My cafe sport 90, stock gearing and picked up top end response

[buddy rawls]

I have not been able to really get decent top end speed out of my Sport 90.  Looking to increase top end, I contemplated a 1 tooth larger front sprocket swap.  but first, I needed to varify what I actually had on the bike.  As it turns out, the front sprocket was actually one tooth larger (according to the B/S parts books), I began retracing the steps of powerband rpm and shift recovery and my automotive racing experience.  What my intuition told me was that I needed to go to higher gearing (lower ratio) to bring the rpms up in 4th gear.  The motor is not strong enough to overcome wind load/speed with the current gearing, so I needed to increase some rpm to gain some response.  I could get 40-45 in 3rd, but when in 4th there was simply no acceleration and recovery, even 50-55 was serious struggle

I contacted Richard to see if he had the stock gear; which he did.  After installing the 14T front sprocket (back to the supposed stock gear ratio), I was greeted with an easy 45-50 mph cruising speed, and acceleration up to 55-57 mph, less drop off on hills and wind, and I honestly think I can get a 60mph max top end.  And this is with a heavier dose of oil in the tank than called for.  I run 16:1, as opposed to the prescribed 20:1. But I may try it out.

Basically lowering the gear, enabled me to get to a better portion of the powerband and actually find more top end.  Increasing the gearing can provide more top end ONLY if the motor can get to the rpms it needs.  If it can't, then you may have less top end.  1 tooth (up front,  or approx 3 on the back) really made a difference.  

Now I am happy.  I cannot ask for more speed than it was originally quoted at, but atleast, I am there.  and heck, I weigh 200lbs.  

[disc_valve]

Hi,

Sounds a familiar story. Someone had fitted a 15T BS100 Sport front sprocket, 1 tooth larger than the BS90.

 I had the same problem wihen I rebuilt my 100TMX using a donor 100 Sport motor for parts. I fitted the 100 Sport sprocket, assuming that alkl 100cc motors used the same gearing. I found that my TMX went like hell in 3rd (50+ mph) but wouldn't pull below 40mph in 4th and topped out about 55. Having fitted a new top end, I just assumed the engine was a bit tight - until, that is, I checked the parts book and found the 100TMX should have had the 14T BS90 sprocket.

I fitted the correct sprocket and the bike was transformed - it now only does 48mph in third, but runs on to 65 in top.

All because of a Tooth!

Graham Weeks

[buddy rawls]

the difference in 1 tooth on the front is about 7%.  That can be substantial, at times.  What really got me thinking was my car racing and camshaft experience.  In the stuff I work with, the gearing is utilized to place the powerband most effectively to obtain the most power to the track within the lap (be it drag or course).  the problem is that I never considered what happens on an extremely low powered set-up when the vehicle itself may struggle to obtain its top speed.  In cars, the top speed that this same situation occurs is way beyond the speed obtained in racing.  Or if it is a street performance situation, the max top end is far beyond the driven speed.  However, what if the car was attempting to run its all out fastest top end when wind drag versus hp becomes an issue, as in salt flats racing.  SO I began rethinking.

The small 2-stroke is quite peaky.  In an under powered situation, it could become "mis-timed" with the needed acceleration or sustained speed.  So I tried lowering the gearing (in this case, back to stock), and sure enough it worked.  I have been able to get a little more top end, BUT with a far better cruise speed and hill/wind recovery or sustaining speed.

I am not wanting this thing to be able to freakin blast down the road, but I want it to atleast be able to carry a 45-52 cruise easily, and I think its about there.  Its a fine line between wind drag, top speed, and power delivery when the vehicle is essentially underpowered for normal "modern'" driving speeds.

The next thing I am working is going to be a readjustment of the valve evnts (or rotary valve) using my own cam calcs I use for my camshaft stuff.  Not looking for the full out documented B/S 90 race build, but rather some slight edge in the current rpm band.  I am likely looking at simply an earlier opening (~7 deg) and a very slight later closing (~3 deg), on the rotary plate.