# HF 2.5 hp engine planned build

Discussion in 'General Questions' started by arkives1, Jan 11, 2010.

1. ### arkives1Member

A planned bike build in spring......
HF2.5 hp engine,5/8 shaft@ 3600 rpm max. Shaft to centrifugal clutch with 10 tooth # 40 sprocket to #40 -17tooth 5/8 bore sprocket on 5/8 jackshaft which goes over to the pedal crank side, then a 9tooth sprocket with 41 chain forward to 44 tooth freewheel screwed onto crank pedal, then 36 tooth #41chain is attached to the 44 tooth equipped freewheel. Chain then goes to the rear wheel which has a 30 tooth freewheel attached. Top speed with a 20 inch OCC chopper wheel is about 30mph.(30.9). For a 26 inch wheel, reverse the positon of the 36 and 30 tooth sprockets, this yields a top speed of 27.9 or use a 48t sprocket on the crank and two 30 tooth sprocket for the drive, this gives 30.7 mph.
These numbers were derived from Jpilot's Gear Ratio Calculator found on this forum and using readily available hardware. I based the wheel diameter as the size given on the tire. The actual diameter may be a bit higher. I do not know if the author of the calculator had figured that in or not. I know the actual diameter of the OCC Chopper wheel is 22.5" approximately.Using that dimension I would use 36 tooth sprockets both front and rear which should give me a top speed of 29 mph.
I used 30 mph top speed as a goal since that's the limit for motorized bicycles here in Wisconsin, other states probably vary.

<
2. ### AussieSteveActive Member

Usually, the true (loaded) diameter will be smaller than the size given on the tyre, because the tyre compresses under load. The radius is the distance from the axle to the ground, the diameter is double this.
The best way to get an accurate diameter measurement is to place the valve stem at the bottom and mark the spot, then sit on the bike with your weight evenly distributed and roll forward one full turn of the wheel. Measure the distance covered for circumference, (for entry into most speedo/computers), or divide that by pi for the true diameter for GearRatio.exe.
Do the front tyre for the speedo and the back one for the drive train calc.
My 26" MB (front) tyre with 40psi and 50kg total load weighs in at 25.57" diameter.
The back diameter is slightly smaller, due to the higher load.

... Steve

3. ### arkives1Member

HF32.5 hp planned build

You are correct about the load etc Steve. The bike I'm currently working on is still on the bench and I cannot do the loaded rolling diameter test. I calculated my numbers on the basis of the unloaded parameters and I presume that Jpilots calculations are also based on unloaded parameters. It's a good estimate either way.In the interest of keeping it legal, I'd rather err on the high side. I did a better measure of the tire on the occ yesterday and came up with 10 5/8 not the original 11.5, not sure if there's an inch difference in the two bikes or not. Can't remeasure the original as it's in winter storage hanging about ten feet in the air. When I get this bike all done, I'll do a road test and see what the actual mph turns out to be, it should be interesting. Maybe we can come up with some adjustment factor for the calculations. I don't know Jpilot or I'd ask him if he considered all this in his software. I certainly appreciate his work, it's a beautiful piece of software. I gave up the idea of using the Harbor Freight engine, it's a bit too big to fit well in the frame of the little OCC without cutting and reforming the frame etc. Still seems a nice little engine for a cruiser perhaps.
I know nothing of all bike speedometers work but I believe the electronic ones use a magnet attatched to the spokes and count revolutions so definitely the load measurement would make a difference. Math is not my best subject, I majored in beer and co-eds. Thanks for the additional information Steve.
Woody

4. ### AussieSteveActive Member

I've always been a bit of a nerd, Woody, and did well only in Maths and English. (You should have seen my results in Sports, Geography, History, French etc...)
I might try to get in touch with JPilot. I have an old version of Microsoft C++ Version 5.0 and could write an updated version, but would't do it without consultation etc. Also not sure if my version of the calculator, written in C++ v5, would run well on later OS. (I'm still running Win98 on this machine - my workhorse.)
If JPilot thinks like I do, the 26" values would be for a precise 26" wheel, since tyre pressure and load make a difference.
An average for each size wheel could be incorporated into the program, along with a 'reverse' calculation for RPM, given speed, gear ratios and diameter/circumference. Maybe the option to enter either circumference or diameter for the calculations.
As an extra thought, it wouldn't be bad to be able to enter all values except one and have the rest calculated.
ie. Enter tyre size, RPM, speed required and one of the gear ratios, (20T to 82T, for instance) and have the program calculate the final ratio.

... Steve

5. ### arkives1Member

That sounds like a good idea Steve. If you decide to pursue this idea, let us know about it. I have so much fun experimenting with different ideas and trying different things on these bikes. Sometimes I even have an "AHAA!" or "Doh" moment. Yesterday I was tinkering with a 5 speed gear cluster and a 6 speed thumb shifter, both Shimano. No way could I get it to shift up and down consistently. Finally the fact sinks in that the cable didn't travel far enough because the shifter was for 6 speeds, not 5 so the indexing was shorter. I have a 5 speed shifter on the way so hopefully that will resolve that issue. I thought a 6 speed shifter should work just using the 5 lowest gear stops and that the difference between a 5 speed gear and six speed gear was just a matter of stack height. The chain width and clearance is the same for all speeds I think. Its a Suntour gear stack so should work with the derailleur. Part of the problem is that the derailleur hits the frame of the bike before getting far enough out from the wheel for the 5th or highest gear to engage. Tried shifting the wheel farther to the right, didn't help. I'd happily trade places with the 14 toother and the 18 and use only 4 speeds...don't think that can be done though. I don't have enough experience with this stuff to know all of that about freewheels etc. Woody

Last edited: Jan 15, 2010
6. ### AussieSteveActive Member

Not that cluey with bike stuff myself. Better with engines. From your description, is the chain a bit short maybe, not allowing the rear derailleur to extend to clear the chainstay?

Regarding the program, I'll follow it up, in a day or two. I'll contact JPilot and go from there.
I was thinking, even if my C++ 5.0 is too old, I should be able to write the same thing in VBScript or JavaScript on a HTML page. Just need to read up and refresh my skills a bit. It's been a few years.....

... Steve

8. ### arkives1Member

I used an online chain length calculator. It's a freebie for the first 30 days. Essentially added 3 links to chain once the length was established for the largest chain ring and gear configuration, normally a derailleur adds two links. I added 3 as a starter because the frame on an occ chopper dips down at the point where the derailleur crosses the frame. A skid plate attached to the frame will eliminate the hang up of the derailleur but I still won't have the right cable travel on the shifter. Any longer on the chain and it gets too loose and will slop aroung on gear changes. I may have to add a forward chain tension device to avoid chain drag on the center mounted kickstand when it's in the up position. Everything gets more complicated on an OCC it seems.:sweatdrop: Let me know how you progress on the software Steve, I'm very interested in that. Woody

9. ### AussieSteveActive Member

Not sure if anything can be done at this stage, but I noticed what I think is a typo in Lou's original post. My HT has a primary ratio of 20:82, (1:4.1), not 22:80.

... Steve

10. ### arkives1Member

Mine is also 20:82. I tried out the newer gear ratio calculator and found I get the same results with either of them. The latest one allows me to start with the engine shaft gear or sprocket and go from there, a very nice feature. Jpilots calculator shows me the gear ratio for each step of the equation and from that I can kinda figure what kind of hill climbing ability I'd have with different gearing. I like both of them. Great work guys, thanks! Woody

11. ### AussieSteveActive Member

No need for a third one, I reckon.
... Steve

12. ### srdavoActive Member

thanks for the heads up on the ratio error. I corrected it on Lou's post.

Since he credited me for the info.... I am probably the source of the misinformation.
:dunce:

back on topic....sorta:

On my HarborFreight 2.5 hp trike, my final gear reduction is 10.7:1 -- this gives me 30 mph @ 4600 rpm's, on a 24" wheel.