loud noise over 35 mph...

[spencercan]

i bought a 2012 Giant (brand, not size lol) mountain bike last year and married her to one of these standard "80"cc 2 stroke engine kits. it sat in the garage for about a year making me angry, until i was told that it wasnt working correctly because i had 34 spokes and i needed 36. thats a huge detail that was conveniently left off of the instructions for the kit, but it did the trick.
it runs great now, still in break in period, running through my last little bit of 24:1 mix. i have several questions, but first i am concerned that whenever i get at or above 35 mph, it sounds like its about to break. it sounds as if the back tire is rubbing on something. it doesnt sound like an engine noise to me, and it only happens above 35 mph (which is about its top speed on slight downhill). anybody know whats going on here and/or how to fix it? it has me scared to test its limits, which makes me mad.
also wondering if i need special tires or brakes for higher speeds? do i need to use any certain octane level gas? id really like to start modifying and making it faster, but not if its going to kill me!

 

[shinjinian]

You don't need special tires or brakes but some cheap bikes have brakes that won't work well at high speeds. As for your issue if something was rubbing you should notice it at any speed, even low ones. Pictures of the bike would help.

 

[Fabian]

i am concerned that whenever i get at or above 35 mph, it sounds like its about to break.

You shouldn't even be going anywhere near 35 MPH on a motorised bicycle - that's simply suicidal.

also wondering if i need special tires or brakes for higher speeds?

You shouldn't even consider riding a motorised bicycle unless it has front and rear disk brakes, with at least an 8 inch front disk rotor, because stopping the bike is far more important than making it "go".

do i need to use any certain octane level gas?

Although it won't make any more power, use the highest octane (non ethanol) pump grade fuel you can get your hands on, then jet the carburettor to suit the fuel.

id really like to start modifying and making it faster,

You don't need the bike to go faster. You should be riding the bike slower if wishing to stay out of the hospital emergency ward.

but not if its going to kill me!

You can only ride as fast as your guardian angel can fly, and motorised bicycle guardian angels can't fly faster than 25 MPH.

it has me scared to test its limits, which makes me mad.

You will be really mad that a hospital emergency ward visit was necessary to test the limits, because you "will" end up in the hospital emergency ward.

 

[shinjinian]

I think Fabian's being a little dramatic. Though he does have a point, there's no need to go over 35, the bearings on bikes wheels aren't made to withstand sustained high speeds and going fast if you have crappy budget brakes is dangerous. My recommendation is to try to cruise around 25mph and not go any faster, disc brakes aren't an absolute need on a motorized bike but they're something you should have if you're going to be traveling on one at high speeds.

 

[KCvale]

I think Fabian's being a little dramatic.

Ya, and that's an understatement...

"You shouldn't even be going anywhere near 35 MPH on a motorised bicycle - that's simply suicidal."

Come on Fabian, thousands of use routinely ride 35MPH or more every day and there have been what, 3 documented cases of some idiot killing himself on an MB because he rode faster than his bike could stop?

"You shouldn't even consider riding a motorised bicycle unless it has front and rear disk brakes, with at least an 8 inch front disk rotor, because stopping the bike is far more important than making it 'go'."

Ohhh come one, Really?
Nothing is safe without DUAL discs and it's needs to be 8" on front?
Tell that to millions of people with a direct drive to a rear sprocket where a disc would need to be that are sill alive.

My recommendation is to try to cruise around 25mph and not go any faster, disc brakes aren't an absolute need on a motorized bike but they're something you should have if you're going to be traveling on one at high speeds.

This topic is about a new Giant build and Giant makes some darn fine bikes that will take 35MPH without bating an eye for thousands of miles.

Show us some pics of your drive-train and maybe we can see what is giving the noise at high speed but these guys are right about brakes, don't skimp and test how fast you can stop in an emergency situation before you have one is my advice.

 

[grinningremlin]

Fabian loves overstating things, "flamboyant" I believe is the word.People have, and still do bomb hills at 50mph+ on touring bikes fully loaded.I personally have done over 60mph down Little Mountain in SBDO CA with a racing 10 speed and calipers.I don't care what brakes you have at that speed, you need MUCH time to THINK about the stop.Brakes are only as good as the users forethought.

 

[butre]

Fabian is being incredibly over dramatic. wear appropriate safety gear at all times and you'll be fine.

 

[spencercan]

ill post some pictures in few...
lots of stuff to respond to here.
brakes - no disk brakes right now, im just using regular v brakes. keep in mind this is a new build and a work in progress. i want rear disk brakes especially badly, as my front brakes work very well at all speeds so far. it seems to me that mounting the rear would be difficult due to the sprocket and cassette. ive never installed disk brakes, so im not sure what i would be getting myself into, but front looks a lot more feasible (but im not lacking front brake power right now...i have too much if anything). any thoughts?

now, about this hospital stuff...why do we have all of this stuff to modify our engines and forums to help us go faster if the engines already go a speed that is too fast to be considered safe? i must say, i was half expecting someone to ask about the quality of the replacement rim i used, but not to be told that i had a death wish. by the way, i do wear safety gear and i try to ride responsibly in places where cars are not driving. and im not sure if it matters, but as the above poster stated, this is a $600 giant mountain bike frame; its not exactly a cannondale, but its not a $100 huffy or schwinn either. i figured it would provide me with a little more durability.
i cruise at anywhere between 22 and 28; and its fine. i definitely do not try to cruise faster than the bike feels safe. i see all this racing stuff. i want my bike to go fast like the racing ones do, but...i do t wanna die! pics coming..

 

[spencercan]

hmm cant really see too much in these photos, but here they are.

 

[Fabian]

i see all this racing stuff. i want my bike to go fast like the racing ones do, but...i do t wanna die! pics coming..

Please don't post pics of your untimely death. I have a weak stomach for that sort of thing.
Racing stuff is for racing and should not be used on the road.

Much better to boost the useable torque range of your engine than push man and machine beyond sensible limits.