Quad Pedal Car - 2025 late Xmas/early Birthday present ?

I can not possibly venture any analysis, except that the threads on that bolt look stretched - Like it was pulled apart.
Maybe they just look like that from being twisted? That must be it - I can't see the forces involved pulling like that, until failure.
If it was caused by it unscrewing, would really strong thread locker fluid have prevented it from happening?
 
undertoad said:
I can not possibly venture any analysis, except that the threads on that bolt look stretched - Like it was pulled apart.
Maybe they just look like that from being twisted? That must be it - I can't see the forces involved pulling like that, until failure.
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All possible , but the why still exists.
Understanding this would be helpful to DannyC as he has built several 2WD units NONE of which have been proven in service.
It is possible his could also fail in an unexpected way if we don't actually understand what is happening to my unit ?
undertoad said:
If it was caused by it unscrewing, would really strong thread locker fluid have prevented it from happening?
Click to expand...
Most probably , it needs someone to try this and put 200 miles on it and report back ?
Don't fancy that !

Paul
 
The bolt most likely has been twisted off as undertoad says above. You can also see on the worn underside of the bolt cap that it been "springy" before snapping. I did a quick test with a m6 bolt and this is the result:

IMG-20250423-074841.jpg
 
pegasus said:
The bolt most likely has been twisted off as undertoad says above. You can also see on the worn underside of the bolt cap that it been "springy" before snapping. I did a quick test with a m6 bolt and this is the result:

IMG-20250423-074841.jpg
Click to expand...
Excellent and many thanks for trying and providing and example.
Can I ask how you achieved this ? as it has striking similarities to my picture ?
For instance was the bolt screwed into something ? if so fully or with some of the thread visible between the underside of the head and the fracture.

Couple of other things puzzle me ?
a) on a normal screw on free wheel if you screwed it onto a bicycle wheel and rode the bike for a couple of miles it would need the axle taking out and a heavy tool inserting into the free wheel and a good couple of blows with a big hammer to free it enough for it to unscrew ?

Yet this set up appears not to be tight when it fails ? surely if it was the bolt threads would be fully engaged for there whole length and this stretching/twisting could not happen ?

Now the only difference obvious between the bike wheel and my set up is the screw on free wheel has a fine thread ?

b) could this failure be triggered by doing something with the car that is not riding it in a straight line ?
I am thinking e.g pushing the car backwards ? the pedals go around so the wheels drive the cassette backwards through the pawls ? What if the left hand wheel was not as free to turn backwards as the right hand wheel ? [ Sticking brake maybe ? ]
This would have the right wheel driving the cassette backwards however if the left wheel was slower it would effectively be unscrewing the bolt out of the left half shaft ?

I think something happens to loosen the bolt in the half shaft , maybe many many times while the car is being used , then on one occasion it happens for to long the bolt comes as far out as it can [ taking up any slack within the 2wd unit then is asked to go further and it then twists and snaps.

Many thanks for you all taking the time to read this and put forward theories.

Quick look at the mileage would imply I did about 120 miles before the bolt snapped , so plenty of time for this to be a gradual process , maybe it unwinds and get stretched till it cannot screw back in under normal driving and the last straw is it snaps ?

Paul
 
Make another bolt and glue it in place to test this "unscrewing" theory.
If it still snaps then you can remove the remains by heating the shaft and stud to break the bond of the loctite. It then becomes possible to unscrew it with a stud-extractor.
 
It's a bad design. Not calling anyone here. Why wasn't the hex machined on the ends instead of using a screwed in bolts. Or am I missing something?
Got some 25mm dia 817M40 bars if someone wants to machine that. (It won't break).
 
pegasus said:
M6 bolt + washer + 2 x nuts in vise.
I've tried a m8 bolt in the same way and was surprised how easy it twisted of, I'm using a 28cm spanner.
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I think you have metaphorically hit the nail on the head.

Because you are turning the bolt in the same direction as my pedaling does then you have proved:-
a) the bolt will snap being driven forward - so no ' unscrewing '
b) it does not shear at the change of cross section under the head
c) so because of b) it leaves the majority of the threaded portion in the hole

More thinking and chewing the cud with DannyC would seem to indicate that it is a combination of being over geared and having steep[ish] hills around here.
Ideally I would like a 1:1 drive for some of the hills
so a 28T crank driving a 28T low cassette gear.and spin .....
Actually I have a 38T crank driving a 28T low cassette gear and so more a slow grind .

The other drive shaft has the bolt welded about where this one twists off , so that weld is doing an awful lot of work in 1WD !

Paul
 
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undertoad said:
What grade bolt?
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I got the bolt from my bucket of reclaimed bolts, nuts and washers.
I was quite surprised how easy it was to over tightening it. There is still some strength in my arm, I thought.
That was until undertoad's question above spoiled my illusion, the grade on the head was 4.8.
 
Ok progress of sorts ?
2nd-broken-bolt-DSCF9741.jpg


I decided the remaining bolt in the hole will be loose in the threads and 10 minutes of careful easing got it out , typical though it was much easier to screw it in with the projections at the break point than out !

The bolt at the back shows the depth of the threads plenty more than the bolt so it had not bottomed out.
right-welds-DSCF9743.jpg


Now this is the right hand half shaft with evidence of not 1 but 3 welds ? You can see the plug weld through a hole at right angles to the bolt's thread. However there are also 2 welds @ 90' to each other around the very very slim rim of the bolt that is screwed in [ 1 can be see at the bottom of the bolt/halfshaft interface ]

left-welds-DSCF9746.jpg

Now this is the failed left half shaft blimey at some point it also had 2 welds along the interface.

So off to tell the engineer I need another bolt , oh and can you drill me a hole in the shaft breaking into the screw tapped hole about where the break point is on the last bolt you made ?

that should be an easy sell NOT !!!

Paul
 
undertoad said:
So, the bolt head was WELDED, broke the welds, and then twisted off?
Click to expand...
Not sure the first time as I had not looked inside the 2WD till it broke and I could not get it apart the engineer did ?
Second time I just screwed the bolt in not noticing the evidence of welds.
undertoad said:
You must have legs like a Percheron, Paul ;)
Click to expand...
Sadly NOT it is the hills and inadequate low gear then has done this.

Paul
 
Popshot said:
A possibility Paul. Equally the broken side could be leess well aligned causing extra stresses.
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Funny you had that thought , the 2 sides are not well aligned [ I have a picture somewhere ] however a hypothesis I tried selling DannyC [ mates rates of course :D ] last week was that these do not have to be ' very ' accurately aligned as their interface to the cassette is via 3 pairs of sprung loaded pawls [ on both sides ] and surely they would lengthen and shorten to take up any eccentricity in the driving dog ?

Awaiting with bated breath ......

Paul
 
stormbird said:
Funny you had that thought , the 2 sides are not well aligned [ I have a picture somewhere ] however a hypothesis I tried selling DannyC [ mates rates of course :D ] last week was that these do not have to be ' very ' accurately aligned as their interface to the cassette is via 3 pairs of sprung loaded pawls [ on both sides ] and surely they would lengthen and shorten to take up any eccentricity in the driving dog ?

Awaiting with bated breath ......

Paul
Click to expand...
Nahhhh....... not biting on this one at all.
Maybe your no-lathe cut down BB will be as successful as this drive shaft, who knows? ;)
Waiting with bated breath for a successful outcome. :)
 
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