Rear sprocket bolts Qingqi QM200GY-BA
Following on from message nš10 on this thread concerning breakage of rear sprocket bolts by shearing and their possible replacement for stronger items, I've had a look at what's fitted to mine.
A simple observation of the heads of the bolts shows they're are marked "7ST". This could be something to do with their strengh. To compare, other bolts so marked include the pillion footrest hangers, engine holding bolts, engine guard bolts, etc.
Other screws for light duties like the chain guard are marked "4ST". This looks so far as if it's some sort of Chinese bolt tensile strengh code, but:
Bolts for serious duties like handlebar clamps, brake callipers, fork yokes, etc, ARE marked using the international ISO markings, and they clearly show "8.8", meaning a quite high tensile strengh grade. So, what does "7ST" mean?. Is it better than 8.8 or worse?.
This is a chart showing the US an ISO bolt strengh markings. No "7ST" there!
http://www.telecable.es/personales/a...tstrengths.gif
I removed one of the bolts after bending back the very strong security tab washer and it's a M8x30 mm with a 6 mm long unthreaded shank.
This 6 mm being the width of the sprocket.
One important note about fasteners. The threaded section of a fastener should never be given side loads or shear loads.
The threads will not be as strong as the shank (unthreaded portion), and can also chew away on the surrounding metal.
This should be taken into account on any critical aplication such as Disc brake rotors, drive coupling flanges, sprocket drives, etc,
so they MUST be replaced with a similar bolt with a 6 mm plain shank and a high tensile rating.
I'll visit my friendly screw peddlar tomorrow and see what he's got.
Here's a (bad) photo of the bolt (bloody useless Macro function on my Cannon IXUS 60...):
http://www.telecable.es/personales/a...ocketbolt6.jpg
Have visited my screw supplier today (28th) and they don't have anything like that (I knew it!). Alternatively he can supply me with the nearest thing that can be modified and it's a DIN-931 M8x40 bolt in 12.9 grade (the strongest) and with an unthreaded shank 17 mm long.
This bolt can be first cut down to 30 mm and then a Die used to cut the thread another 10-11 mm along the plain shank until only 6 mm remains.
This is all very easy to talk about, but 12.9 steel is tough and after doing one was exhausted. (It's hot and humid here today). So I did the only thing a red-blooded Spaniard does when confronted with a difficulty ...gave up, drank some wine and had a siesta.
I did talk to a keen bicyclist in the shop and he confirms that the cause of the sprocket bolts breaking is letting the spokes come loose, it happens to bicycles all the time apparently. So I think that I'll just keep an eye on my spokes and see what happens.
Qingqi (Qlink) QM200GY sprocket bolts.
I've just received my genuine Suzuki 44 tooth sprocket to replace the stock 48 tooth and so raise the gearing. It's a Suzuki part number 64511-48001 made to fit models DS100, DS125, TS100, TS125 (1978-79).
If you prefer JT Sprockets have an equivalent listed with reference number JTR 809.44
Site at: http://www.jtsprockets.com/
Some of us, in this and other forums, have had trouble with sprocket bolts shearing or threads in the hub stripping. We've speculated about the bolt steel quality, poor hub alloy, etc, but I think I now have the answer and it agrees with ChinaV's suspicions also. (See post number 2 on this thread).
At the same time as I ordered the sprocket I ordered a set of genuine Suzuki bolts, according with the parts catalogue where I found the sprocket. Their part number is 09100-08094. They come as a set of six.
On opening the packet, the difference is obvious, they're M8x30 grade 8.8 (marked "7" as per Japanese practice) same as the Qingqi's but, the unthreaded shank is 14 mm long rather than 6 mm.
Good engineering practice advises not to let a bolt work in shear load, only in tension, but if the designer chooses to do so, to ensure two things:
1- Ensure the tensile load applied to the parts by the bolts is sufficient to stop relative movement between the parts and so avoid movement and the transfer of the shear load to the bolts.
2- If this happens through overload or under-torquing of the bolts, then the shear load must not fall upon the threaded part of a fastener.
This is the reason why there's an unthreaded portion in the bolt holes of the hub. (see ChinaV's photos in post 2).
The idea is to use a bolt with a plain shank that will go through the tab washer thickness (1 mm), the sprocket thickness (7.2 mm), and have the remaining plain portion sufficiently long to penetrate the unthreaded hub portion.
In one South American forum they thought that this unthreaded portion in the hub meant that threads had been stripped and have carried out a repair using a thread insert threaded right to the top surface. This is wrong.
In normal usage and with the bolts torqued up correctly, the two parts (sprocket and hub) should not move relative to one another. This can only happen if the bolts come loose.
The recommended torque for these bolts is 22Nm (16 ft.lbs.). The procedure I would use is to torque the bolts, wait a while before bending up the lock washer tabs, to allow the joint to settle, re-torque and bend the tabs over one of the bolt head flats.
http://www.telecable.es/personales/a...ketsection.jpg