Simple (and practical) streamlining?

[woodsrat]

Last night I left work on my Lifan 125-powered Honda C-70 Passport and in a hurry to get home to watch the Isle of Man TT races decided to risk life and limb and take the loop interstate home (55 MPH speed limit). I got the bike up to a "normal" fast cruising speed of around 50 and decided to tuck in and see how much that helped. Amazingly despite the CRF-80 fender, floppy jacket and other wind catching things on the bike it immediately and easily went to 55 getting as high as 58 without running full throttle.

The trip home went well but it got me thinking. If something as simple as tucking down on the bike helped this much I wonder what other things I could do to it to improve both it's cruising speed and, as a side benefit, gas mileage.

I've seen the Vetter economy run bikes firsthand and I'm not sure I want to build a full streamliner so much as I'd like to make what I have slicker.

My question to you--how much do things like spoke covers (like used on Bonneville bikes) help streamline a bike? Any other ideas on relatively simple and easy to create modifications that would actually give me some real world results?

[Grant-53]

There are articles on human powered vehicles such as zzipper.com and triathlon or time trial bikes that give a breakdown on each modification. Changing the handlebars to get a comfortable position is the start. After that making a tail box is the biggest single improvement.

[jkv357]

Post a photo of your bike as it is now and we may be able to give you some ideas for easy aero improvements.

[woodsrat]

Sorry--I don't have the means to post a picture of the bike. It's an '80 Honda Passport sans the "splash guard" and currently has a CRF-80 complete front end installed. As mentioned it has a semi-auto Lifan 125 motor with an XR-70 pipe on it. The original bike, purchased on eBay for .99 (!!!) without a front wheel, cylinder head, seat and other bits it was built using a lot of the leftover parts lying around from my pit bike racing days. It has a simple AC lighting system with no battery or blinkers.

Originally a spur-of-the-moment purchase (how could I pass it up?) it was originally intended to be used as a runabout when we go to the races. On a lark I decided to ride it to a lunch gathering about fifty miles away and had a ball doing it. This led to other trips including one to Chicago to see the Allman Brothers Band and another to Mammoth Cave.

My riding buddies liked it so much that about half a dozen other step-throughs were built up from CT-90's. I enjoy riding it so much that the "big bikes" have been parked for the last couple of years and these little bikes have become kind of an obsession. I know I really enjoy riding them as I have three up and running now. With the $31 24mm Mikuni carb available from eBay I use their only liability is poor gas mileage--generally between 70 and 80 MPG. I have it jetted pretty fat and I can start it, ride off and turn off the enrichening circuit immediately and the bike runs great. Currently I'm running 17/37 gearing.

Strictly based upon my run home on the autobahn the other night it got me thinking about sort of aerodynamic improvements I could do without compromising it's go-anywhere utility.

[jkv357]

From looking at stock pictures, I would say some type of small fairing on the headlight to direct the wind over you may help. Lower bars and a lower seat wouldn't hurt either.

[Poodwaddle]

Have you considered a fairing behind your back to help the air slide around you smoother? It could be like the back half of a teardrop behind your back. It would be pretty challenging to design because it would have to take into account various riding positions, but it might strap right onto the back cargo area, if you have one.

Or you could go all out and build the Mythbusters bubble bike shroud.

[elhigh]

I'm seconding the partial tail. Let the hole in the air behind you close more gently. While you're building it, build in pannier capability and have a secure, weather-resistant trunk. You'll pick up speed and utility.

Allert gained a mountain of fuel economy in addition to speed with his full-on, no-limits FFW scooter. He changed his gearing twice if I recall, and his top speed is now way beyond the usual 125cc region. If I'm reading his website correctly, his streamliner will now hit 85mph, that's plenty fast. I reckon adding a pannier-tail to your bike would get you comfortably into the 60s without forcing you into a full tuck to get it.

[Grant-53]

Use the HPV Shell Design software at recumbents.com to make a shell for your machine. The handlebars on a bicycle are 7/8" too so any extension bars would accept the throttle and lever controls. Various sheet materials could be used so fiberglass resin is not needed.

[woodsrat]

Thanks for the ideas.

Here's a question for you. At what speed does aerodynamics really start making a difference on a motorcycle?

This particular motorcycle spends the bulk of it's time at 35-45 MPH with occasional short runs up to 50+ to flow with traffic on the faster roads.

For the way this bike is used and the speeds it runs it may be impractical to add much in the way of aerodynamic improvement without sacrificing utility and ease of use. The only alternative there is going to a full blown Vetter-style streamliner that would both allow easy access as well as carrying capacity.

I haven't ruled out building such a bike. I have lots of raw material in the form of other step throughs in the barn and Lifan motors are dirt cheap. It would be great to have a bike for the open road that would maintain real world highways speeds with only a 125cc motor. I'm just not sure I want to ride it in everyday in-town traffic.

[jkv357]

Because you are dealing with a very limited amount of power, you may see improvements from aerodynamics with speeds in the 35 to 45 mph range.

Put a medium-size fairing on the handlebars to deflect the air over and around your body for starters. That should give you some improvement, and it's simple.

A tail would help, but I wouldn't say constructing one was simple. I expect it would also show less improvement, by itself, than a handlebar fairing at those speed.