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Author Topic: Subframe bolt breakage failure  (Read 3767 times)

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#20

Offline Telecaster3769

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Re: Subframe bolt breakage failure
Reply #20 on: 14 Jan 22, 05:21:48
I don't know. The first I knew was when the back end collapsed. I hadn't done anything. It's a 69 plate bike. My point is, if you rely on 2 bolts to hold subframe on then it shouldn't need checking every 7600 miles ?

#21

Offline Scotty001

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Re: Subframe bolt breakage failure
Reply #21 on: 15 Jan 22, 11:27:20
*Originally Posted by Telecaster3769 [+]
My point is, if you rely on 2 bolts to hold subframe on then it shouldn't need checking every 7600 miles ?
Playing devil's advocate...the entire bike is held together by bolts, do you think no bolt should ever be checked? vibration can and does make nuts and bolts come loose (hence threadlock) but dare I say overtime that can deteriorate (again dut to vibration)

Yes it is rubbish that this happened, there are many reasons that a bolt can shear off (fatigue being one, a minute stress fracture at point of  manufacture / over tightening at some point, being loose at some point and gforce being applied theuch weight movement....)

When it comes to if it can't handle a  top box then it can't handle a pillion, the weight distribution is completely different and physics become involved.

I agree with what I thunk all on here would, a true tourer should be able to carry panniers a pillion and a top box (with a max weight limit for each item based on the configuration of use) all at the exact same time.

Its nothing that the R&D team can't work out and design to spec, but this would affect other design aspects of the bike.

#22

Offline AndyR

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Re: Subframe bolt breakage failure
Reply #22 on: 17 Jan 22, 07:21:41
It seems to me the question shouldn’t be whether it’s safe to carry panniers and/or a top box we should be questioning the safety of the pillion. When considering the potential arrangements it’s clear a pillion is likely to apply more load to the connection than panniers and top box ever could.

Yes critical bolts should be checked periodically but it seems to me there isn’t sufficient redundancy in the connection and the bolts are working too hard.

#23

Offline Scotty001

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Re: Subframe bolt breakage failure
Reply #23 on: 17 Jan 22, 12:56:03
This is where the physics come in, pillion although weighing more sit closer to the mounting point.
A top box sits right at the back of the subframe and can exert more preasure on the bolts with a lower weight limit.

It's the same as structal buildings with a cantilever, the further out you go the less weight can be added as it causes more stress at the tie in point.

Say you fixed a plastic ruler to the edge of a desk (using a 500g weight) with 50mm of the ruler being on the desk and remaining 250mm hanging over the desk in mid air.
put 2nd 500g weight on the ruler at the 100mm point (50mm past the desk edgel, the ruler will barely flex and the 1st fixing weight won't move. Then take that 2nd 500g weight and swap it with a 200g weight, put it at the tip of the ruller (300mm point, 250mm out from the desk) the ruler will flex and the 1st weight holding the ruler to the desk will have so much force on it, it will be lifted off and the ruler will drop off the desk.



#24

Offline AndyR

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Re: Subframe bolt breakage failure
Reply #24 on: 17 Jan 22, 02:45:29
Totally agree about the mechanics….

However, a few very simplistic figures (you could treat the below a lot more precisely):

A top box normally has a maximum load of 10kg. Add in the the box itself and the rack and assume a total weight of 15kg acting as a point load at around 1m from the connection. This gives a reaction of 15kgm split between both sides/connections on the subframe.  Clearly this ignores any additional wind loading/drag the box might introduce at speed.

A pillion of say 75kg sat at around 0.5m from the connection is 32.5kgm split between the two joints.

From experience my panniers have weighed around 8kg each in total when loaded. They sit at approx 0.6m from the connection so 0.6m x 8kg x 2 = 9.6kgm, between the two frame connections, and again ignoring wind loading/drag.

So, in broad terms a normal sized pillion is always going to be responsible for the biggest single load on the subframe connection.

For the bolts to be braking in “normal use”, so with or without a pillion and/or either form of permitted luggage then they’re working too hard. In this scenario correctly torquing the bolts should mean they’re only acting in shear and are not subject to cyclic loading, which could cause trouble. The loads I’m talking about really aren’t much and should be well within a competent design.



#25

Online CraigZ

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Re: Subframe bolt breakage failure
Reply #25 on: 17 Jan 22, 05:04:19
I'm no physicist, but are we forgetting that the load isn't static?

Hitting a bump will generate additional downforce. That force may be multiplied on the bolts depending on the point of fulcrum.

#26

Offline AndyR

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Re: Subframe bolt breakage failure
Reply #26 on: 17 Jan 22, 06:40:14
*Originally Posted by CraigZ [+]
I'm no physicist, but are we forgetting that the load isn't static?

Hitting a bump will generate additional downforce. That force may be multiplied on the bolts depending on the point of fulcrum.

Totally agree, dynamic loading can/does change things massively and will be extremely complex, well  beyond my understanding. The subframe is effectively be a dynamic cantilever thats dependent upon speed, road conditions, suspension etc., etc.

My point being, the bike is designed for two people with one form of luggage system, but it’s the passenger that is likely to have the greatest effect on the subframe connections. Kawasaki to my knowledge don’t quote a maximum pillion weight yet they do restrict the rather modest luggage loading limits. Yet anecdotal evidence is suggesting the connections are failing under normal usage.

I haven’t had an issue with my 2020 bike yet, but would like to know the bike is safe for its designed use without additional checks and maintenance.

#27

Offline Telecaster3769

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Re: Subframe bolt breakage failure
Reply #27 on: 17 Jan 22, 07:14:46
I get all the involved "scientific" explanations, but as the previous  person (Andy r) has eluded to, you should be able to carry a pillion without fear of the rear end falling off. Kawasaki state that bolts that hold the rear subframe should be checked as part of the 7600 mile service ? Is this an admission that there is a problem ? Why 7600 miles ?
Fulcrum's, loads,  physics should be irrelevant to the integrity of a bike advertised as a tourer. That was my point in the beginning and still is. It was also a heads up to riders like me that bought the bike to enjoy and not have in the back of your mind all the time, will the back end fall off. Also, if you look up on kawasaki Web site, part of a 7600 mile service does not include removing fairings and checking frame and sub frame bolts ? These checks are never mentioned.

#28

Offline Steve CJ

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Re: Subframe bolt breakage failure
Reply #28 on: 17 Jan 22, 07:32:54
 :461: I'm concerned about carrying a pillion as how do you explain that if the back drops don't panick  :005: just make sure you dont pull me off with you  :028: It should be highlighted on the schedule and if it's not then how long have they known and as stated why at that milage i would say that the bolts should be replaced regularly if they are just snapping.

this is very serious and they should take it seriously.

#29

Offline ed

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Re: Subframe bolt breakage failure
Reply #29 on: 17 Jan 22, 11:19:45
Thanks everyone for bringing this safety issue to our attention.

Has anyone got a guide that they could share on here describing how to gain access to these bolts to check that they are correctly tight?

Thanks again
Last Edit: 17 Jan 22, 11:44:31 by ed