Weekend Video Clips

[65ShelbyClone]

I like bench racing. There are, I am now positive, more than one size horse. Long ago when I came to that realization it cleared up a lot of confusion in my mine. The shop manual for my 41 Knucklehead Harley notes it has 9 or so HP. As the old girl has 74ci it is producing .1216 [note where the decimal point is at] horse power per cubic inch. As it can run 85 MPH [flat out, layed down] it is possible the pre war engineers were likely talking about a few reasonably sized plow horses. Last quarter I was at a local college and a student was looking at a website that stars a killer Toyota four banger. The dyno pull that they were crowing about showed 3000 HP. Almost 34 HP/ci. Probably Lilliputian ponies.

Bwahaha! I guess the average benchracer Briggs has 15 tiny horsepower, but only five regular horsepower. :001_tongue:

On a side note, I'd like to know what Toyota engine that is that you mentioned. The highest power density automotive piston engine I know of is BMW's turbo F1 qualifying engines of the mid '80s. 1.5L(90ci) and in excess of 1400hp. That's 16hp/ci and they were only able to do it with experimental gel fuels and 80psi of boost. I find 34hp/ci rather unbelievable.

 

[enigma]

Which one is lighter? Weight is everything! The go-kart with me in it is 270 pounds. A mini bike with me on it is around 130 pounds.

[ It all depends on weight of a driver on (both machines) and weight total of the kart. (with driver in it). We already know who is lighter

I weigh 100 pounds

Minibikes can be anywhere from 80-200 lbs

So, mini bike with you on it is going to be between 180 and 300 pounds, kart is 270 according to you. Now figure the power to weight ratio, you probably believe the Briggs is a 30 HP monster and the shifter kart puts out a humbe 45-50HP, I tend to think with all the billet pieces you talk of, that the engine, a rather large engine at that, is going to put you at around 250 pounds /30HP for the mini and 270/50 for the shifter,
so the kart would have .185 HP/pound and the mini would have .12HP/pound

[Which one is lighter? Weight is everything

these are your numbers and claims.
Then figure the aerodynamics....

You know, they say an open mouth is a windoe to the mind, it sure is breezy in here!

 

[5hpbriggs]

So, mini bike with you on it is going to be between 180 and 300 pounds, kart is 270 according to you. Now figure the power to weight ratio, you probably believe the Briggs is a 30 HP monster and the shifter kart puts out a humbe 45-50HP, I tend to think with all the billet pieces you talk of, that the engine, a rather large engine at that, is going to put you at around 250 pounds /30HP for the mini and 270/50 for the shifter,
so the kart would have .185 HP/pound and the mini would have .12HP/pound
these are your numbers and claims.
Then figure the aerodynamics....

You know, they say an open mouth is a windoe to the mind, it sure is breezy in here!

With me on my kart!
Billet=Lighter part!
But anyway as minidragbike stated a mini bike has beat a shifter kart!
So that is that!

 

[minidragbike]

I don't race for money. I'm simply to heavy unless I'm racing agianst another minibike with a heavy rider.

That BZ3 motor makes way more then your 30hp estimate. It's just like I said it very close to the same, or maybe even the same as the kart the 250cc shifter kart it was going to race against.

One that kart is a old model
Two that motor wasn't a fresh motor

So he is not making the same dyno power as the modified 270cc motor you read up on.

That BZ3 minibike is 220 lbs with the jockey on board.
You miss understood what I was saying about 80-200 lbs. I wasn't talking to you. I was comparing complete minibikes with NO ONE onboard.

This is one of my minibikes. This minibike with no one on it is 90 lbs.

This is my friend Dobie minibike. This minibike with no one on it is about 200 lbs. Very big heavy motor, big gas tank, big tires, longer, wider, disc brakes, steel live axle, etc.

 

[minidragbike]

Here is the minibike that your talking about. It's 100 lbs without the jockey on it. Check out the little Tecumseh plastic gas tank. The motor is the same O.D. as a regular 5hp Briggs. The inside of it is just bigger. So it's big power in a small package.

Here is the jockey. He's the guy in the middle starting the race. His name is no#1. http://www.youtube.com/watch?v=N92YriBjFZU

http://profile.myspace.com/index.cfm?fuseaction=user.viewprofile&friendid=55529761

 

[Oldsalt]

Aero

That is an interesting aerodynamic comparison....a Kart and a mini bike. Bear in mind that wheels are really dirty. Open wheel cars like Indy and formula stuff have more drag related to the wheels than the body [ignoring the stupid downforce wings]. Same with Lakesters at the salt flat. They are the same thing as a Streamliner except that the wheels are out in the breeze and they are a LOT slower. The next issue in the comparison a mini bike and a kart would be who got the most frontal area. I am kinda thinking the Kart would because of the engine and big tuned pipe being off to the side of the driver. The next issue to look at would be downforce devices. They are extremely dirty. The shifter I was looking at some time ago seemed to have 'strikes' and etc. to generate downforce. I suppose the rule don't allow big ugly wings. In any case the mini is innocent of any downforce device. Lastly there is rolling resistance. Heavy = worse. It is also possible that two tires [vs. four] generates less rolling resistance even if the vehicles were the same weight.

 

[minidragbike]

I was thinking somewhat of the same thing. I know when no#1 races and others race there minibikes. We tuck are knees in and get down as close to the handle bars as possible. It does help a little, and the faster your going the better it works.

I was thinking something like the total height of the minibike VS the total width of the kart. If the kart were to be laying on it's side, or the minibike were to be laying on it's side they would be pretty close.

The minibike could maybe benefit from a body fairing kit? But that's something we have never tried yet.

Before I sold my old 2 stroke kart it use to seem much faster being that your right next to the ground then when your sitting up high on a minibike. Is there more wind on the ground? Maybe I have never did a wind turbulence test.

 

[enigma]

Billet=Lighter part!
!

Billet parts are not necessarily lighter, as a matter of fact, a billet steel crank is probably heavier than a cast crank, it is just stronger because it is cut from a solid piece of steel. People throw the term billet around and some get confused as to the benefits and draw backs of billet parts...
I have to disagree with your "billet is lighter" theory.

Nice bikes, MDB

 

[minidragbike]

enigma is right billet is heavier, unless it has been lighten to save weight. But if it's the exact same size it's heavier.

 

[Oldsalt]

I was thinking somewhat of the same thing. I know when no#1 races and others race there minibikes. We tuck are knees in and get down as close to the handle bars as possible. It does help a little, and the faster your going the better it works.

I was thinking something like the total height of the minibike VS the total width of the kart. If the kart were to be laying on it's side, or the minibike were to be laying on it's side they would be pretty close.

The minibike could maybe benefit from a body fairing kit? But that's something we have never tried yet.

Before I sold my old 2 stroke kart it use to seem much faster being that your right next to the ground then when your sitting up high on a minibike. Is there more wind on the ground? Maybe I have never did a wind turbulence test.

You are probably right about the frontal area being similar between a mini and a kart.

Tucking in will help, no doubt. Flat track motorcycles in the old days [I don't know about now] had a little grip mounted down on the left fork. On the straightaways the rider would abandon the left bar in favor of the 'tucked in' fork mounted grip. It was necessary to retain the right bar because of required throttle control.

Resistance goes up by the square of the velocity. Judgeing by the speeds I see in minidragbike's videos, a fairing would be a big help. Also a skinny front tire is worth considering.

As for there being more wind near the ground: In effect there is. The air must be moved out of the way of the moving object. Near the ground there are fewer directions that the air take to get 'out of the way'. Karts are as close to the ground as possible [too close aerodynamicly] because they need as low a center of gravity as possible for cornering. As a rule of thumb most land speed cars cars should not be closer to the ground than 2-1/2" or so.

I been a wondering about the wheel base used for drag mini bikes. Is there some sorta agreed upon rule that keeps them so short? A longer bike would allow a more efficient frontal area near the top end. Wheelie bars are a BandAid. They tend to 'unload' the drive wheel. A longer frame would also, I think, allow the rider to better control the bike's tendencey to revolve around the rear axle at the start of the run.

 

[enigma]

I'm still curious of the max rpm and gearing of the mini...

 

[minidragbike]

Oldsalt
The longer frame do seem to handle better at high speed. But longer is also more weight. I have a friend that has a stretch minibike that's made from chromoly that runs pretty good.

 

[minidragbike]

I'm still curious of the max rpm and gearing of the mini...

There is no max RPM, or spec gearing anyone uses. There are to many different motors used 2 strokes and 4 strokes, so it's always different.

 

[enigma]

I am referring to the BZ3, how was it set up to race the shifter?

 

[minidragbike]

14/45 gearing
9,000 +++ RPM
38" tire circumference

 

[Oldsalt]

14/45 gearing
9,000 +++ RPM
38" tire circumference

I don't know much about fast mini bikes but that strikes me as really tall gearing. I had expected sprockets that provided shorter legs. 9000 RPM X 14 teeth divided by 45 teeth = 2800 RPM for the rear tire. 2800 RPM X 38 inches cir. divided by 1056 = about 101 MPH if you could pull the full 9 grand at the top end on a really long straightaway. That situation sound scary. Hell. if you could get only 72 hundred on the clock you would still be doing 80 MPH. That is not one bit less scary.

 

[enigma]

I don't know much about fast mini bikes but that strikes me as really tall gearing. I had expected sprockets that provided shorter legs. 9000 RPM X 14 teeth divided by 45 teeth = 2800 RPM for the rear tire. 2800 RPM X 38 inches cir. divided by 1056 = about 101 MPH if you could pull the full 9 grand at the top end on a really long straightaway. That situation sound scary. Hell. if you could get only 72 hundred on the clock you would still be doing 80 MPH. That is not one bit less scary.

That is very tall, my newest build is running 17/48 with a 75 inch tire circumference, but it also has a primary reduction of 4.055:1
It would seem that mini would not have very good take off, especially paired next to a shifter with 6 speed...

 

[minidragbike]

You guys forget large 4 stroke make way more TQ the 2 strokes. It leaves fast enough so that it's very close to popping a wheelie. The first 15 feet it's moving almost like any other minibike. But once the power band comes on it's all over.

It we were to raise the clutch stall 400 RPM it will come right from under the jockey. The clutch is setup to come on 400 RPM before peak TQ.

It averages around 9,400 RPM depending on weather.

The other minibike I had with the wheelie bars was running 16/45 gearing with a 36" tire circumference. It was also turning over 9,000 RPM too. This motor made even more TQ then the BZ3. It would carry the wheels the first 500 feet.

 

[enigma]

guess I haven't watched the right videos, I haven't seen one with outstanding take off, I remember seeing them actually pushing with their feet on one, video can be very deceiving though...

 

[minidragbike]

That's because your not here. Even those slower "push off" 65mph-70mph minibikes are fast on the take off. If you were to have a stock minibike next to them it will seem like your motor is just idling and not moving.

The minibike that beat the quad was setup for the 1/4 mile. So the quad guy was thinking that it would take off so slow he would kill it in a 1/8 mile. But the quad never got in front the whole race.

The minibikes the run over 80mph you can't push off with. If you push it will shoot from under your body.

Either way today I broke something on my Yamaha again. It's either the cam, lifter(s), or crank gear. Either way it's bad news. And I had it running so good, then pow. I was just glad it wasn't the block again.