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The changes made by the super stiff contra spring are a wash and choosing to change it will depend on your intended use of the scooter. The big advantage is increased 5-40 acceleration. I haven't done any timed runs in this range but the spring makes the trans instantly down shift over 1,500 rpm when cruising at 20 mph and gunning it. With the initial increase in speed that is also occurring, the rpms jump right from 5,000 to 6,800 and continue to wind up to 7,400 by 40 mph. The 0-50 times are down some to between 12.5 and 11.8. call it 12.2 which is .5 seconds lower than before the spring but this maybe too broad a test to really show what is happening between any rolling speed and 40 mph. The problem with 0-50 as a test is that the launch is very slow. There is a programmed delay in the engines output on take off. I haven't timed it but it is at least half a second. Maybe more. No changes will ever effect this unless you get into a replacement fuel ECU or something. So this is always dead data right in the front of the timed run. The next issue with using 0-50 as a yard stick for playing with the trans is that by 50 you are close to max gearing and will be near the power peak so changes in 40-50 times will be very much less affected by the small amount of gearing change that is left at the top of the gearing. 50 mph used to run about 7,000 under load and now with the spring it runs at 7,400 as it is still getting 400 rpm of kick down under high load. It used to be much less active with only about 150 rpm of change between light cruise and gunning it. The change in gearing (as indicated by the change in rpm for any given speed) is much less between 40-50 than it is between 5-40 mph. But by far the longest portion of the run is from 40-50 which isn't going to change much regardless of what you do to the trans operating points. What I am trying to say is that most of the .5 second improvement from the stiff spring is between 5-40 mph at which it becomes pretty significant. This is more beneficial to shooting through city traffic than climbing a hill on the highway. For blasting around in the city, having a stiff spring to spool up quickly and launch you out of corners is really fun.
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But in just a day of riding I can see several down sides to a super stiff contra spring. There is an obvious increase in the running losses in the belt. I think the increased compression on the sides of the belt, and the increased tension, are creating added friction and heat losses. The loaded hill climb speeds are down a percent to 49 mph. Top speed is often way down to the 63 mph range. Some of this my be due to the tighter belt riding down lower in the front pulley but I am also sometimes not making it to redline, indicating some loss somewhere. Fuel economy is also down 6% to 90 mpgUS. Partly due to higher rpms everywhere but I think also due to increased friction losses in the belt. Maybe a stiffer Kevlar belt could give some of this back but for highway commuters it looks like the light factory spring with lighter sliders might be the better option. An after market torque pulley with more aggressive slots could give a similar amount of kickdown using the stock spring as what the stiff spring gives now, without increasing the losses in the belt, but I haven't seen one for less than $160 and I am not ready to go there.
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