Pursuing Maximum Velocity

I'm trying to push this little bike to maximum potential velocity in a generally stock scenario. As a refresher, I've got an '81 C70 with the Dr. ATV Long Duration Power Cam (XR70 grind I believe) and the 88cc jug and piston. Running stock carb, exhaust and intake.

I had been running a 15x37 setup, but just recently switched back to a 15x36 which is the standard longer geared setup. The bike seems much happier here than with the 15x37, but I still cannot get it to crack the 40 mph mark. I'm actually becoming suspicious that my speedo will not move past 40 as I really let it wind out on a long downhill today and couldn't get it past the 40. At this point its not even about going faster or gaining more power, the 88cc setup with the 15x36 is a really nice kit for in-city riding. I have room to go from 35-40mph in a reasonably short time frame (i.e. I can pass people), and I've got good torque in 3rd on all but the worst hills. The difference from the 37 rear to the 36 rear is notable, I haven't really given up any power but it just seems to roll along at a smoother clip.

Am I fooling myself in thinking I can hit 45 or even 42? At this point its an exercise in what I can make the bike do and still be tractable in the city. Should I switch to a 16T front? Or keep the 15 front and go to a 34T rear? I know the answer is to just try it, but swapping out the rear is a particular pain.

Thoughts? I know these are even beyond the normal 1st world problems, but I'm fixated on crossing the 40MPH threshold. Tomorrow I'm going to test my speed with a speedo app on my phone just in case there is a problem with the speedometer itself. but I know I'm just being ridiculous in that assumption.

Hal (pursuing my age in MPH) Landvoigt

Try the speedo app and post the results. If you do not like the idea of a slightly bigger turn out intake and a k&n air filter, no clue if there is a better flowing exhaust available or if that particular exhaust is too restrictive, ie the baffling system in it? You may have to deal with the top speed it currently is running. I was able to hit 52-53 with a 88kit 19mm carb free flow filter and I slightly opened up the last baffle 10 inches inside the K0 exhaust on a CT70..

With an 88 kit you should be over 45 mph!

I regularly hit 42 mph with a 15/33T set up on my stock 1991' with the 72cc engine.

IMO, there's something wrong with the speedometer.

I hit 44 today according to the speedo app. The feedback speed sign said 46. So at least its not the bike, its the speedo.

Hal

Those are more reasonable numbers. None of these mechanical speedometers are linear, in terms of indicated speeds, across their ranges. They tend to be most accurate in the middle of the dial..."generous" at the lower end, "stingy" at the high end. There's a ton of variation between models & years...as well as individual specimens. I've "found" as much as 7-10mph, at the top end, simply by swapping speedometers, on the same bike...no other changes. Anything under 5% error is good accuracy; 10% is not ultra-rare and is a lot...especially when indicated speed is slower than actual. Anything beyond 10% is shite, I wouldn't put up with that. And, there's also the possibility of input error. First things I'd do, 1.) ride a circuit of known distance, to verify odometer reading 2.) "pace" the bike with GPS (which can be off, too, but should usually be within a few mph of actual) and another vehicle. Automotive speedometers are usually pretty accurate; even 40 years the D.O.T. requirement was no more than +/-3mph, at any speed. If the odometer reading is accurate, and they usually are, you can probably just source another speedometer, from another Honda. Go for something calibrated to a top speed that's 10-20mph beyond your machines capability.

I've had a number of longterm studies going. #1 on that list is gauging...including speedometer, odometer, tripmeter, tach, fuel...and how that affects the riding experience. After decades, I am now 100% convinced that most of us are too focused on the quantitative side, while 99.9% of the riding/ownership experience is qualitative. There is a simple reason for this, numbers (hp, mph, dollars) don't invite much verbiage; that makes for short answers...and a lot of problems. Quick answers are oftentimes incomplete answers and those gaps lead to assumptions. It's not that all assumptions are wrong, or even invalid; they form the basis of trial & error testing, which leads to improvement. That is, unless one stops too early in that process, in which case "assumption is the mother of all f***ups". That's not meant as an insult, btw. What I am saying is, look at all the frustration you've put yourself through looking for mph you thought you weren't getting, due faulty assumptions.

Time for a reality check. Based on top speed for a bone-stock 72cc Honda, of 43-47mph, depending upon model, year, and resource you consult, for the most part, we're all talking about 5-15mph above stock. I don't want to go too far down this road, the point is that the statement "it's sad how much one will feverishly invest in the pursuit of another 5-10mph" really fits. The reason should be obvious, it's a simple measurement of what, too often is considered "success". A speedometer/odometer can be a powerful psychological tool; consider devices such as "Fitbit", or even a simple pedometer. Now, consider actual speed, indicated speed, and perceived speed. Not having a working speedometer/odometer doesn't really cut it, for a few reasons. That said, without a speedometer, do you really think you could accurately tell how fast you're going, when keeping pace with traffic? With rare exception, the best of all possible worlds is a speedometer that reads 2-5% fast...that's a whopping 1.0-2.5mph error at 50mph. Your speedometer error is at least 12% and look how long it took you to discover that fact. The happiest solution would be an electronic, programmable, speedometer. That can be calibrated to within 1% accuracy with nothing more than a measuring tape and a calculator. Consider the impact on your riding experience...

racerx said:

After decades, I am now 100% convinced that most of us are too focused on the quantitative side, while 99.9% of the riding/ownership experiequalitative. There is a simple reason for this, numbers (hp, mph, dollars) don't invite much verbiage; that makes for short answers...and a lot of problems.

Click to expand...

As usual you always have very solid advice, in this case more about the experience than the facts and figures. I can tell you with the latest set of changes (going from 15x37 to 15x36 and reducing the main jet from 96 to 94) has made this bike into an absolute dream to ride. The reduction in vibration from the longer gearing is extremely significant as is the small change in the jetting. Even if I'm only going 4MPH faster than I thought, it feels like and rides like I picked up another 5HP and am going 15MPH faster.

I agree with you that sometimes we forget about the goal and focus on the numbers, for me the goal has just been to try to squeeze out the maximum performance using only cheap and easily bolted on parts. I feel like I'm finally there, at least until I get the itch to change things again. maybe switch to a 16x36???

Hal

whereshaldo said:

As usual you always have very solid advice, in this case more about the experience than the facts and figures. I can tell you with the latest set of changes (going from 15x37 to 15x36 and reducing the main jet from 96 to 94) has made this bike into an absolute dream to ride. The reduction in vibration from the longer gearing is extremely significant as is the small change in the jetting. Even if I'm only going 4MPH faster than I thought, it feels like and rides like I picked up another 5HP and am going 15MPH faster.

I agree with you that sometimes we forget about the goal and focus on the numbers, for me the goal has just been to try to squeeze out the maximum performance using only cheap and easily bolted on parts. I feel like I'm finally there, at least until I get the itch to change things again. maybe switch to a 16x36???

Hal

Click to expand...

You get it, that's half the proverbial battle..."the big 50%" I've been into serious road riding on these little Hondas longer than all but a couple of others and can only think of one other US-based owner-rider-customizer who has worked, steadily, over the past 2 decades to extend the capabilities...and define just what a successful project means, in more than one context. It is impossible to build a single bike that can do everything, so step one is setting a goal...you've done that and the project has pretty well dictated itself. Once you've gotten the jetting & gearing dialed-in, that's it, there's no further improvement to be had, without changing your goal(s). Funny (peculiar, not ha-ha) thing is that it's largely the same with 50, 60, 70mph+ bikes, too. Oh, they'll cover more ground per unit of time. But, they all have their optimal configurations and none of them, even the fastest, is really all that fast. A 100mph small Honda would be a death trap...and still not fast enough to cover a flying mile as quickly as a Honda Fit(!). And this is where you've realized an "aha moment"...it's all about the journey, not reaching the destination as quickly as possible. Now, within this framework, you're seeing for yourself how ostensibly small changes can have a big, cumulative, effect, from the saddle...and it sounds to me like you've found some of those elusive mechanical sweet spots. As ridiculous as it might sound to some, there's artistry to be found in mechanical perfection...when that means successfully reaching your goal. A machine that has balance and delivers the goods reliably, has a beauty of its own.

I began with a different goal...which changed along the way. Ultimately, I returned to the goal I set better than 45 years ago. (I mostly stepped away from bikes from the late `70s till the late `90s) Circa 2001, I was set on building an 80mph CT70...for a daily rider. That machine turned out beautifully, it even won show awards. It has very few miles on its odometer. What was originally intended to be a "beater" bike, has covered close to 26000 miles, 96% of which are on one motor. 60-65mph top speed (under favorable conditions) isn't even noteworthy anymore; it doesn't matter. Being able to cover 200-300 miles in an afternoon, comfortably & competently...that's what keeps the experience fun, over the longterm. Getting to that point is, essentially, the same process you've followed...only the scope is different - more time, money, and expertise thrown at it. This isn't for everyone.; few will ever take things this far. Still, I've gained a lot of insights and some of them could be useful, on a piecemeal basis, to a lot of riders. Sounds like a few of those have helped you.