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The Size of Things

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The Size of Things

Postby Greg » 23 Feb 2010 23:16

Here are some notes I've gathered about how the sizes of various parts of the bike effect it's speed, power band, mileage and tuning.

Peak Torque RPM
For drag racing you want your peak torque to occur at a higher RPM so that you can accelerate longer before needing to shift. For hopping curbs and doing wheelies you want it at a lower RPM.

Peak torque on our stock MadAss engines based on the dyno readings at Moto-Scoot looks to occur around 5300 RPM. Peak horsepower looks to occur around 6500 RPM. The performance kits form Moto-Scoot all move the peak torque and horsepower up to a higher RPM.

Power Band
The power band of an engine is the RPM range where it is most effective. This is generally measured from the peak torque RPM to either the peak horsepower RPM or to redline. The broader this band the easier it is to ride efficiently in each gear. Performance bikes are often tuned for a more narrow power band which requires shifting gears more often in order to remain within the optimum RPM range of each gear.

Air Filter Dimensions
On the same intake, a large diameter, short filter will flow more air than a small diameter, tall filter.

Intake Diameter
It is beneficial to have the intake air moving as fast as possible into the cylinders. This increases the turbulence and mixes the fuel with the air better. One way to increase the air velocity is to use a smaller diameter intake. Since roughly the same volume of air enters the cylinder each cycle, if you pump that air through a smaller diameter pipe it will have to go faster.

The downside to using a smaller diameter intake is that at high engine speeds when lots of air is going through the pipes, the restriction from the smaller diameter may inhibit airflow. So for the large airflow at higher speeds it is better to have large diameter pipes.

Intake Length
A longer intake (the entire length between the engine and the air filter) creates more low end torque. A shorter intake moves torque up the power band higher.

Carburetor Size
Smaller carb size will give better low end. Larger carb size will give better high end. If your engine isn't feeling snappy with a quick twist of the throttle from idle you may have a carb that is too big.

A rough guide to carb sizes:
50cc – 18mm carb
88cc – 20mm carb
110cc – 22mm carb
125cc – 22/24mm carb
140cc – 24/26mm carb
160cc - 26mm carb
172cc - 26/28mm carb

Jet Size
Bigger jets equal more gas, a richer air/fuel mixture.

Main Jet
The main jet effects how the engine runs from about half throttle to full throttle.

Pilot Jet
The pilot jet has the greatest influence for idle to 1/4 throttle. If your bike is sluggish or unresponsive until past 1/4 throttle that is a sign your pilot jet may be too rich or too lean. A properly tuned pilot jet will help you accelerate off the line and out of corners.

Air Screw
Tightening the screw makes the mix richer; turning the screw out makes the mixture leaner.

Needle Clip Position
Your carb needle affects throttle settings from 1/4 to 3/4. This is the range we are in most of the time we are riding. The needle jet on many carburetors will have a clip at the top of it. Moving the clip up makes the mixture leaner. Moving the clip down makes the mixture richer.


Good carb tuning links:
- TBolt USA - Simple Guide to Jetting
- ScootRS - Carb Tuning
- OORacing - Tuning Tips


Engine Size
The maximum amount of power generated by an engine is determined by the maximum amount of air ingested. Two things dictate the RPM potential of an engine (less intake and exhaust systems); the cam and the breathing ability of the head.

Cylinder and Piston Size
Increasing the cylinder and piston size will increase the displacement of the engine and thus the power that the engine can produce. However, installing a big bore kit alone will mostly increase low end torque, with only a marginal high end improvement, since most of the potential power is wasted because the standard head probably won't have big enough valves. Also, when increasing the engine size the camshaft duration will change so that a standard cam will allow the engine to rev less with a big bore kit. Adding a race head and hotter cam will allow you to get the most out of your big bore kit.

The stock piston on the 125 is 52.4mm with a 14mm wrist pin. The stock piston on the 50 is 39mm with a 13mm wrist pin.

Engine displacement is measured in cubic centimeters:
bore x bore x stroke x .0007854 = cc's

The diameter of the cylinder block

The depth of the cylinder block


Short Stroke Engine (also called an Over Square Engine)
The Stroke is "shorter" than the Bore. A short stroke engine revs fast due to a relatively shorter stroke, good for bikes where fast build up of Power is essential (eg. track / race bikes).

Long Stroke Engine (also called an Under Square Engine)
The Stroke is "longer" than the Bore. A long stroke engine makes good torque at relatively lower engine revs, good for bikes where good torque is desired at lower/relaxed engine revs (Eg. commuter bikes, touring bikes).

Square Engine
Engines where the bore and stroke are almost the same size. These engines theoretically balance the best of both worlds.

A hotter cam will increase the RPM limit of an engine, and subtract horsepower from the lower RPM range of the engine. Since making the cam hotter does not increase torque, to take advantage of the higher RPM range may require installing a rear sprocket with more teeth.

Valve Head Size
A big valve head will allow the engine to breathe more efficiently.

Compression Ratio
Think of Compression Ratio this way: When the piston is at the bottom of the cylinder, say you could pour 100cc of water into the spark plug hole (both valves closed) and it would be full. When the piston is at the top of its stroke, you can only pour 10cc of water into the hole to fill it. The compression ratio would be 100 to 10 or 10 to 1. Higher compression ratios (in general) will let the engine make more power, require higher octane gas and will be harder for the starter to turn over.

Exhaust Size
Exhaust size has influence on power band timing, duration, strength and torque.

Pipe Diameter
If the existing torque peak is at bit lower RPM than you prefer (typical in under-cammed or stock motors), it can be “bumped” a bit by increasing the primary diameter. If the torque peak is too high (motor is “peaky”, with no range and poor recovery from gear changes), the peak can be adjusted down by using a smaller diameter pipe. There are equations for approximating a good pipe diameter that use the engine size and desired peak torque rpm as variables.

Pipe Length
Pipe length generally will not change the peak torque or the RPM at which it occurs. A length change has the effect of improving the torque on only 1 side of the peak by “borrowing” it from the other side. A shorter pipe improves the torque after the peak (reduces it at lower RPM), preventing the curve from flattening out so quickly as speed increases. A longer pipe extends the torque curve backwards to improve the engine's flexibility, at the expense of after-peak torque. Less stall speed is required, and the motor will pull taller gears.

Sprocket size will change at what speed(mph) peak torque is reached. Less teeth on the rear sprocket increases top speed. More teeth on the rear sprocket makes it easier to do wheelies. Changing the number of teeth on the front sprocket acts just the opposite. Increasing the number of teeth by one on the front sprocket is roughly like removing 3 teeth from the rear sprocket. So if you were going for top speed alone you would decrease the number of teeth on the rear sprocket and increase the number of teeth on the front sprocket. With the MadAss, people tend to be pretty happy with the 39/15 and 39/16 combos. This makes first gear taller and will increase your top speed. The smallest rear sprocket for the MadAss is a 37 tooth.

The large center hole on the rear sprocket of MadAss 125s and late model 50s is 93mm. Older MadAss 50s have an 108mm diameter hole.

A rough guide to sprocket sizes:
125cc – 15/39
150cc – 16/39
160cc - 16/39 or 17/39
172cc - 17/39
184cc+ - 17/37

Wheel and Tire Size
Due to the flywheel effect larger, heavier wheel and tire combinations require more torque to accelerate, and more braking power to stop than smaller, lighter wheel and tire combinations.

Whether or not a taller tire will give you a higher top end speed seems to involve too many variables for a simple answer. Diameter, weight, rolling resistance, and the flywheel effect all come into play.

Wider tires give you more grip while increasing rolling resistance, and thus decreasing mileage.

Higher-profile tires have a longer and narrower contact patch with the asphalt, whereas low profile tires have a contact patch that is broader, but shorter. The higher-profile tires give you better stability at high speeds and the lower profile tires give you better cornering ability.

Wheelbase has a major influence on the longitudinal stability of a bike, along with the height of the center of mass of the combined bike and rider. A bike with a short wheelbase is much more likely to perform wheelies and stoppies.

Fork Rake Angle
The amount of rake is significant in determining a motorcycle's handling characteristics. In general, more rake provides greater straight line stability, less rake (the forks are more vertical) makes the bike more responsive.

Shock Size
The stock rear shock on the MadAss 50 is 225mm. On the 125 it is 235mm. The top of the shock can be moved to the mounting hole directly below it in order to lower the seat a bit.

Master Cylinder
Smaller MC yields more braking force from same lever pressure by spreading the work over longer lever travel.

Too big an MC for your caliper(s) and the lever will feel wooden/firm, too small an MC and the lever will feel mushy/soft and you'll get too much lever travel.

A MC that is way too big may feel like an on/off switch.


Handlebar Inner Diameter
Our stock handlebars have an 18mm inner diameter. If you upgrade to aluminum bars, they generally have a 14mm inner diameter and will require different bar ends. If you get fat bars you'll need riser adapters. I haven't tried it, but you can probably take the stock top riser mounts, flip them, and install the fat bar adapters on top of them.
Last edited by Greg on 27 Feb 2010 19:31, edited 1 time in total.

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Re: The Size of Things

Postby MilwaukeeMadass » 24 Feb 2010 08:27

You're becoming the MadAss wikipedia. 8-) Thanks for droppin' the science.
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2010 125 Madass "Hugo Stiglitz" | Graphite | Stage I Kit w/ Asspipe
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Re: The Size of Things

Postby Stikfas » 12 Jan 2011 10:35

there is one thing i am eager to know and that is the Engine dimension.

i need to know the length of the engine and the height of the engine. although i can measure it myself but my bike is away from me right now.

Thank you
May we always put our Bike stand down as many times as we put it up!
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Re: The Size of Things

Postby StanO » 13 Jan 2011 14:29


AWESOME info for noobs, and a great refresher for more experienced types.

Good on ya.
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Re: The Size of Things

Postby Paya » 28 Jun 2012 05:14

Jose (Amahoser) helped me alot with the info about Fork seal sizes:

HT = 11mm
OD = 50mm
ID = 37mm

It could be helpful, since I couldn't find that info anywhere...

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