will the standard crank last with a70cc kit?

Hi,
I was going a fit a stage 6 sport pro 70cc kit to my bike, and i know that i need to upgrage the crankshaft when doing this. I can fit the 70cc kit in a weekend, but im not sure how to fit a crank so if somthing goes wrong and its not fixed by monday i wont be able to get to work. If i was to fit the 70cc kit do you think the standard crank will last about 2 weeks?

 

Provided that your crank is in good order Take it easy and it should be okay for a short period.

 

i wouldnt risk it tbh ..if it does let go you will regret it

 

iv read that if its a low mile non abused crank then it may last for however long... could be a day or 3 could be a week or 2 lol..

Just dont rag it evrywhere n you should be fine.... ( hope so, just fitted my Polini sport without upgrading me crank

 

What will kill your crank isnt the power you're putting through it, the rods are designed to cope with much more pressure than you'll put on it with that kit. What these cranks CANT cope with is the speed.

Mean Piston Speed (Ft/min) = S x 0.00654 x RPM
S = Stroke in mm.

Aerox shall be used as an example... On a standard crank moped, 3,500ft/min is the absolute limit as a rule of thumb, with 3,000+ being unsafe.

Now a YQ50 has the stroke of 39.2mm so 39.2x0.00654=0.256368

So lets say we run this engine at 12,000 RPM... 0.256368x12,0000=3076.416ft/min.
so when you think that your piston is sat at the end of your crank, moving at that speed then all of a sudden reversing direction, it's going to put a lot of strain on your crank. Once you get a nice mid range crank, your safe piston speed will move to about 4000ft/min with limit being at 4,500.

 

I bow to your knowlege and your ability to use a calculator, but arn't you overcomplicating things a bit. The poster knows that his crank will not last with the extra stresses involved in the 70cc upgrade and has accepted that he needs to fit an upgraded crank, he simply asked for advice whether his standard crank would be okay for a short period of use. The concensus was that provided that the crank was in good condition and he took things easy it should be okay for a short time.
I'm not having a dig at you as you obviously know your stuff but sometimes it's more helpful to answer a question directly and help the poster than to baffle them with science.

 

theres also quite a few good guides on how to fit the new cranks on here another scooter forum, and the Shiny red tuning website,

 

Yeah fair enough, I guess I'm so used to dealing with things like this on my bike that I seem to just let terminology fall out of my mouth, and I must say from the other side, it must be quite annoying =( Sorry guys!

 

That's cool mate (I understood it all and it's good to have someone with a good grasp of engineering on board)

 

I only have it all down in theory, In practice I lack experience =(

But back on subject, I do feel that an entry level polini crank will help this build, but for the time being try and avoid a stuffed one as these sharpen your powerband a bit which is undesirable on a street setup unless you don't mind spending some time on variator tuning (Worth getting used to)

 

I've got a top racing (doppler) full circle crank (10mm small end) coupled to a stage 6 streetrace 70 kit and both a Leo Vince zx and stage 6 pro rep kit. the combination is really well balanced as the streetrace revs well and produces a very wide powerband which the crank easilly takes. The exhausts are very closeley matched with the stage 6 tuned for the streetrace producing a broad powerband but the Leo Vince a bit more lower down but a beautiful deep note. Combined with a 17.5 dellorto which is more than adequate at this stage of tune and a well set up variator and clutch I've got a wheely happy ped capable of taking most 125's but still proving reliable and well balanced.

 

Ive had a stage6 racing 70 kit, stage6 variator, stage6 clutch and 21mm carb with a standard crank for nearly six months on my zip done around 1600 miles.

 

I dont know why people always try to overcomplicate everything.

It isnt recommended to use the standard crank on that kit. Your standard crank will probably last for so long however there is no guarantee on how long that is. Is it worth risking your kit on it? In my opinion no. Keep the kit aside until you can get the crank changed

 

I was just trying to explain what causes them to fail in greater detail as having underlying knowledge as to WHY something fails is (in my opinion) more useful than just being told it'll fail.

 

This is the newbie section. People only want simple, easy to understand answers Tailoring the answer to the audience is key here.

Although I really liked your answer

 

sport pro is a bit much for a standard crank really because they like to rev and rev !
if you had a low revving pipe like a TT it would probably last quite well because it would limit the kit from being able to rev properly at high speed, but even with something like a ZX it would still easily reach 12,000rpm at top speed

if you're going to use a standard crank with that kit the best bet is to choose rollers so as the bike runs at around 9,000-9500rpms (that kit makes the same peak power all the way from 9000 to 12000 rpm) *and* to fit a gear kit. That way the revs will only be up to about 9500rpms at 50mph but you'll still be seeing the full peak power-
the danger area is if you keep going at high top speeds for long periods because that's where the revs will be in the danger zone for the crank !!!!!
it will then probably last quite well unless you're doing long distance high speed commutes but no guarantees!!

 

I don't think this equation is particularly useful when considering moped cranks- the reason is that whenever you change the crank, unless you are at the very top end of tuning then the stroke is going to be exactly the same each time so we may as well remove the stroke from the equation
this means that for our purposes the equation is only really saying that piston speed is directly proportional to the crank speed, which is correct but it doesn't tell us anything more useful than to look at the max rated crank rpm when choosing a crank -as crank manufacturers (mopeds) quote their cranks in terms of maximum rated rpm rather than in terms of piston (conrod) velocities i would agree this is overly complicating things and it would be easier to just to talk in terms of max rpm
but definitely i agree with the point that the speed (either of piston or rotational speed as they are directly related) is what kills the crank conrod bearings more so than any increase in rpm- the reason is that if you fit a kit with a 20% increase in power this increases the loading on the conrod bearings by a small amount, but if you increase the revs the acceleration/ deceleration forces as the piston changes direction have a massive 'pummelling' (a bit like a pnematic drill) effect on the bearings of an additional perhaps several thousand times per minute- our single piston engines are especially vulnerable because there is just one big 'thump' per crank rotation instead of multiple 'smoother' firings as on multicylinder engines

 

tnx and 1 more thing my friend has a kit 70 dr evo malossi multivar and leovince tt its an aerox and he told me that it goes 70mph but i dont think that dr evo can generate so much pover is that possibel

 

Ive done 60mph on a stock aerox cylinder....