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  1. #1
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    New question here. uncertainty regarding stem installation

    Hey,

    in the manual of the Thomson X4 stem it says: "Steerer should extend 3mm above stem requiring a 5mm spacer".
    I have no clue how to interpret that sentence...there is no drawing to clarify it.

    With steerer I presume they mean the steerer tube?
    I thought the stem should extend 2mm above the steerer? As shown here: fork11.jpg

    And what's that about the 5mm spacer?

  2. #2
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    They are wanting you to put one of the spacers that go underneath the stem above it. if the steer tube is 3mm above, then you will need a 5mm spacer to leave room for the 2mm that is required when putting the cap on.
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  3. #3
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    Thomson has done a lot of engineering to make their parts as light AND strong as possible and I'm sure have found a weakness by mounting the top of the stem 3mm above the steerer like most people do. Having one spacer on top of the stem looks pro as well as providing the most clamping force on the steerer.

  4. #4
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    If possible, I ALWAYS put a small spacer above my stem. Especially on carbon steer tubes. It's a good practice and ensures proper clamp force on the sturdier parts of the steer tube.

  5. #5
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    Sorry, was away for a few days, thanks for the answers, I get it now.

  6. #6
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    Now I understand why my headset ran loose twice a day during my holidays and I had to replace my headset eventually. To save 2grams in the stem they add 20 in the spacer and extra fork length. I don't want to lower my handlebars. So 80mm Thomson stem for sale.

  7. #7
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    Quote Originally Posted by XXLspot View Post
    Now I understand why my headset ran loose twice a day during my holidays and I had to replace my headset eventually. To save 2grams in the stem they add 20 in the spacer and extra fork length. I don't want to lower my handlebars. So 80mm Thomson stem for sale.
    Thats not really it at all but I guess by reading my post you could get that. Thomson is all about engineering their parts to be strong first, light second, and beautiful third.

    On any stem, you want your full clamping force to be on the steertube, the highest position you would put your stem would be with the top of the stem equal to the top of the steertube, then put a 5mm spacer on top to allow room for the top cap to tension the headset bearings. If you need your bars higher than this, look for higher rise bars or a higher rise stem; cant think of how many times I've seen a bike come in to the shop with the top steerer clamp bolt above the top of the steertube. At this point the top clamp bolt isnt clamping anything but air and the top cap and poses a safety risk.

    Your headset probably worked loose because you didnt have enough space above the steertube to properly tension your headset bearings. Or the tension was off to begin with.

  8. #8
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    Quote Originally Posted by XXLspot View Post
    Now I understand why my headset ran loose twice a day during my holidays and I had to replace my headset eventually. To save 2grams in the stem they add 20 in the spacer and extra fork length. I don't want to lower my handlebars. So 80mm Thomson stem for sale.
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  9. #9
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    Nope. After replacing the headset I verified that the forktube ended only slightly below the top (just over 1mm distance with the top cap). I put some grease on the bolts and tightened them up as far as I dared to not strip the hex hole. Of course I used proper hex tools. Still it comes loose. I suspect that the area that has been cut away from the stem and the nearness of the top of the fork cause a deformation of both the top of the fork and the upper part of the stem near the bolt, that causes it to slip easily. Stems that do not have this cut away will suffer much less and in my practice stay seated. To avoid this deformation, it will help considerably to add a few mm's of fork tube above the top of the stem and therefore the installation instruction is completely logical from a mechanics point of view. I suspect this is a design choice by Thomson to make their stem lighter in the ads.

  10. #10
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    Just put another 3mm spacer on top. Doesnt really matter how your stem and spacers are arranged, you just need to have 3-5mm of spacers above the top of the steertube in order to be able to tension the headset bearings. If you dont have enough space you'll just end up bottoming the top cap out on the steertube and not tighten the bearings any more.

    Thomson stuff is great, plenty of other stems have similar cutaways and do not have any issues. This is not a hardware issue, this is a user issue. If you cant safely work on your own bike take it to someone who can before you hurt yourself.

  11. #11
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    The headset bearings are properly loaded. The 1mm is plenty space to not bottoming out. I actually checked the distance by looking through the split at the back in properly tensioned state. And don't worry I do know how to work on bikes and have very rarely issues. I am swapping forks 2 to 3 times a year already for years and this is the first time I have problems. Note that I already tightened the screws already more then I used to do and still it does not hold. It may be a user error not to place a fake spacer on top, but I did not have it with my previous stems. Therefore I blame the Thomson stem. As I don't want to lower my bars the stem will go.

  12. #12
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    Try another spacer. Guarantee you 100% that it will help the problem. This isnt a stem issue, its a setup issue.

    Thomson stems normally have less stack height than other stems, this means that you'll need to add 4-5mm of spacers either above or below the stem. Sounds like in your case it needs to be above the stem for safety's sake.

    Most stems have a 40mm stack height, Thomsons are 36mm IIRC. If you change forks frequently you should have some extra spacers around, throw another real spacer (I dont recommend using fake spacers) on top of your stem and retension your headset bearings.

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