Throttle twist rate...

Hi All-

A bit of an absurd/arcane question here, but I'm wondering if anyone happens to know the twist rate of a typical late 70's airhead throttle. If a specific model is wanted/needed, let's use an R80/7 as our example.

I ask, because I am working on an R50/2 conversion (using an R75/6 engine) and will most likely be using an aftermarket throttle assembly (for a wide variety of reasons I won't bore you with here), and want to compare aftermarket options against the traditional rate. I have always generally liked the throttle rate on old airheads, but want something maybe just a little bit quicker for this conversion, and so need the traditional baseline as a comparison.

Thanks in advance!

ECJ

Isn't the rate as fast as you can rotate your wrist! Does this have to do with how much the carb butterfly moves for each movement of the grip? Note that the /6 engine throttle isn't directly connected to the carb slide...it opens/closes the butterfly downstream of the slide. I'm pretty sure BMW made the movement such that you could finer control at low throttle openings like coming away from a stop and then it got coarser when running on wide open throttle. Can't say as I ever seen any specs for this.

The Bing manual has a small diagram showing the amount of throttle twist as it relates to which jet the carb is operating on. It's a little complicated:

- throttle 0 to 25% -- idle jet is in operation
- throttle 15 to 80% -- needle jet is in operation
- throttle 60 to 100% -- main jet is in play

As can be seen, there's overlap with throttle and various jets.

throttle rate

ecjjce said:

Hi All-

A bit of an absurd/arcane question here, but I'm wondering if anyone happens to know the twist rate of a typical late 70's airhead throttle. If a specific model is wanted/needed, let's use an R80/7 as our example.

I ask, because I am working on an R50/2 conversion (using an R75/6 engine) and will most likely be using an aftermarket throttle assembly (for a wide variety of reasons I won't bore you with here), and want to compare aftermarket options against the traditional rate. I have always generally liked the throttle rate on old airheads, but want something maybe just a little bit quicker for this conversion, and so need the traditional baseline as a comparison.

Thanks in advance!

ECJ

Click to expand...

Not sure about 1974 /6 models, but the R90s (mine) has a full 1/2 turn from idle to full throttle on the stock Dellorto's. I believe that changed in 1975 to a 1/4 turn rate.

I think what complicates this is that there is a non-linear aspect to the throttle. So, stop-to-stop might be something easily measured, there are variations that as the throttle is twisted. Snowbum mentions that in the Section: Why the complicated throttle assembly design?

https://bmwmotorcycletech.info/controlcables.htm

Kurt-

You are exactly correct. Thanks for pointing this out. It is a non-linear control, set by the cam. This is definitely at least part of what makes airhead throttle responses so appealing to me (especially at take off and slower speeds), and also answers the question as to why there will be no specified 'twist rate' (usually measured in degrees of movement for every millimeter of rotation, e.g. 3.4°/mm), like you see on most modern throttles.

I know that Motion Pro makes at least one throttle whereby you can change the twist, or quickness rate, by inserting different 'control discs' in the assembly. It even includes a non-linear disc (or perhaps two), and so may be worth my looking into.

Appreciate all of the answers and perspectives!

ECJ

Report back once you've had a chance to check things out. Having the ability to swap out control parts could help recreate what the Airhead throttle does.

Only certain models have a cam that makes them non-linear. It's shown on my website.

Thanks, Anton-

From your article (http://largiader.com/articles/throttles/) it would seem that all of the throttles whose feel I like are non-linear. Thanks for directing me to it.

I'll report back later with what I ultimately set up in case anyone proves interested.

Thanks to all,

ECJ