I have a modified 26″ Bicycle which we STRETCHED 18 1/2″ are going to add 2cycle China Bike Motor 70cc to. :devil:
TO EXPLAIN WHAT WE HAVE DONE:
we lowered bike (top of seat is 20 1/2″ from street surface) because we are using 17″x3.50″ rear Motor Cycle rim, tire and 10G/36H spoke coupled to a Custom Bicycle hub. which has 6 speed free wheel gearing and disc brakes. the original BB or where pedal/chainwheel and crank were is now right side removable (2) chain wheels 42t attach by chain to rear wheel, 24t attached to new BB chain wheel 38t and crank which is 18 1/2″ forward from original location, to give leg room to pedal bike. Now on left side of bike on original BB axle is 18 or 22t free wheel to attach to drive gear on motor. We actually extended bike from seat post to front fork 18 1/2″ stetching out frame and then double walled all tubing. We are using a Honda M/C front fork (suspension) which actually let use use either 26″ or a 24″ disc brake bicycle rim and tire (Double walled rim) We were actual able to fabricate a bracket to mount caliper to and original caliper fork mounts….lined up perfect with rotor.REASON FOR SOME OF THE ABOVE (OTHER THEN LOOKS
:
What we have done is added freewheel to center BB to let motor sit still while pedaling and vise versa let the pedals sit still while under petrol power. (because of pedals it is considered a bicycle still and with specific additions is street legal with no license needed.NOW THE PROBLEM:
When we extended frame we gave Head Tube more rakeactually just turned it over. Not wheel wants to dart here and there. (when you turn fork ground contact point varies) We had Nice 26″ x 1.38″ tire and rim. We switched to 24″ x 1.75″ and that seemed to improve handling, Why?
If we go to 1.96 width tire or 2″ or even 2.25″ will it continue to Improve? Is it because wider tire is more contact surface? With 26″ as we turned fork tire contact seemed to roll from center tread to off center. Could I even go to 26″x 2.25 or 2.50 and get same improvements? (would look more Radical )
David
Hey David – sorry for the delay – I’ve been out riding!! 
Every bike, bicycle, motorcycle, etc will have a unique steering geometry and performance based on the rake, wheel size, fork length, and axle mounting position. The longer you stretch the front end of a bike the more the tires contact point with the ground will shift side to side when the handlebars are turned.
Ever notice that when you take a hard right corner on your motorcycle you actually turn the wheel to the left? Thats due to the offset, there’s not simple way around it. That’s also why, when you park your bike on the street you always turn the handlebars in, because the front tire will then be further to the outside of the bike’s axis allowing it to lean more firmly into the kickstand.
The slower you are traveling the more pronounced this effect will be.
If you want a more sharply steering and controllable bike, then bring your forks closer to vertical. A vertical fork is the only one which will not exhibit an offset in ground contact when the bars are turned.
You can also use a smaller tire, which again will reduce the side to side offset.
Keep me posted on your progress!