คำตอบที่ 25
Front axle pinion and caster angle considerations:
For a leaf-sprung front axle, you can use the above measurements, but they only consider the pinion/driveshaft angles. IMHO, you really want to first address steering angles up front (unless you have a trail-only rig) first then worry about pinion angles. Afterall, if you have to choose between having proper steering geometry or a proper pinion angle (so that you can use 4WD at speed), I would choose proper steering angle any day? Why? Well if you can't drive the truck in a straight line on the road, having 4WD available at high speed becomes a moot point. I have had bad caster angle on my truck and I can tell you that the term "death wobble" is aptly named! When that front end starts hopping all over the road and seems to get progressively worse as you drive is not fun. Give me good solid steering anyway and I can live with a little front driveshaft vibration if I need to use 4WD at higher speeds every once in a while.
So how do you measure the steering caster angle? Best bet is to go to an alignment shop and have them put your truck on the alignment machine and give you a printout of the angles. You'll get a list of toe-in, camber and caster as-measured and they may tell you what the angles should be. If not, consult your owners manual or a good repair manual for your vehicle. As far as what angle to use, its a direct relationship between the number of degrees your caster angle is off and the angle of the shim. For example if your vehicle specs +2° of caster and you are at +5° or -1°, you'll need a 3° shim, and just install it in the proper orientation to correct the angle.
Here is a link to Toyota solid front axle steering alignment specifications. On the Toyota front axle, an angle of approx. 6° up (front of perch higher than the back)on the spring perch provides a decent caster angle.
It is not uncommon for the caster angle to be off different amounts on each side of the axle. If so, this means that the axle housing itself is not quite aligned side to side or that it is slightly bent. Best bet is to get a shim to correct the average of the two angle errors. Short of cutting off one steering knuckle and turning it to align with the opposite side, its unlikely that you'll be able to "twist" the housing by using two different angle shims.
And if you want to correct BOTH the caster and pinion angles at the same time, the only way to do that properly is to cut and rotate the steering knuckles on the axle to set the proper caster angle and if needed cut and relocate the spring perches (or add shims) to set the proper pinion angle. Here is a writeup on doing this modification on a Toyota solid front axle. On some axles, like the Dana 44, you may find that one of the spring perches is cast into the side of the differential housing and is therefore difficult to modify. In that case, you can either use two shims to change the pinion angle, or cut/relocate the separate perch and then add a single shim to the cast perch to set the pinion angle.
สำหรับคานที่ติดตั้งเรียบร้อยแล้ว และ ต้องการปรับมุม caster ให้ได้ตาม spec เดิมๆ(ต.ย. VX80 +2 ถึง +4) ก็ทำได้โดยการรอง shim ไปใต้แนบ(ทางแก้ที่ แรก) แต่เมื่อรอง shim แล้วก็จะไปเกิดผลกระทบของมุมตัวเฟืองหน้า ถ้าได้มุมตัวเฟืองหน้า ต่ำลงนิดหน่อย ก็โชดดีไป
แต่ องศาเงยมากกว่ามุมดิ่งของเพลาขับ หรือ อาการมุมตัวเฟืองหน้า ต่ำลงมาก(มากกว่า กี่องศานั้น ต้องศึกษากันต่อไป)
ทางแก้ที่ดีที่สุดต้องปรับใหม่ทั้งหมด โดยการกรีดคอ ปรับหัวกระโหลก แล้วปรับเริ่มใหม่ทั้งหมด
สำหรับที่กำลังจะวางคานเริ่มต้นถูกต้องก็ไม่ปวดหัวทีหลัง
อ้าวอิง http://www.4crawler.com/4x4/ForSale/Shims.shtml#FAQ1
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