A BTR Build in Progress: Lets reach 100mph

[Rebuilder]

How hard was it to integrate mounts/fittings for the larger engine such, Yamaha XT500....?

Mounting the motor wasn't too bad. I did basically what you're doing and built the frame to fit as close as possible to the engine. This way the mounts were short and it looks allot better. The hardest part was figuring out how to step the chain over from the wide motor to the narrow rear hub. Not sure if you'll have that problem or not.

Regarding the wheels and tires. I only say this because I went through all the same options as you when planning my build. I figured either go with bicycle scale wheels and tires and have a rad and fun bike to cruise around on or go with motorcycle wheels and tires and have a rad and fun bike that you can actually go fast and be "safe" on. Potholes happen and at 50mph a bicycle rim may or may not hold up. At minimum I would go with Coker tires and Coker rims. I believe the rims come in 36 and 40 hole and they're made for the clincher tires. Nothing wrong with bicycle scale builds but if you're talking 100mph it better be tough. Hope that didn't sound too preachy. Just my opinion.

 

[DareDevil]

Hi .... If you respect you life you need to use light motorcycle 100cc to 200cc hubs, rims, axles and tires and I dont think that they are designed even for 100MPH...as for bicycle rims at 30 MPH are not safe...ride at 30 mph straight at a curb which is probally the same as a pot hole..the wheel will collapse ...

 

[Lurker]

PS, Im really glad to see that you started a thread for this build. I have been anticipating it for awhile. I like where you are going with this and I can relate to your need for speed and or the spirit of the original btrs. I'm very flattered that my gx200 build could be of some inspiration to you. I have read through the thread and I have a few suggestions.

The engine: If you are looking to make about 18hp with a gx200, Why not just use the gx340? It already make about 12-14hp stock and would be cheaper to make more power when its all said and done. I was going to use one on my last build but realized that was a dumb idea on a bicycle.

The frame: Its looking really good so far but I think you have miss jugged the physical size of the engine. From what I remember it is slightly larger than what you have pictured. Below you will find a quick rendering of a few changes I made. Please take note of the seat tube and the loop. I like the look of the front end being really compact but the back looked sloppy, so I made a few adjustment to make it more visually appealing and more period correct. Then loop I sketched would allow you to run either a 200 or a 340. I would also like to mention that in my opinion repeating visual angles make for a nice looking bike which is why I changed the top tube. I am a designer by trade and these are just suggestion on aesthetics not a personal attack on your design. I believe Mr.B is also a designer and he might have some input as well, if you ask him.

Wheels/tires/brakes: Please do not use bicycle tires for this, we are talking about your life. Spend the money and invest in either Simplex tires speed rated at 92mph or use 21"motorcycle tires with Avon Speed Master tires. For brakes I suggest a Honda cb100 or similar

Looking forward to seeing more from you, keep it up.

 

[truckd]

Hey! PS.... Glad you talked to Dan, talked to him today as well, even though he seemed a little hesitant give him time to think about it, he loves a challenge, keep him in mind in the future as you will have questions.

 

[DareDevil]

Hi Lurker....Your infomation for P.S, both on design and saftey can only help him along with his design and have a safe outcome with his B.T.R.......DD

 

[projectstarter2012]

Hey DD and Lurker, thanks for the advice. I am going to make the appropriate changes to both the wheels and frame of the bike. I appreciate the feedback and heads up on safety. Like I said, my knowledge base surrounding motorcycles is limited so that's why I kept my design in the bicycle realm but as you guys pointed out, the materials that fall under the bicycle category are not meant for such speeds and are unsafe. I wish to survive the 100mph ride.....

On a different note, this is what my quest to build a BTR is all about, design, re-design, constructive criticism, etc. I am going to build this bike, if I have to go back to the drawing board 100 times before I get there to make it safe and practical I will. But it won't stop me from building this motorcycle. Thanks for all the feedback guys, I really appreciate it, keep it coming. I want to hear more of what you guys think....Mr. B you out there?

 

[Rebuilder]

That's definitely the right attitude for this type of project projectstarter2012.

Not sure what you have available for tube bending but when designing my frame I found it best to figure all your bends based on what mandrel dies are available. For mine it worked best with 10" center line radius dies. Yours will probably be smaller which will be nice because smaller dies are more common. I had these guys make mine and it turned out perfect. I just sent them a 2D drawing showing the shape with the degree of each bend and straight distance between bends. It was a little spendy but it was really my only option for the larger die.

http://www.sharpeproducts.com/capabilities_services.html

Here's a list of their dies.....

http://www.sharpeproducts.com/downloads/Sharpe_Tooling_List.pdf

This is what I sent for mine.

 

[projectstarter2012]

Rebuilder,

Thanks for the info on the mandrels. I am still between doing the fabrication myself and having someone else do it. Truckd recommended a guy named Dan (Taylar Bikes) who builds BTR and has been building bikes professionally for a while so I might use him. If not, I will probably have a lot more questions on the bending process....a buddy of mine also recommended having an exhaust pipe manufacturer do it/auto shop's exhaust bender...?

Also, here are a few rough drafts of the redesigned bike...I took what Lurker suggested and repeated angles/lines, I also incorporated 23" motorcycle tires (which ones I should use, I am not sure...suggestions? the O.D. on the Avon Cobras is only .5" taller than the Avon Speed Masters (21"). I kind of wanted to keep a 23" wheel size though...). Also, on the redesign with a smooth rolling top tube, the vertical line on the rear tire of that bike represents 51" (the wheelbase of all the bikes is 54" and the rake is 27 degrees). Is there anyway I could get the bike down to 51" without sacrificing to much design/look? Also, the engine height is approx. +/-.5" to scale, the base is a little wider than normal. Its the GX340 Engine.

 

[Rebuilder]

I like the top one with the arched top tubes. Nice flow from the top tube into the rear seat stays. The fab issue there is you'll need to find a tubing roller which is different than a bender. If you're having someone else fabricate it it shouldn't be a problem.

As for the wheelbase... the rake on my bike is only 20 degrees and I haven't had any stability issues at 75-80mph. You could decrease your rake a little and loose a few inches of wheelbase. With that said I'm definitely not an expert in handling geometry. I just winged it on mine and it worked out.

I found this helpful....
http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry

Also the only tires I've found available for the 23" rims are....
https://www.denniskirk.com/bridgestone/front-trail-wing-9-3-00-23-tire.p651746.prd/651746.sku

 

[Mr.B.]

The easiest place to lose wheelbase in your designs would be in the rear triangle...

But in my opinion I wouldn’t be quick to lose too much as I think you have the proportions pretty good there.

Even if the tires aren’t much larger, the over all look of 23” rims will seem bigger and closer to the actual size of antique 28” wheels. But tire options are limited.

And since I’m already freely spouting opinions, I would recommend doing your own fabrication if you’re able, even if (when) it takes longer you will have satisfaction of doing it yourself when finished.

-Kirk

 

[LouieMCman]

Great and ambitious project. Looks like you got the drawing part down now it's time to build. You say you have no experience in fabrication, well I think your going to find it tougher than the drawing part.

Let the learning begin; are you going to start with a frame or make your own from scratch? I have some bikes with old clincher rims and tires, stay away from them especially if your not familiar with them. And they are also real expensive to buy. Good idea to use light motorcycle wheels, tires, and BRAKES.

Front end; 20 degrees seems shallow to me. You thing you can do is find some side pictures of real board trackers and draw lines and measure the angle they used, should give you a good idea. Remember too that real board trackers just went around a banked track and did not have to turn much at all, unlike you riding on the road.

100 miles per hour; you must use some sort of transmission, you can't do 0-100 with a single speed unless you push it off for the first 40 mph or so. Good luck.

I'm not familiar with the engines your looking at but be careful on how you mount them, some motors may not be designed to stand straight up.

Well just some ideas to bounce around, check out some of my work in the DIY section. Have fun.

 

[Rebuilder]

I agree on the motor angle although I assumed it was a pretty common setup for these bikes. If it's not you might want to make sure everything will oil correctly at that angle.

Headtube angle.... yeah I have too much time on my hands....

The other important thing you'll need to add, which will change your current design a little, is the rake out at the bottom of the forks. You have to kick it out down there or move the fork tube forward by offsetting them at the top (like modern motorcycle forks). On mine I did a little of both.

 

[Lurker]

These new drawings all look much better ps,
1. reminds me of batman in a good way.
2. looks good but I think the top tube is too tall, and the seat post jutting out like that looks weird to me.
3. is my favorite but I think that is because it is more of a traditional btw.

Glad to see that you are going with the gx340. This will allow you to have electric start and the ability to run lights, which will be good if you decide to make it street legal.

 

[projectstarter2012]

LM, thanks for the advice! The motor I am using is basically an upscale of the GX200 that Honda makes. Both Lurker's Cobra Cycles Flat Track Racer and the Sportsman Flyers have the GX200 (Clone) motor mounted the way I have my GX340 Motor in the picture, it should perform fine with an upgraded carb....I'll will have to look into it more once I get to that stage, all I know is that it will work in that position. It should also have a top end of roughly 7,000 RPMs if I mod it correctly and roughly 16-17 HP, so my theoretical speed should be 20+ mph or so over 100 mph with 28" wheels....then energy loss due to friction occurs and puts me closer to 100mph....approx.

RB, Yeah I forgot about that. How much of an offset up top do you think I'll need? If I bump it out say, 1-1/4", will that be enough? I don't think I will offset them at the bottom because it will change up the design to much....

Lurker, thanks for the compliments man. The advice you gave me about the reoccurring angles worked out nicely. I cleaned up the top design a little and attached it below...

 

[agk]

A GX340 pumped up to 16-17hp will not likely reach 100 mph. You can setup your gearing for 100 mph based off of what the speed calculator says however it doesn't mean the engine will be able to pull that gear ratio. We are aware of many karts and mini bikes making much more power and cannot reach 100 mph. Not knocking your quest just trying to to share information that may help.

There are a few things to note when comparing a GX200 to a GX340. The 340 weighs nearly double what the 200 weighs. A 200 pumped up to 18hp will walk all over a 340 making 18hp. The rotating assembly in the 340 is much bigger and heavier than the 200's rotating assembly. Because of this the 340 is a slow revving engine compared to a 200. On the track the 200 will easily out perform the 340 at this HP level. The 200 will also spin more rpm.

To take a 340 to 17hp there are a few things we would consider. We'd start with an upgraded carb and exhaust. Next would be upgraded valve springs, billet flywheel, increased compression and of course governor removal. And we would base this engine off of a GX390 instead of a GX340. The two engines are physically the same size however the 390 makes a couple more HP. An even better choice would be the 420cc clone engine. These can be purchased at Home Depot online for a really good price.

Another important piece of information is the 7000 rpm you mentioned. The 340, 390 and 420 engines can reach 7000 rpm but just about everything in the engine needs to be upgraded. You need heavy valve springs for that rpm. The stock cam cannot handle the heavy valve springs so you need an aftermarket cam. You definitely need a billet rod and flywheel. And all of the valve train needs to be upgraded to handle this kind of rpm. All of these parts for the bigger engines cost much more than they do for the GX200. So your looking at a lot of upgrades and a lot of money to get 7000 rpm out of a 340, 390 or 420.

I'm not trying to discourage you. I would love to see a forum members bike do a legitimate 100 mph. Just sharing some of our knowledge on these engines.

 

[MotorBicycleRacing]

A 200 pumped up to 18hp will walk all over a 340 making 18hp. The rotating assembly in the 340 is much bigger and heavier than the 200's rotating assembly. Because of this the 340 is a slow revving engine compared to a 200. On the track the 200 will easily out perform the 340 at this HP level. The 200 will also spin more rpm.

Hi agk
How fast do you estimate your 18 hp HF race bike would go
if you geared up and maybe used better tires?

How fast would you trust those Maxxis hookworms at?
thanks

 

[projectstarter2012]

AGK, thanks for the info. I know you specialize in working with these engines so I really appreciate the advice. Knowing this, do you think my idea of reaching 100 mph with a 340 or 420 is plausible? I am looking into using an actual motorcycle engine now, either 500cc or 450cc out of an Enduro Bike/Motocross. The GX series 390/340 really fits my design nicely, but if you think it will cost me more to mod the engine then the approx. cost of an actual motorcycle engine then I may need to rethink my build.....

Motorbicycleracing...send a PM to AGK next time...

 

[Rebuilder]

If your goal is to simply go 100mph then go with an XT500 motor like mine. It won't be that difficult and unfortunately it won't be that big of a deal. If you want to be the first to do something build your bike for the GX motor and give it a try. Get crazy.... put 2 GX motors on it.

 

[charliechaindrive.]

If your goal is to simply go 100mph then go with an XT500 motor like mine. It won't be that difficult and unfortunately it won't be that big of a deal. If you want to be the first to do something build your bike for the GX motor and give it a try. Get crazy.... put 2 GX motors on it.

Two of them, with the base plates bolted together! (not plausible but it would look cool!)

 

[Rebuilder]

Two of them, with the base plates bolted together! (not plausible but it would look cool!)

Anything is possible....

http://thekneeslider.com/archives/2007/01/15/twindian-twin-engine-indian-motorcycle/

 

[agk]

AGK, thanks for the info. I know you specialize in working with these engines so I really appreciate the advice. Knowing this, do you think my idea of reaching 100 mph with a 340 or 420 is plausible? I am looking into using an actual motorcycle engine now, either 500cc or 450cc out of an Enduro Bike/Motocross. The GX series 390/340 really fits my design nicely, but if you think it will cost me more to mod the engine then the approx. cost of an actual motorcycle engine then I may need to rethink my build.....

Motorbicycleracing...send a PM to AGK next time...

One of the great things about the GX390 is its power potential. We can get 42hp with simple bolt-ons. Some of our customers have built 390's that put out over 50hp! The parts for the 390's are expensive when compared to the smaller GX200. Building a 42hp 390 will require about $1300 in parts.

Reaching 100mph with a built up 390 seems like it would be possible. However I have no experience going 100mph with a single cylinder engine. The down side to doing this with the GX390 is having to run it as a single speed. There isn't a gear box setup to be used with the GX engines. So if the gear ratio is set for 100mph, that's about all the bike would be good for. You wouldn't have any bottom end for just cruising around.

With all this in mind, using a motorcycle engine seems more practical. You can get a solid engine with a gear box. I have a DRZ400 super moto bike that can almost hit 100mph. The engine is stock and with a gear change 100mph would be no problem.

I'm sure you are aware of the safety concerns going 100mph on some sort of bicycle so I won't preach about frame concerns, wheels, tires and brakes...just be careful!