Post by lobarrett on Mar 3, 2014 21:24:49 GMT -5
CVT Operational Theory – do I have it right.
I ran across an article several months ago on the Polini official website, titled “VARIATOR WORKBOOK” – It was billed as a first time tuners guild for setting up their automatic transmission. The above “VARIATOR WORKBOOK” is linked to the English translation of the document.
I always have issues reading and understanding translated documents – so I did not pay to much attention to it. After all, I all ready had a ton of information on the subject and there wasn’t any point in trying to understand the “poorly” translated Polini document.
So, after putting all the information together from my other sources and for the most part thinking I had successfully “decoding” the magical world of the CVT into a theory of operation – I decided to compare my “new” knowledge with the “true” but somewhat cryptic knowledge outlined in the Polini document. I thought from this, I could reorganize the Polini document into something more logical and hopefully more understandable. In the end, I wanted to say, “I got it - right.” That’s when the “shit” hit the fan – I didn’t get it - right. My decoding of the mysteries surrounding the CVT appears to be wrong.
At this point in time, I am not is a position to debate the gospels of the CVT according to Polini. They certainly understand it more than I do. In fact, the Polini doctrine agrees with the majority of the information shared by several members of 49ccScoot.com. And, I did believe and I had faith in this information – I understood, it’s just that somewhere along the path – my thinking got screwed up.
Guess you could say, it is time to get back on the path. This time, I thought I would try to put what I think I know into one document and post it to the forum in the hope members would read it and advise me if I am still missing the point - if you got any advise on the subject, I am ready to receive it.
I will start by recapping what I think the Polini “VARIATOR WORKBOOK” is saying.
According to Polini, the functioning of a CVT can be theoretically divided in four well defined phases:
1) Section A-B neutral or standing scooter (short gear) phase,
2) Section B-C clutch engagement/skidding (short gear) phase,
3) Section C-D variator ratio change (shortest gear to the longer gear) phase, and
4) Section D-E extension phase.
I didn’t feel comfortable copying Polini’s document (copyrights) as part of this posting, but I did copy the primary graphic that has blown my CVT theory all to hell. I than redrew the graphic and added the verbiage Polini had used to describe each phase or sections. Both are copied below.
My Graphic including Poilin’s Verbiage.
I have a little bit of an issue with section B-C, “the clutch engages progressively “skidding” till it reaches point C @ 7000 rpm.” The word “progressively” doesn’t seem to fit. But that might be a translation issue. And, in fact the clutch may start out “skidding” then progressively decline until it reaches point C @ 7000 rpm. The other issue I have with section B-C is the length of time the clutch “skidding” takes place – 0 to 15mph seems like a long time. However, this maybe a condition that is indicative of a stock clutch like the one that was originally installed on my 50cc Piaggio engine. I would think that a performance clutch like the Malossi Adjustable Delta Clutch could drastically reduce the time it take for the clutch to fully engage.
At this time, I don’t have any issues with section C-D, having the maximum torque available and the CVT in a shortest gear position is a major plus. The front pulley in the lowest position and the rear pulley in the maximum position would ultimately give me more torque at my rear wheels at the time I need it most.
I then compared the above with a document put together by HGT titled, “Rollers and Springs... How to keep the balance” and the accompanying video titled, “Variator transition”.
According to HGT, “To get the best performance from a constant velocity transmission (CVT) based two stroke scooter such as the SR50 DiTech, the optimum RPM band must be maintained throughout the entire transition of the variator as seen in this video”.
Time sequence of the video:
0 to :17 Malossi Logo,
:17 to :25 Section A-B speed 0, standing scooter, short gear position,
:25 to :26 Section B-C clutch engagement, progressive “skidding” until max rpm,
:26 to :32 Section C-D clutch is fully engaged, the engine is at max torque and the variator starts changing ratios from the shortest gear to the longer gear (1:1 ratio),
:32 to :42 Section D-E the variator has reached the longer running position and maintains it – the engine rpm starts increasing up to max.
There seems to be a correlation between the video and the Polini document. The :25 to :26 Section B-C seems shorter than Polini Section B-C. I am still a little confused about :32 to :42 Section D-E, I thought this was called the Over Revving section.
HGT write up is pretty good – they do not really discuss the video – so I am doing the correlation between the two sources by sight and sound. It sounds like the engine goes to max rpm almost instantly after the clutch engages, and it seems to maintain it.
Anyway, that’s where I am at in my thinking. I will have to go back an run my torque, velocity, and acceleration calculations all over again, but maybe I have it right this time.
If you disagree or have any other comments about all of this, please let me know.