影响自行车性能的因素除了上述人的因素外，还有许多机械因素，如图1-3所示。为了获得自行车较佳的性能，必须把人的因素与机械因素有机地结合起来，以使人一车协调。为此，着重分析与人体相关的结构要素。
图1-3 影响自行车的机械因素 1(速比
大小链轮的齿数比，与链轮直径比相一致，一般控制在2.3-4.0的范围内。利用速比关系可取得骑行时所必要的功率和必要的速度。速比要合适，如果太小，无论人的肌力有多大，由于不能充分提高转速，所以就得不到大的输出功率。也由于速比小，在限定的曲柄转速下，得不到必要的骑行速度(后轮转速)。速比过大时，要求的踏力也大，容易使人疲劳。为了保持不疲倦的持续骑行，希望肌肉的负担约为最大肌力的10% ，按此选择速比和曲柄转速，可得到比较好的效果。
2(曲柄长度
传统的自行车设计，一般从杠杆原理考虑比较多，对人研究少，认为曲柄越长越有力。曲柄过长后，为了不使脚蹬碰到前泥板，不得不加大中轴至前轴的距离(前心距)。这样势必加长车架，影响了正确的坐车姿势，使人感到臀部痛。若能按人的身长或下肢长来考虑曲柄长度，则可使人省力和舒适。通常曲柄长度的基准，取人体身长的 1,10 ，也相当于大腿骨长的 1,2 。
3 (三接点位置
正确的骑车姿势，是由骑车人和自行车三个接点位置决定的，如图1-4中所示的鞍座位置A 、车把位置B 、脚蹬位置c 。按三点调整法，AB 和AC 约等，一般 AB = ( AC 一 3 ) cm , A 点略低于B点，约为5cm 。
图1-4 三接点位置
Million. Water quality standards the dredging of river boundary section obtained provincial environmental protection Office special bonuses for wetland construction ... ... Million. But sober see, these achievements were made in the case of decreased rainfall for two consecutive years, we realize that environmental protection work still faced many problems in our County. Mainly in the PM2.5 and PM10 air quality monitoring indicators often exceeding, dredging of river section's water quality is not stable, and rural non-point source pollution has not been radical, Wade through the corporate environmental risk, livestock pollution rebound from severe, a wealth of protection and use of water resources is not well. If this year's rainfall, these problems will be exposed, and raw materials caused a great deal of pressure to go green. To solve these problems, we focused on three aspects of the work of environmental protection: (a) in accordance with the ... ... Real good green. According to the \
lined, mountain forests of\piece, scale\barren hills green, River afforestation increased, City Green, farmland and other greening works to ensure complete afforestation this year ... ... Acres and acres and afforestation in the plain ... ... Acres, new nursery wood ... ... MU.
4 (鞍座位置
鞍座装得过低，骑行时双脚始终呈弯曲状态，腿部肌肉得不到放松，时间长了就会感到疲软无力;鞍座装得过高，骑行时腿部的肌肉拉得过紧，脚趾部分用力过多，双脚也容易疲劳。骑车时适 当的用力部位是脚掌。设计或校正鞍座位置高低最常用的方法，是使手臂的腋窝部位中心紧靠鞍座中部，使手的中指能触到装配链轮的中轴心为宜。人体各部尺寸都有一定的联系，只要腋窝中心至中指的长度确定下来，鞍座高度便可大致确定。行驶较快的车，鞍座位置要向前移动，行驶较慢的车，鞍座位置要向后移动，否则都不利于骑行，如图1-5 ( b )、( c )所示。
图1-5自行设计的结构要素 5 (车闸
设计时，闸把开挡、力率和闸把力要与人手的大小和握力相适应。灵敏度高的车闸，随着闸把上力的增大，刹车力也按比例地增加。如果闸把力到达某一程度不
发生刹车作用，继而又骤然生效，说明这种车闸设计不良。在紧急情况下操纵时，理想的施闸力和减速度见表1-1 。
表1-1 理想的施闸力与减速度 闸把施闸力相对握力% 减速度 说明 /N
控制下坡速60 10 0.1g 度 350 70 0.6g 全刹车 500 100 0.8g 紧急全刹车 2注:g=9.8m/s
Million. Water quality standards the dredging of river boundary section obtained provincial environmental protection Office special bonuses for wetland construction ... ... Million. But sober see, these achievements were made in the case of decreased rainfall for two consecutive years, we realize that environmental protection work still faced many problems in our County. Mainly in the PM2.5 and PM10 air quality monitoring indicators often exceeding, dredging of river section's water quality is not stable, and rural non-point source pollution has not been radical, Wade through the corporate environmental risk, livestock pollution rebound from severe, a wealth of protection and use of water resources is not well. If this year's rainfall, these problems will be exposed, and raw materials caused a great deal of pressure to go green. To solve these problems, we focused on three aspects of the work of environmental protection: (a) in accordance with
the ... ... Real good green. According to the \lined, mountain forests of\piece, scale\barren hills green, River afforestation increased, City Green, farmland and other greening works to ensure complete afforestation this year ... ... Acres and acres and afforestation in the plain ... ... Acres, new nursery wood ... ... MU.
四、人一车动态特性分析 1 (动态稳定性
自行车的稳定是行驶过程中的稳定，是一种动态平衡的稳定性。动态稳定性影响到自行车骑行中的动作，包括直进稳定性和前后左右方向的稳定性，见图 1-5(a)。显然，稳定性对安全行驶是必不可少的特性。
2(力学特性
自行车行驶在平地上转弯的条件是侧向力(与离心力平衡)与自行车总重量(人和车的重量)的合力作用线要通过轮胎与地面的接触点。这当然与骑车人有关，但更重要的是自行车的造型要有适合这种力学特征的结构形式。
3 (转向特性
自行车转弯时可能有三种情况:
图1-5 人-车系统动态特性
自行车设计中人机工程学案例分析
