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MikeHolland · Posted: Sat Jul 28, 2007 12:22 am Post subject: Dr.Pulley sliders

In response to my introduction I was asked about Dr.Pulley sliders. So I have started a tuning topic for them.

First, the theory. I won't go into how a variator works, as that has been covered in dozens of topics. But the important fact to grasp is that rollers don't necessarily roll. They are a bad design. The two surfaces (half-pulleys) that the roller is clamped between are angled, and move closer or further apart as engine revs change, with the result that the rollers move towards or away from the centre. If the roller rolls on one surface, then it has to skid on the other. And the result of this skidding is the wearing of flat spots.

Dr.Pulley sliders are not round. They are a sort-of five sided shape which provides flats for the working surfaces, so that there is a much larger working area, and consequently less wear. They are also made of special slippery material to reduce wear. So they should have a much longer life.
Their design makes much more sense than rollers do in a variator.

But additional claims are made for them, that they give a wider range of ratios by being able to slide further in each direction. I am not yet convinced of this but my reasoning gets rather technical. Skip the next paragraph if you don't already understand variators.

If you assemble your variator and hold it in your hand, you will find that the rollers will rattle a little because there is metal-to-metal contact between the half-pulley and the ramp plate. The plates cannot go further than this, and so the sliders cannot extend the ratio in this direction - towards better take-off acceleration. I don't know whether they can move further out, giving a higher top speed, but due to their wedge shape this is quite possible. But this in turn depends on the half-pulleys at the clutch end being able to move further apart, so that the belt moves further in. So I remain unconvinced.

Now for my experience. I fitted 8.5 gram sliders, replacing 12 gram rollers. The result was terrific acceleration off the mark up to about 90kph. But I cannot say how much (if any) of this was due to the design of the sliders and how much to the lower weight. I suspect the latter. There was no drop in my top speed, but my GT has always been a slow one and with all my kitting still has a top speed of only 120kph (130 downhill).

Unfortunately, the saga does not end there. After a week of enjoying this lovely acceleration, the variator came loose on the shaft, and did a little damage to the inside of the transmission cover. And I had to wait ten weeks for a spare part. When I got my scooter back on the road, the acceleration was gone. Felt like I was taking off in third gear! Either the clutch was taking too soon, or the engine was failing to get up revs. I have given the scoot a complete service, cleaned the carb, dusted the clutch, checked the operation of the petrol pump that operates when the throttle is opened. But I cannot find the reason. Probably something to do with the scoot sitting idle for ten weeks. Any suggestions welcome.

So, do I recommend the sliders? Well, they cost a little more than rollers, but will probably last at least three times as long. For performance, you will have to try them. But try the same weight as your rollers and you will end up better informed than I am. And make sure you tighten that variator really tight.

Hope this hasn't been too long-winded.

Mike
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Tinman · Posted: Sat Jul 28, 2007 9:20 am Post subject:

Thanks Mike, and good luck with getting your performance back. Confused

Innocenti · Posted: Sat Jul 28, 2007 9:59 am Post subject:

im tempted to try them to see if there is any difference Wink

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MikeHolland · Posted: Sat Aug 04, 2007 2:08 am Post subject:

Ok, several of you have said you plan to try them. I hope you don't do what I did, and change the weight at the same time, so that you can't tell what difference they make!

BUT a word of warning. The three ramp plate guides that come with the sliders are too loose. They have an 8mm gap where they fit onto the plate, and it should be 6mm, with the resultthat they wear out very quickly. So chuck them out, and keep the original ones.

Mike
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Vespa GTS250 Vintage Red