them crack in the past, but it’s always when the plastic seat sleeve inner has worn and the rider has kept tightening the nut to stop the post from slipping. I can’t recall one failing in normal use.
What happens is that dirt and grit from the rear wheel gets on to the seatpost and as the post is lowered and raised the grit/dirt slowly wears away at the plastic sleeve over time. People tighten the nut to take up the wear and this stresses the clamp area.
Just pay attention to seat sleeve wear. It’s cheap and easy to replace (the part is around £5) and should be replaced as necessary.
God answers here, fatigue point of steel starts to show when maximum load exceeds around ⅓ of maximum strength. Just think about springs , they flex a little and don’t break , used to do design work and used high quality springs , you have a chart which shows the % of the length reduced in compression, keep it below a certain threshold and the life is in millions of cycles , exceed other thresholds and the expected life decreases to hundreds of thousands and then then 10’ s of thousands of, same principle as the bike clamp , just right enough and it’ll last forever.
Aluminium for example has no such safety point , flex it and it will always fail eventually…
Rest assured that after twenty-five years, a minimum of 2,000 miles a year, and six folds/unfolds a day, it hasn’t failed.
Just appreciated that I’ve done far more than 50,000 miles, and folded/unfolded at least 35,000 times.
Had to check my fold/unfold maths.
Thank you all! These replies have been exceedingly helpful and reassuring – I’ve definitely made the right choice for a bike!
This reamer specification matches to the Brompton reamer.
https://www.ebay.com/itm/351502316470?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=xdI85ULqQu6&sssrc=4429486&ssuid=s6Qu69mSSv-&var=&widget_ver=artemis&media=EMAIL
Can confirm, I have cracked my frame there because a mechanic in the past didn’t do too great a job fitting a new seatpost shim and I overtightened the whole thing to compensate. Lots and lots of little cracks formed. Thankfully, I took my bike to Brilliant Bikes and they spotted this and have sent up the frame to Kinetics and they’ve just repaired and reinforced the area. I am very relieved! Please never overtighten that bolt and listen to mechanics if they pull you up on it- I wish I had but I’m glad there are experts that can sort these problems