Hacker News Reader

Hacker News

Comment by Dylan16807 | Hacker News Reader

Parent Full thread

Dylan16807

What's the point of the weight ratio? If a 20kg motorcycle can do 4kW why is that a concern?

View on HN

The latest generation of RAV4 has power of 176 kW (236 hp; 239 PS):

* https://en.wikipedia.org/wiki/Toyota_RAV4#Sixth_generation_(...

A fairly recent iteration of BMW S1000RR motorcycle has 152 kW (204 hp; 207 PS):

* https://en.wikipedia.org/wiki/BMW_S1000RR#2019_(K_67)

* https://en.wikipedia.org/wiki/List_of_fastest_production_mot...

One can set lap records, the other cannot. Power-to-weight can determine performance a lot (it's why high-end sports cars often use carbon fibre).

I don't think you can compare a car and a motorcycle that easily. Especially at high speeds where air resistance is so important.

The current-gen Miata is 135 kW (181 hp; 184 PS):

* https://en.wikipedia.org/wiki/Mazda_MX-5_(ND)

It's about ~third of the weight of the RAV4. Which one do you think accelerates from zero faster (not even reaching "high speeds")?

Now adjust things so the driver weighs more than the car and we'll be close to motorcycle territory. And even here it's like 6 versus 8 seconds for 0 to 60.

So the Miata gets there faster even with a >20% power disadvantage. I think that shows why power-weight is important.

It does something, but it's not so important when you make the weight difference smaller and then strap on a rider that weighs more than the vehicle.

I'm asking why there's some kind of significant safety issue and why it's around .1kW/kg vehicle weight in particular, not whether it does anything whatsoever.

I mean, you can, but it's not apples to oranges. I would feel like a car with 200hp would be a sporty fun car to drive, but I will go down fighting if you tried to force me to ride a 200hp motorcycle.

Then again, that's in the 1000+cc category so it would be a big sporty bike, but still, that's a capital S Scary amount of power for the average non-thrillseeker like myself.

> If a 20kg motorcycle can do 4kW why is that a concern?

Because it would go too fast.

The alternative that follows the rule is a 40kg motorcycle that can do 4kW. Is there really going to be a significant speed difference?

The 20kg bike would accelerate roughly twice as quickly as the 40kg bike. Accelerating super quickly is not safe.

A motorcycle accelerating at full speed without a rider is very dangerous no matter how many horsepower. I don't think that should be the test method.

Also twice the power doesn't give you twice the acceleration as you pick up speed. Each mph takes more energy than the last.

On the other hand, getting hit by a 20kg bike would hurt a lot less than a 40kg bike at the same speed.

Reply on news.ycombinator.com