 This topic has 5 replies, 5 voices, and was last updated 15 years, 10 months ago by Anonymous.
Using (Bad) Math to figure out if “I am too big for a 250cc”

AuthorPosts

April 30, 2008 at 5:28 pm #1320MattParticipant
I wrote this out to answer a question in the introduce yourself forum. I’d like to share it here.
Basically, I’m using oversimplified math to show how heavy you need to be in order to be “too heavy” for a small motorcycle.
This is probably the most common question asked on this forum – am I too tall or too heavy for a 250cc bike.
Are you too tall? Only way to find out is to sit on the bike in question and find out.
Are you too heavy? Well, maybe we can help with that here.Strictly speaking, you are not too heavy for any bike until you are above it’s load capacity. I can’t speak for the cruisers, but the load capacity on a Ninja 250 is 330 pounds (almost the same weight as the bike itself).
But, will it be fast enough? Or is the salesman / my friends correct in telling me it’ll be too slow and I’ll get bored / run over?
Okay, so now I’m gonna go and play with numbers.
If you don’t like math feel free to ignore this next bit.
If you really like math, please ignore this bit (it’ll make physics majors squirm)…Just how slow is a slow motorcycle?
For comparison, I’m going to list some 060 times for well known cars (I originally did these from memory, now I’m sourcing wikipedia, modernracer.com and other websites for my numbers):
2004 Hyundai Tiburon (V6, 181hp, 3032 lbs) – 7.5 seconds
2004 Mazda 6 (V6, 220hp, 3311 lbs) – 6.9 seconds
2005 Imprezza WRX (Turbo Inline 4, 227hp, 3085 lbs) – 5.6 seconds
2000 Firebird (V8, 305hp, 3440 lbs) – 5.0 seconds
2004 Mustang (V8, 260hp, 3379 lbs) – 6.1 seconds2000 Ninja 250 (Parrallel twin, 35hp, 350lbs (with fluids) + 150lbs rider) – 5.7 seconds
2000 Ninja 500 (Parrallel twin, 52hp, 410lbs + 150lbs rider) – 3.7 secondsFor reference, most family cars today do 060 between 8 and 13 seconds. All of the above cars are fast enough to get you into trouble with the law. (For laughs, the original Beetle did it in 27 seconds!)
Now, this is all very rough, because really, Torque is what accelerates you, and tires dictate how much of that power can actually be used. But pounds per horsepower (lbs/hp) is a pretty good metric for comparing acceleration between cars. It is part of how my local Auto X organization figures out classes for cars, so if it is good enough for them…
So the Tib gets 16.7 lbs/hp
the 6 gets 15.0 lbs/hp
the wrx gets 13.6 lbs/hp
the bird gets 11.3 lbs/hp
the stang gets 13.0 lbs/hp
the ninjette gets 14.2 lbs/hp
and the mild ninja gets 10.8 lbs/hpI should explain that I’m using a high set of hp numbers for the tiburon, and a lower 060 time than some publications use. I’m doing this because those fall more in line with my experience with the Tib, and seems more accurate when doing the math later on.
Having roughly 1617 lbs/hp for a car gets you in the 7 to low 8 060 times. This holds true if you look back to older cars like the 86 (Second generation) RX7 and the 86 Prosche 944 (non turbo versions) which each had weights and times in this area.
Having in the 1011 lbs/hp range gets you up close and personal with the 5 second range. The WRX which can do a really hard launch thanks to its all wheel drive, also gets close to that magic 5 second number, but in rolling starts, behaves more like you’d expect, with acceleration somwhere between the firebird and the more mundane FWD cars.
Now, with bikes, we see that for a similar lbs/hp range, we get better acceleration than for cars. We also see that even the lowly ninja 250 is in fact pretty quick by street standards. And the ninja 500 is downright fast (for reference, a 500hp Dodge Viper has a 060 time of 3.9 seconds).
Now we check to see if the math is consistent.
The equation I’m using is:
X lbs/hp / Y lbs/hp * Y 060 time
Where X is the unknown vehicle and Y is the known Base vehicle.Using the Tib as a base (16.7 lbs/hp, 060 time of 7.5), we expect that the firebird will get 5.0 seconds and the Stang will get 5.8 (pretty darned close). The Mazda is also close at 6.7. So I’m comfortable saying the math is accurate to about half a second.
And for bikes, using the ninja 250 as a base, we expect a time of 4.3 seconds from the ninja 500 (no doubt, the wider tires and broader torque curve help it more than the math suggests).
So, now lets make stupid guesses at how fast other bikes are.
A ninja 250 with a 300 pounds rider would have 18.6 lbs/hp giving us a guessed 060 time in the mid 7 seconds. Still fast enough to keep up with rice rockets and family sedans putting the pedal to the floor.
A rebel 250 (17hp, 330 pounds with fluids+ 150lbs rider) returns 28.2 lbs/hp, giving us a 060 time of 11 seconds. So now we are hanging out with minivans.
This is however, where a hole in our math shines through: The Rebel makes less than half the power of the ninja, but makes almost as much torque (Ninja 250 makes 16 lbft, the Rebel makes 14 lbft). So, the Rebel should get better than 11 second 060 times. And as my mother owns one, using the good old buttdynometer I’m pretty sure it does. My buttguess puts it in the 910 second range… Heck, my mother wheelied her rebel just last week!Putting a big boy (300 pounds) on a rebel 250 gives us 37.0 lbs/hp or, again very roughly, just shy of 14 seconds to freeway speeds. While I know it’ll be faster than that – that is definitely into the “I’m not entirely comfortable with how fast she pulls” range for me.
Putting the same big boy on a Vulcan 500 (it has the same engine as the ninja, but weighs 440lbs) gives us 14.8 lbs/hp – the same range as the ninjette with a skinny guy on it. Plenty fast.
If you were skipping my mangled math, you can starts reading again
So, if I was over 250 pounds, and wanted a cruiser, I’d stay away from the 250s and move up to a 500. More than 10 seconds to reach freeway speeds is just longer than I’m comfortable with (but then, I’ve owned both the Tib and the Mazda 6 above, so I’m used to getting on my freeways pretty fast). Of course, if you never plan on taking the bike on the freeway, that might affect your view.
Anything more than 50hp (the 750cc+ cruisers, 600cc+ standard & sport) really is way more power than you need, no matter how much you weigh.If I was over 250 pounds, looking for a sport bike, I think the Ninja 250 would still be fast enough for my wants. Moving up to a Ninja 500 would certainly get me going faster, though the bigger bike is more work to learn on.
If I was under 250 lbs (which I am) I’d feel pretty comfortable with the power from any 250cc engine (which I do).
I hope you found this useful.
April 30, 2008 at 7:00 pm #5917smadaakramParticipantgreat post! i’ll have to say i’ve never seen it broke down quite like this b4
//@R!<
April 30, 2008 at 11:02 pm #5922datattooedParticipantThat is definitely a different way of looking at it. I have to admit, I was one of those people saying the exact same thing mentioned above, is the 250 enough, or should I go for something larger!!
May 1, 2008 at 9:04 pm #5927BenParticipantGreat post!
I’m going to throw this on the front page tonight. I think everyone should read this.
Ben
~Best Beginner Motorcycles AdminMay 2, 2008 at 11:11 am #5939swedeParticipantThis is why I’m never getting a gasgulping sports car I can only drive in the summer, still loosing value sitting in the garage, doing nothing.
I never knew acceleration could be this good at these prices before I got on a bike.
Good post!
smörgåsbord
JonathanMay 4, 2008 at 5:44 am #5972AnonymousGuestIf air resistance was equivalent for skinny person and a larger person, and torque curves were strictly linear than this would be accurate. Unfortunately neither is the case. At lower RPMs your power and torque curves are close enough to linear that this isn’t negligible but the drag coefficient is a far more dramatic change.
A bike with riders twoup with minimal increase in drag or weight in a tail bag won’t affect your acceleration nearly as much as side paniers even if empty. Put a passenger in a WRX, even a 250lb person and it will make little difference to the acceleration. Put a bike rack on the roof and you’ll definitely notice it.

AuthorPosts
 You must be logged in to reply to this topic.