Non-trivial in Tightening the Headset Cap 鎖緊頭碗蓋非小事一件

Non-trivial in Tightening the Headset Cap 鎖緊頭碗蓋非小事一件

前言:

我在獅頭山行這篇的[*]提到頭碗卡卡的。原因是我picky,在飛鳳山試車後自己又鬆開重新鎖了一次。獅頭山行回來後,又重新鎖了一次 (將上蓋螺絲上油)。轉動把手還是覺得頭碗卡卡的。

其實自從店家說我頭碗沒鎖好 (參見後記1),我回來後有查書 (Zinn’s xi-4 & 古川(二) p.46),但我沒有看到詳細說明如何鎖頭碗。像Zinn’s xi-4 step 7 也只是簡單說將上蓋螺絲上油然後以5mm hex key鎖上。Park Tool’s Repair How-to並沒有針對Headset Tools部分的保養維修做解說。

今早我比平常晚起 (好像身體還虛虛的),躺在床上還不想爬起來,隨手拿了Will Peveler’s The Complete Book of Road Cycling & Racing來讀。翻到Ch3 Bike Maintenance的Headset (頭碗) 部分,看到了這段:

It is important to understand how these parts interact. Although the top cap pulls up on the star nut or compression plug inside the steerer tube, you still need a gap between the top of the steerer tube and the stem so that pressure from the top cap is applied to the stem. In turn, pressure from the stem is applied to the spacers, down through the bearing races and to the bearings. Tighten the top bolt until it feels snug. You want the top cap tight enough to hold the fork in place with no play but loose enough to allow the handlebars and fork to steer smoothly. Next, ensure that the stem is straight, then tighten the bolts on the stem.

A-ha! That’s it! 我鎖得太緊了! 難怪我覺得頭碗卡卡的。這下子我全醒過來了,巴不得趕快到辦公室來重新調整頭碗。

在試之前,我先Google一下。一下子就看到有討論這個問題,“What happens if you overtighten the Stem cap bolt?”。amahana1的回答比較完整:

If you overtighten the headset cap bolt you will def ruin the headset bearings/bearing races and you will be buying headsets quite often. The bolt should be tightened just to the point where there is no play in the fork and no tighter. (skip)

可是這麼重要的事,怎麼Zinn和古川都沒有提起呢? 他們不是沒有提,而是我沒有看到後面他們說的。Zinn在xi-13 “Check Headset Adjustment”還有xi-15 “Adjusting a Threadless Headset”就有提到了。尤其是xi-15 a.3:

Adjust the headset. Be careful not to overtighen the compression bolt, which will put too much pressure on the bearings and eventually pit the headset.

古川則是在組裝公路車部份中p.131有提到:

在鎖緊龍頭上蓋時注意不要鎖得過緊,只要讓頭碗培林與車架頭管之間沒有間隙,頭碗不會產生搖晃的情形即可。

我原文照抄,不過我覺得這裡不應該說成”龍頭上蓋”而應該是頭碗上蓋,因為英文全名都是headset top cap。龍頭是Stem,沒有包含這個蓋子。我一開始也是龍頭和頭碗分不清楚。 看英文書比較沒有名詞混淆。

我後來之所以會鎖的這麼緊,一方面是因為店家說我鎖錯了 (順序),二來當我鬆開螺絲時也頗費力氣的 (不過剛試車時是沒有卡卡的)。書上又說這前叉的固定是藉著頭碗上蓋螺絲 (再次強調不是龍頭)。因此,在鎖回去時我就用盡蠻力來鎖緊它。想說,這樣前叉才不會因為撞到坑洞而掉了。怎知是鎖過緊了。

至於怎麼鎖得剛剛好,依Zinn的說明就是以1/6圈轉螺絲做細部修正,或是用扭力扳手以22 in-lbs (~ 2.45 N*m 非常低的扭力) 為出發點。

調整頭碗看似一件很簡單的事 (Level 1 by Zinn),其實還是不能太輕視。 我也不是沒看書就亂DIY,可是跳著看總有疏忽之處。 還得像Peveler把重要的部分精簡提出,我才會知道。So, what’s the lesson I’ve learned? Be more careful and if you’re not so sure, check with the lbs mechanic. (I’m too lazy to go to my lbs) Plus, listen to your bike so that you’ll notice the problem more quickly before it gets worst.

後記:
騎乘完簡單擦拭車子也是有幫助的。像我昨天擦拭 (我沒有在洗車的) 就發現我前輪左邊的煞車皮歪了。應該是我騎乘前有重新調整時沒有鎖緊。

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