Bike Bodies - Improving Throttle On Light Load.

[Dream Weaver 2]

I've spoken to a few peeps about this, but thought I would throw it out to the wider audience on here to get some ideas flowing.

 

The bike bodies on my Mi appear to be working well, they are smooth once you are on the throttle, idle is good, and the noise at full throttle is addictive

 

The big problem is low down throttle response - the bodies have a small cam on the end which i've attached the throttle cable to directly - they work fine when travelling at full pelt, but on light throttle, i.e. when cruising, setting off from standstill, or even trying to apply light throttle on sharp bends, its a nightmare

 

Its starting to ruin the experience a lot now, as its impossible to drive on light throttle without snapping off - after changing gear, you go to lightly press the throttle so as not to spin the wheels, and you almost have to wait a while to get the pedal moving, and it usually ends up moving to much and jerking forward.

 

So, what ideas do we have to help improve the situation, such as progressive throttle cams, bigger cam etc, over to you.........

[Kev-G 1]

This will relate to something you posted on PPC Forum about 'ramp angle' of bike ITB's not being suitable for cars IIRC. I think that you have hit the nail on the head re: cam on the end of the throttle cable....

 

I'd try changing that to see if you can introduce more modulation.

[M3Evo 0]

I spent quite a bit of time designing a progressive throttle linkage for mine which wouldn't be too difficult to transfer onto yours I'm sure.

 

In essence, the throttle cable attaches to an actuator rod which has a big long arm sticking out. A link then takes movement of this arm to another smaller arm on the throttle spindle(s).

 

What's important is the geometry which causes the throttle to open very slowly for the first bit of pedal travel, and ramps up the rate of opening thereafter.

 

Anyway, s*ite piccy paints a thousand words!

 

 

Your foot is connected to the black bit and the red bit is connected to the spindle. with the green bit being our link

Edited June 13, 2006 by M3Evo

[24seven 104]

which bike bodies are you using?

[tom_m 0]

can you not extend the cable mount point away from the pivot, giving yourself a longer lever and making your throttle openings smaller for the same amount of pedal travel?

[Ahl 4]

can you not extend the cable mount point away from the pivot, giving yourself a longer lever and making your throttle openings smaller for the same amount of pedal travel?

Yeah, what I don't understand about bike bodies is how this ramp angle can be steeper. The spindel on my 1.9 is attached directly to the throttle plate. How do bike bodies differ?

[Sandy 191]

If you have enough resolution in the map, you can back off the ignition advance at the inital sites to de-power the engine deliberately and make it naturally drive over more loads sites. You can then trim the ignition and fuelling to make a more progressive transition up to half throttle.

 

This is advanced mapping though and will require a great deal of time and persistance to get right!

[pugrallye 0]

what about just fitting a larger cam? surely they still use a sqaure section end like weber carbs etc??

[Dream Weaver 2]

Thanks all, it confuses the hell out of me!!

 

The ramp angle thing was mentioned by Wayne Schofield (Chipwiazrds) on another forum at some point, but when you look at the bodies (GSXR btw), the butterflies are all vertical, and they look exactly like a Jenvey or KMS setup, so i'm not sure what the difference is, unless it means the throttle cam???

 

The cam could be swapped, but I need to check how its fitted in place, and also how much room there is as its quite close to the filter base plate.

 

M3Evo - that idea sounds interesting, but cant get my head round it from your drawing.

 

AHL - agreed, I dont get it either - before I had the bodies the old cable was "loosely" fitted to the std throttle body and it was fine.

 

Sandy - good idea, but that's kind of masking the problem, which is mainly that the actual pedal is very stiff to start off.

 

Think its time to start playing about with the cams, may try my old std cam on it if poss.

 

Any other ideas chuck them in the pot

[Ahl 4]

There must be something funny about the way the throttle cam turns in relation to the butterflies in that its not linear, but, er, inversely progressive or something.

 

Can anyone confirm how it works?

[Sandy 191]

Sandy - good idea, but that's kind of masking the problem, which is mainly that the actual pedal is very stiff to start off.

You hadn't said the pedal was stiff, i'd read it as part throttle transition problem, which sounds like it might be a factor as well.

[M3Evo 0]

The basic premise is that the black lever arm is nearish to vertical when the throttles are shut.

 

When you start to press the pedal down, the end of the lever is moving more forwards than upwards, again piccy paints a thousand of 'em:

 

Each graduation at the lower arm is 10 degrees. you can see that the rate of movement of the upper arm is not linear

 

Edited June 14, 2006 by M3Evo

[Richie-Van-GTi 71 2 Cars]

I think the easiest solution would be to extend the butterfly arm that runs through thew bodies and attach a standard cable cam of a gti TB on the end, this will give the same movement as a normal gti then as its matched to the pedal travel. You can then leave the bike cam in place with spring to return the butterflies to closed. Should be easier than tryingtoadjust the ramp angle on standard cable cam as access is quite restrictive.

[charlie_sav 0]

I have the same issue with bike T/B's as said it seems to be the cam on the throttle bodies is too small, very small pedal movment and it's off, also gone through two throttle pots, don't know if my spindle is slightly bent but they seem to bugger up and start sticking,

[Rob_the_Sparky 9]

Can anyone post a photo of the throttle set-up on bike TBs? I have a fair idea what the problem is and think M3EVO is probably correct if it what I think is going on.

 

On an 8v 1cm of cable travel = #degrees of throttle plate movement where # is a constant.

 

Seems that on a bike TB # is variable and larger at low throttle openings (which would be a pig to drive)

 

On many cars this is reversed (like the Mi) by having a smaller throttle open first to make them easier to drive. The key will be how the cable attaches to the spindle unless there is some complex linkage system. I'd expect to se something simlar to the 8v connection but with a varying diameter...

 

There are a few bikers at work who might know so will have a chat if I can.

 

Rob

 

P.S. The cam in question I think is the one where the throttle cable attaches, a constant diameter on an 8v but to produce the effect described it would be shaped like a cam lobe.

[Dream Weaver 2]

Its quite oval on the bike bodies, i'll try and find a pic now.

 

Here we go, the read transparent bit is the stuff I had to remove so the filter baseplate would fit on:

 

 

And here is one of the cam as fitted with the throttle cable - it just uses a Weber throttle adjuster thing.

 

[Rob_the_Sparky 9]

The red bits in the first shot look like a manual choke as they would hold the throttle slightly open.

 

The pulley is the sort of shape expected but not quite! For a fixed length of cable a larger diameter will result is a slower opening. Looking at the shot above I'd expect the early opening to be slower, mid opening to be fast and slower again towards fully open. E.g. pretty much what you want.

 

Only way I can think of getting finer control is by using a larger pulley or by employing some sort of lever system, as described by M3EVO...

 

Rob

 

P.S. According to my biker mate at work they are generally adjustable so it is possible that different pulleys are available at bike shops but I can't guarantee that.

[Alan_M 66 1 Cars]

Could'nt you swop over the pulley/cam for another, say for example off an 8v or a defunct Mi16 TB?

[Dream Weaver 2]

Pretty much resolved this issue today

 

Pulled the throttle cam off, and swapped it for the old cam off my std throttle body. It needed cutting down to clear the filter, but seemed to fit fine and pretty much bolted straight on.

 

Its a hundred times better now, and much easier to drive on light load - still not 100% perfect but so much nicer than it was on the std GSXR cam

 

Just the popping exhaust to sort now before the mapping session is booked.