When push comes to shove

spook · Post by **spook** » Mon Mar 30, 2009 12:36 am

Can anyone tell me why modern road going sports bikes have push-pull cables? For instance my CBR929 (undergoing rebuild (lol)).. has such a set-up, though the butterfly rack also sports quite a heavy spring, that is responsible for the throttle valves return to idle seat. The return cable is never in tension. The only thing the push cable is doing that I can see is adding drag? Can anyone enlighten me? Cheers.

hotcam · Post by **hotcam** » Mon Mar 30, 2009 1:16 am

Yes it does seem redundant. Off-road motocross racers have only a
single cable.

I heard that some old carbs could hold the throttle valve open in
the carb by suction if the return spring broke, thus the rule for
a pull-close cable.

It might be a requirement of safe design practise,
or it might be enshrined in legislation somewhere, but I couldn't find
it in ADRs.

spook · Post by **spook** » Mon Mar 30, 2009 5:10 pm

Yep, I think your spot on Cam. I found this page in relation to RS flatslides. I don't envisage a problem with the 929. I'm going to run a 1/4 turn MX unit and see how it goes.

http://www.mikunioz.com/rs_accessories.htm

sickle44 · Post by **sickle44** » Mon Mar 30, 2009 5:29 pm

It is matter of safety, reliability, and precision, as more accuracy can be obtained with throttle position opposed to a single set-up which can at times get a little sloppy and loose, therefore, not as accurate positioning of the butterfly.

hotcam · Post by **hotcam** » Mon Mar 30, 2009 7:30 pm

more accuracy can be obtained with throttle position opposed to a single set-up

True, but don't Yamaha normally recommend running the pull-close cable
adjusted with fair slack, so there is zero tension on it, to allow for
handlebars to turn etc?

If one did adjust one's cables so as they were precisely positive-acting
on the butterfly (same as the Exup cables), there would definitely
be precision.
But would the tension change slightly as handlebars were turned?
EXUP cables don't have to deal with any relative movement between the
master and slave pulleys.
Hmmm interesting aside but it probably doesn't make any difference.

spook · Post by **spook** » Mon Mar 30, 2009 8:39 pm

Single cable works fine, no worse than the dual. The bike only has 4000 klm on it, the butterfly's etc are in good condition, and the spring quite firm. They in fact bang shut. I don't think Stan is running the second cable on his fizzer either. It was Stan who suggested a thread on the subject... Interesting.