# Who's Got A Quick(Not Fast) Bike?

Discussion in 'Performance Mods' started by 5-7HEAVEN, Nov 30, 2008.

1. ### 5-7HEAVENGuest

A fast bike has a big number top speed. It is measured by a speedometer.

A quick bike accelerates at low speed, wanting to buck you off the saddle. Quickness is measured by a stopwatch.

Depending on gear ratio, a fast bike can also be quick or not.

A quick bike can also be fast or not.

Brute torque in a vehicle produces fast times for the first 60 feet in a quarter miles. On a bicycle it's probably wayy less than 60 feet.

Does anyone know of a way for all of us to measure and compare our motored bike's quickness?

2. ### seanhanMember

Are we talking about the Mid range power band ???

3. ### 5-7HEAVENGuest

Quickness

Low AND midrange power from a standing start.:scooter:

You want to measure acceleration (a). All you need is a speedometer and a stopwatch.

Start your stopwatch and note your current velocity (if it's not zero) or your starting velocity if you were already moving, at the same time crank hard on your throttle. Then, some short time later, stop the stopwatch and note your new or final velocity.

acceleration or a = v/t (where v is final velocity and t is time) for starting from a standstill

OR

a = (Vf - Vs)/t (where Vf is final velocity and Vs is starting velocity) when measuring acceleration between one speed and another.

a's units are commonly in feet / (second * second) (or feet per second per second or feet per second squared)

Example:

you go from 10 miles/hour to 20 miles/hour in 5 seconds

a = {[20 (miles / hour) - 10 (miles / hour)] X (5280 feet / 1 mile) X (1 hour / 3600 seconds)} / 5 seconds

= (20 - 10) X (5280/3600) / 5

= 2.93 feet / (second*second)

[ the (5280 feet / 1 mile) and (1 hour / 3600 seconds) just converts miles/hour into feet/second, a much better unit to use for the acceleration calculation ]

This measures your average acceleration over a period of time, so the shorter the time you take for your measurement, the more accurate it will be for the acceleration available near that particular velocity, but the greater the error induced by poor stopwatch handling, etc... just experiment to see what kind of time works best for you and generates repeatable answers.

This certainly gives you the ability to determine your bike's acceleration performance and compare to other's if they wish to join in.

Hope this didn't put u to sleep.

Good Luck and Good Riding

Last edited: Nov 30, 2008
5. ### misteright1_99Member

My MB was very quick with a 50t sprocket, but topped out at 20. I like to cruise around at between 15 and 20 so I put the 44t back on.

6. ### 5-7HEAVENGuest

[.

This certainly gives you the ability to determine your bike's acceleration performance and compare to other's if they wish to join in.

Hope this didn't put u to sleep.:sleep1:

Good Luck and Good Riding[/QUOTE]

VERY good info, which I need to absorb.

7. ### DIYMarkMember

Ah yes but I can cheat all this lol!

Get a huge rear sprocket. Good acceleration but low top speed and that low speed means less drag. So to compare our bikes "mathematically" we would probally need to set guide lines or "classes". Say have a "10 - 20 MPH Class" and then a "20 - 30 MPH Class" and even a 30+ MPH class?

So far my bike with a 44T could do wheel stands/mono/wheelies, with a 22T I could do 80+ Km/h (Ive raised my compression to ~8:1) and then now Ive dropped it back to a 36T which I took for a test run today - with this sprocket It allows me to climb hills (on the very steep ones some assitance is needed to keep the engine In the "sweet spot") and I reach a top speed of 65Km/h.

HOWEVER, I still want gears (the 36T is just a quick fix because I could hook it up to the bike in a flash just for kicks). One day Ill get a shifter kit and then I'll have a motor bike lol!

In the near future Ill try to meausre my acceleration and post it up!

8. ### beachcruiserMember

I know DIMark defined quick and fast motorized bicycle correctly. If it can do a wheel stand/ wheelie on take off, you definitely know your bike is quick.

Absolutely! So if we were to start comparing acceleration rates over several different speed ranges, as Mark said, we'd have to compare apples to apples by including top speed.

I cheat also, with a SBP shifter kit. I can wheelie in 1st with just a tiny bit of pedal, yet top at about 35 mph in 7th and, due to the 'gearbox', I can reach that top speed rather quickly. This is why drag races are always done over a fixed distance. It's not how fast you're going at the end that matters, it's how long it took you got there.

So, perhaps a simpler comparison of bikes would be to use elapsed time from a standstill to 0.1 mile, and to 0.2 mile, etc. (Of course, now we have to define the flatness of the course, etc., for a fair comparison) This may be an easier way to compare apples to oranges.

However, if you want to 'tweak' your bike, acceleration is usually the best way to quantify the performance gained/lost due to any changes you've made.

11. ### beachcruiserMember

Try your local 1/8 mile Dragstrip. See if they will let you get a qualifing time this spring and just post your time with your set-up. Just an ideal

LOL.. then there have to be 'with' and 'without' pedal-assist categories.

13. ### DIYMarkMember

Ah that would be a KX 125 motor strapped to a bike lol.

I meant when I HAD my 44T I could do wheelies if I rode at 15 Km/h and either hit a small bump or pulled on the handle bar and gave it gas - I found this out accidentally once lol, I was flying through the bush on some trail and hit a bump which caused me to hit the gas. As a result the bike was about to tip back over on me lol but I saved it with the rear brake.

My engine is a so called "80cc" HT that's had extensive mods done. Lately Ive actually re machined the combustion chamber to give me a slight squish band and to raise the compression to a calculated 7.8 :1. Also my homemade boost bottle have me a 7.5Km/h increase in top speed (I'm surprised it helped this much). Other future upgrades, perhaps when "santa" comes will be a new exhaust and carb.

I doubt Ill be getting the acclaimed 4k Miles on a HT, maybe 2000Km then it'll krap out on me. I don't think they last long with 8:1 - I dont know, Ill just have to find out lol.

Speaking of which, I so want to stick a 80cc 2T Dirt bike engine on a mountain bike! Power and a gear box in one small package. I might have to take a trip to the wreckers :devi2l:

Rode a friend's kid's honda CR85 2stroke over the weekend.. man, when u hit the power band even in 3rd, that front wheel lifts off the ground even with my 250 pounds on it! should be **** on wheels on a mountain bike.

15. ### 5-7HEAVENGuest

I agree.

Good idea, since some bikes don't have pedals.

You could also have runs with pedal-assist and runs without pedal-assist.

16. ### ZnsaneRyderMember

Pedal assist makes a huge difference. If I pedal hard when accelerating the engine, I can leave most cars on a takeoff from a redlight.

IMO gearing matters even more than HP for takeoff.

With, engine only, it takes me a short while to get to 30mph. My gearing is high, so engine by itself isn't that quick on a start, but once I get to 20mph, it pulls REAL hard up to 35-40. For that reason, I pedal a LOT at lower speeds, and save the engine for anything over 15 mph. I'm quick in the middle speeds, and FAST at high speeds.

A 6.5HP trike I built for a friend, only topped 30-35mph, but with the lower gears, it would take my bike on acceleration EVERY time.

I'm pretty sure many of you smaller engine guys could easily beat me on a standstill with your lower gears and higher RPM.

Last edited: Dec 3, 2008
17. ### seanhanMember

My GEBE 32cc Tanaka

Is Slow on take off (without peddling) Has real good midrange powerband but after I get to 28MPH it takes about 15 more seconds to max out at 32MPH.
And thats with the stock Gear. I was thinking about buying the other 2 Gears and experimenting with it to find the best Ratio for my riding style. But I think there really is very little diferance maybe a MPH or 2 in top end, and better take off with the trail gear. :stuart:

18. ### MotorBicycleRacingWell-Known Member

Yamaha GT 80 and no its not mine, some guy in NJ was going to send me more/better pics but never did

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19. ### DIYMarkMember

That would be so cool! Pity there's no more pics.

It would be kind of weird being pulled over on that. I guess you could argue its just a mountain bike with an engine - but then there's the power/speed limits.

20. ### machiasmortActive Member

DIY,

Where you at boy??? I'm in Ny. I gotz an 80 also that I upped the compression on. Pulls like the devil. I took the lower gasket (tween head and block) out. Figure I'm even over 8 to 1, probably more like 10 : 1.

Either way, put higher octane gas in it and you'll notice a huge difference! Try it nomatter what others say or you read. If you've really upped the comp. you'll notice the difference. My bike reminds me 13mph-23mph like the rm80's when I was a kid!
I blew past a brand new moped this summer!!!