1-----------:-------------------------------------------------------r,. .
'"
"
16'
e,
,...
M
en
\
OF FAST RIDING
AND OTHER SEMI-LITERATES-
-
-
-
U
,
, ....
I
/
/
-
t3 ' I:H II
I?
R
F
Fig. 3: Th e f irst ins ta n t of braking as pra ct iced by a seaso ne d rider. Th at firs t
tap of rear brak e, enough t o pr o du ce goo d so lid w eigh t transf er t o the f ront
w heel. bu t no t so mu ch that the rear w h eel lo ck s up et'ell after weight transf er.
An y tim e hereaft er he can call on u p to 254 [t .slb. of b rak ing torq ue wh ich is
equ ivalent to 254 lb . thrust and th is leads up directly t o. . .
So in road bi kes at least one o f the
swingarms , Any or all th ese thin gs ca n
m o s t c ri tica l factors in frame design is
act to sc rew up a m o to rcy cl e 's han d ling
t or sion al st iffness: th e ab ility t o resist
twist ing. The reaso n this is so critica l is
t ha t any t ime the fra me twist s the fr ont
and rear tires arc twis ted out o f plane
with ea ch other (again , see Fig. 6 ). What
an d saf e feel.
To ยท recap, t he co nside ra b le grou nd
I've c overed in wor ds an d p ic t ures so far
re al ly c a n- b e b o i l e d d o wn and
conden sed in to the things y o u 'll w an t t o
ponder in sele ctin g o r modify ing a hi ke
for fast ro ad rid ing. Yo u 're lo ok in g fo r
som eth in g with a tau t. resp on sive
suspensio n in a sti ff. preci sely aligne d
fra me and with a se t o f well ba lan ced
brakes th at match t he weigh t, balance
and suspension o f the bike. Yo u also
want somethi ng which wi ll lean 45
degrees or be tter wit hout ground ing ,
This is so you can co rner easily w ith out
having to crawl all o ver the motorcycle
like J showe d you in Part I. Now here 's
this c auses is "rea r en d st ee ring." an
ef fec t o f the tires trying to ro ll on
divergent paths. It is an eff ect which
m ak es som e m otorc y cl es se e m spoo ky
o n w avy surfaces and wh ich makes
others seem never to go precisely where
they're pointed . T his co ndi ti o n comes
about fro m inade q uate frame st iffness
b ut also from slop and wear in th e
s us pe nsion, be nt or m i s a li g n e d
suspens io n and fro m loose pro du c tio n
to lerances in so me st ock fo rks. axles or
liS t
t;6# ~ ;""' T
_
.2"+ l'
L I": ,
b O"
&C
r 51
:tl
F S 3~ "
Fig. 4: Th e ins tan t after front br ak e is applied. This is w here th e reall y hea vy
brak ing pow er lies sinc e th e great er th e wh e el loading, the grea te r th e available
brak ing f orce at tha t w heel. When th e lim it is reach ed both tires are pulling
th eir lim it , deceleration is on e g (32 [t .Iscc, per sec) an d that almigh ty fron t
brak e is pulling alm ost 86 % of th e load. This should tell you ch op p er freaks
wh at you throu: awa y when y o u p ut a sp oo l h ub o n t he f ro n t.
a h o o ker . J ust sitti ng at rest and leaning
the bi ke over . w atchin g to see wh en
so m et hing touc hes grou nd wil l NOT tell
you abo ut i ts ground clea rance a t speed .
To realize wh y lo o k b ack a t Fig. 5 a
min u t e. That resolved fo rce R: it 's
p r e ssing st rai gh t down o n th e
suspens io n. right ? Now , with trig and
all, a guy can prove tha t a t 4 5 degrees,
all cran ked ove r a t speed , that force
pressing down o n the s usp ens io n is
go ing t o b e 1.41 4 tim es th e sp rung
weight which will meas ur abl y co m p ress
th e suspensio n. So a bi ke th at Mil not
ground at rest m ay gr ound at speed . The
way t o chec k without go ing o ut o n t he
road and pressing until y ou ground
so m e thing and fall o ff is t o use a go o d
set o f tiedowns o r a co up le o f cha ins
wi th turnbuckles t o jack th e suspension
down a co uple inc he s front and rear.
Then lean th e bi ke an d lo ok .
O ne more thing abo ut co rn ering
clear ance. Aside from tu cking in t he
e x h aust , t h e f o otpe gs and othe r
attach ed hardwar e , o ne o f the basic
limiters of co rnering clear ance is the
e n g i ne it self. Obvio usly then the
narro wer the eng ine is in the crankcase
the better it is for cornering clearance .
To mo unt a wid e case multicylinder
engin e in a frame so it has good
cornering cl earance y ou have to mount
it co nside ra bly high er o ff the gro und .
This, d o ob y , raises the overall CG of the
bike with unpleasant effects o n a th ing
called " top hamper " which we 'll d iscuss
more full y in Par t V .
- , ---
--o-----~T"'~ - - --
Fig. 5: Free body dia gram of a corne ring m ot orcy cle ill a stable att it ude. R is
th r sum or resul ta n t of the weight II' and centrif ugal f o rce F and with the rider
tucked in p oin ts straight d oum th e cen terline of th e m oto rcy cle, niht th ro ugh
th r co n tac t p oint.
Fig. 6: Tu nsting im pa ct due to a b u mp w he n cran ked over. N ote d ue t o f rame
flex ing, [ron t tire k nock ed o ut of plan e wi th rear tire. lI'hell this hap p ens th e
tw o tires roll in diue rging paths.
