Viper Clutching RPM, shift rate and tach recall

[TD Max]

I’d like to start by thanking Straightline for their excellent lake ice run video showing the tachometer and speedometer. The reference to their video in this post is designed to be analytical and not critical of their work. https://www.youtube.com/watch?v=cLQ...3916&feature=player_embedded&x-yt-cl=85027636




There has been a lot of talk about using the tach recall function to gauge what your clutching RPM is. I tend to disagree with this method. What this method is doing is catching any sort of spike that may occur be it on a launch, or at the climb at the end of the shift on a top speed run. It always seems to come back significantly higher than the normal observed numbers. It is hard to watch the tach while driving. When running various clutching setups if my sled runs in the 8600 range I will often see recalls on the order of 8900. When it runs in the 8800 range I will see recalls on the order of 9100.



Watching the Straightline video I see a characteristic that I have seen in my sled as well. If you watch in the early to mid part of the run, you’ll see RPM come up, pull back, come up, pull back then come up again. This is on the order of 8900, 8360, 8800, 8400, 8800 then it seems to settle in and run in the 8800-8900 range before eventually climbing to 9100 range. To my novice eye what I am seeing is that during the 8400-8800 oscillation either the clutch is under/over shifting, or there is a belt slip of some sort going on or both? Once past this window in the 40-60 mph range the RPM stabilizes quite a bit. Then as it crosses 100 mph and we hit the end of the shift, the RPM stabilizes even further while it still climbs steadily to peak. To me the sled is clutched to the 8800-8900 range. My guess is that a recall with be on the order of 9200+




I see the same oscillation after launch with my sled. Then the RPM stabilizes somewhat and tends to hold in this window. What I don’t see with my sled is the RPM climb at the end of the shift. I suspect this will come as the break in continues and as I find better conditions with less drag.



So my questions:


What is causing the RPM fluctuations, particularly the 40-60 mph ones? I’ve never ran the stock clutching to see what the MPH was, but it seems that this is a tamed down version of what the stock clutching does when it hits the limiter shortly after a launch.


Why is it so hard to keep the RPM in a nice narrow window like a typical 2 stroke where you would expect to see the needle of a tach hidden behind a narrow strip of tape?

 

[superb]

Exactly right!! When you clutch a Apex it pulls 10500 all the way through and finishes at 10800. These Vipers the rpm's are up and down.

 

[NOS-PRO]

Gotta love digital tachs.....LOL

Might be investing in a playback tach. Use them all the time on my other race sleds....just didn't know how to hook one up till now.

 

[TD Max]

Gotta love digital tachs.....LOL

Might be investing in a playback tach. Use them all the time on my other race sleds....just didn't know how to hook one up till now.

So we can expect to see a 5" Auto Meter Pro Comp with memory playback in a temporary mount on the bar pad of your sled?

 

[NOS-PRO]

Exhaust Gas Technologies full playback tach.

Right on the bars

 

[TD Max]

Boy if you could get them to build as a 10k dual range...

Bury the 0-4 or5k at the bottom then spread the rest of the scale.

 

[shagman]

Gotta love digital tachs.....LOL

Might be investing in a playback tach. Use them all the time on my other race sleds....just didn't know how to hook one up till now.

Terry you can record with your POD 300 on your dash. And send to a lap top. I haven't had time to play with mine yet.

 

[stingray719]

I’d like to start by thanking Straightline for their excellent lake ice run video showing the tachometer and speedometer. The reference to their video in this post is designed to be analytical and not critical of their work. https://www.youtube.com/watch?v=cLQ...3916&feature=player_embedded&x-yt-cl=85027636




There has been a lot of talk about using the tach recall function to gauge what your clutching RPM is. I tend to disagree with this method. What this method is doing is catching any sort of spike that may occur be it on a launch, or at the climb at the end of the shift on a top speed run. It always seems to come back significantly higher than the normal observed numbers. It is hard to watch the tach while driving. When running various clutching setups if my sled runs in the 8600 range I will often see recalls on the order of 8900. When it runs in the 8800 range I will see recalls on the order of 9100.



Watching the Straightline video I see a characteristic that I have seen in my sled as well. If you watch in the early to mid part of the run, you’ll see RPM come up, pull back, come up, pull back then come up again. This is on the order of 8900, 8360, 8800, 8400, 8800 then it seems to settle in and run in the 8800-8900 range before eventually climbing to 9100 range. To my novice eye what I am seeing is that during the 8400-8800 oscillation either the clutch is under/over shifting, or there is a belt slip of some sort going on or both? Once past this window in the 40-60 mph range the RPM stabilizes quite a bit. Then as it crosses 100 mph and we hit the end of the shift, the RPM stabilizes even further while it still climbs steadily to peak. To me the sled is clutched to the 8800-8900 range. My guess is that a recall with be on the order of 9200+




I see the same oscillation after launch with my sled. Then the RPM stabilizes somewhat and tends to hold in this window. What I don’t see with my sled is the RPM climb at the end of the shift. I suspect this will come as the break in continues and as I find better conditions with less drag.



So my questions:


What is causing the RPM fluctuations, particularly the 40-60 mph ones? I’ve never ran the stock clutching to see what the MPH was, but it seems that this is a tamed down version of what the stock clutching does when it hits the limiter shortly after a launch.


Why is it so hard to keep the RPM in a nice narrow window like a typical 2 stroke where you would expect to see the needle of a tach hidden behind a narrow strip of tape?

I know you have had some issues and to be honest I am sorry you did. Both my Vipers have a good solid shift without rpm hunting.

 

[Studroes144]

Ya I must say I have no big rpm jumps like that at all, this year with the turbo or last year all stock. The only change I have in rpm is when conditions change, on perfect hard packed it'll run right in the 8900-9000 range, out in loose snow it runs 8700-8800, but it's consistent and has no big drops in the rpm range. The more and more I've adjusted mine the more I've realized that one of the best forms of tuning is your ear. Watching the tach is good, but that digital tach as we all no doesn't register as good as it could. After owning my viper for over a year I can tell when it's running at peak rpm and when it's running a bit low, you can also easily tell if there's drops in rpm where it shifts too fast and then falls off. Nothing at all against the video shown, but those drops in rpm are not suppose to happen and if they do they def shouldn't be that drastic, especially on ice where there is no resistance.

 

[ViperBill]

flyweight clutches were origonally designed for the narrow power band of a 2 stroke. The torque of these engines seems to overpower the the clutching at lower shift ratios then they catch up and overshift and pull down rpm and then stabalize. I have had good luck with leveraged weights (currently running drag n flys) as they seem to react quicker and really flatten out the shift curve and match the power curve of this engine. I may be wrong but they really work great. it comes up on rpm quickley and holds rpm on snow as well as ice regaardless of condition. It may run slower in snow speedwise but rpm holds at 9000 all the time. just what ive seen

 

[stingray719]

RPM hunting is almost always in the secondary clutch, if the weights have an even profile that is.

 

[scott32]

What helix are they running? And what are you? A good spread in helix angles and you will see a noticable rpm change during shift but a closer angles make this less noticeable and a straight not at all. A radial multi-angle helix shifts smoother and will help the shift patter of a the angle act more like a straight helix.

 

[NOS-PRO]

Terry you can record with your POD 300 on your dash. And send to a lap top. I haven't had time to play with mine yet.

I need to get my software working so I can download my runs

 

[Blue Dave]

With my COTC set up I have very little "rpm hunting". When racing from a dead stop it launches hard at around 8700 RPM and climbs steadily to around 9,000 rpm topping out at around 100 mph.