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  1. #1 Rear sprocket bolts Qingqi QM200GY-BA 
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    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!



    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...):



    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.
    Last edited by forchetto; 07-28-2008 at 07:33 PM.
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  2. #2  
    Life Is Good! ChinaV's Avatar
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    Thanks forchetto for the thorough information and research. I had the rear wheel off last night to change the sprocket and tire. I think you can see from the pics that the stock bolts (silver color) do not have a very deep shoulder, even the sprocket is almost resting on the threads.

    I had a couple of other bolts lying around to compare, but have not found a perfect match yet. I'm going to measure the hub thread depth and try to have a local screw shop make the correct bolts. I'm thinking that the shoulder that rests in the sprocket and top of the hub should have a slightly larger OD than the rest of the bolt.







    Cheers!
    ChinaV
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  3. #3  
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    Thanks for those great pictures, chinaV, they help a lot, as I only took off a bolt while the sprocket was still on the bike.

    I assumed the thread went to the top surface of the hub, but it seems there's something like 6 mm unthreaded. There must be a reason for this, as it entails cutting the thread first and then a second machining operation to remove 6 mm of it. I can't see the reason, although someone suggested it stops the bolt tension from raising the aluminum, making the sprocket seating face not flat.

    This means that, as you say, the bolt could have a longer unthreaded shank, and it makes it easier to find standard bolts that could fit. Looking at a chart of preferred sizes of DIN-931 bolts, there's one that requires minimum modification and is a M8x40 with a 28 mm threaded portion. This means it has a 12 mm plain shank, and with just cutting off the surplus 10 mm in length to 30 mm total, would fit and present this plain 12 mm shank to the tab washer (1.00mm), sprocket (6.75 mm) and the remaining plain part (about 4.25 mm) would sit nicely inside the hub, just before the threads start.

    As you suggest, it would be ideal if the plain part fitted better inside the holes. It would have to be made specially and look something like the bolts that are used as handlebar lever pivots:

    Last edited by forchetto; 08-01-2008 at 04:55 AM.
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  4. #4  
    Administrator-tron CrazyCarl's Avatar
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    Hey Guys,

    I found some possible replacement bolts for the QingQi 200gy rear sprocket.

    These come from the excellent Mcmaster-Carr company in the US. They have a partially threaded M8, 30mm, 1.25pitch, Class10.9 cap bolts which I think may work.

    They are about 12.00USD for a pack of 25 zinc plated and 9.00USD for black finish.

    - Click image for site link and search "Metric cap screw" -


    If anyone is serious about trying these, let me know.

    Paz,
    CC
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  5. #5 Qingqi (Qlink) QM200GY sprocket bolts. 
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    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.

    Last edited by forchetto; 02-08-2009 at 02:39 AM.
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  6. #6  
    Administrator-tron CrazyCarl's Avatar
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    That makes sense! I'm sure it will be a great improvement over the stockers.

    How deep is the unthreaded portion of the QingQi hub? From ChinaV's pics it's didn't look as deep as indicated in the drawing. Would you have to countersink-drill it?

    Also, cool illustration! I didn't know you were also an artist!

    CC
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  7. #7  
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    Nice work forchetto, Who needs shop manuals when you have forchetto the artist at MCM

    Cheers
    ChinaV
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  8. #8  
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    Thank you, thank you. I thought the drawing was rubbish really. I do have a little chap at work that can work an AutoCad program to do it properly, but being another Spaniard all I got was: "Maņana..." and I wanted to publish it as quick as possible.

    By the way, the bit about not working fastener threads in shear was more than confirmed by the NASA Fastener manual RP-1228 which states on page 24:

    "Standard design practice is to choose a grip length such as the threads are never in shear".

    So all those billions you've spent on Space work are useful eh?. We're always told that a by-product of the US Space program is the non-stick frying pan. I ask myself: Surely the one thing you don't want in space is a frying pan that allows your sausages to fly off in zero gravity all over the capsule?. One that would encourage the food to stick would be far more useful to your astronauts...
    Last edited by forchetto; 01-31-2009 at 01:40 PM.
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  9. #9  
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    Quote Originally Posted by CrazyCarl View Post
    How deep is the unthreaded portion of the QingQi hub? From ChinaV's pics it's didn't look as deep as indicated in the drawing.
    CC
    Have now done the sprocket fitting and proper measurements. The sketch has been edited to read 7mm deep unthreaded portion of the hub, and the Suzuki bolt has a 14 mm plain shank rather than the 15 mm stated previously. The chain had to be reduced by 1 and 1/2 links.

    Not road-tested yet, as bike still on the work bench waiting for a chain tensioner that I'm hoping to adapt to the swing arm.
    This mod was carried out by "frog" in http://www.chinariders.net forum and apparently transforms the feel of the transmission in exchange for a very modest outlay. The universal tensioner shown is only about $10.

    The swing arm has some brackets welded on that hold some sort of chain guiding loop near the sprocket. I can't really see what it does, so I'll try to fit the tensioner to those brackets.
    Will report on progress.



    Additional findings for the pedants amongst us :

    Everytime I work on this bike I never fail to be impressed by the design, fit and finish, and build quality. It feels just like working on a Jap bike. I can think of no higher praise.
    However, I have noticed yet another error in the rear axle shaft. This is entirely decorative-chromed throughout its length: head, shaft, threads, nut, the lot.

    I have never seen this done on any Jap, Italian or British bike I've ever worked on. The reason being what is known as "Hydrogen embrittlement".

    Hydrogen embrittlement (or hydrogen grooving) is the process by which various metals, most importantly high-strength steel, become brittle and crack following exposure to hydrogen. It's caused by the Hydrogen evolved during electroysis being, to some extent, absorbed into the crystal structure of the steel at its surface. It has a tendency to increase crack propagation sites in steel due to the micro cracked structure of the coating . There is also a drop in the fatigue strength of the material which can be as much as fifty percent.

    Well known examples of this error were those special bikes in the past with chromed or nickel-plated frames, they were discontinued due to this weakening of the material leading to breakages.

    Let us pray that it's strong enough...
    Last edited by forchetto; 02-01-2009 at 11:23 PM.
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  10. #10  
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    Hey Forchetto,

    First post for me, but can I just say thanks for the excellent tips regarding Qingqi 200S.

    I'm in the UK and own an RMR200 that I'm really happy with although I have suffered from the shearing bolt problem from the rear sprocket. I've just collected my swingarm from the welder as the stud left behind once the bolt head had sheared, worked it's way out and cut a gouge in the swing arm!

    I'll be ordering the 44T sprocket for a bit more poke but will definitely order the suzuki bolts you specified as well!

    Thanks again for the info about these fine bikes.

    Cheers

    Ian
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