Musings on optimum power to weight ratios

[Easy-X]

I'm just watching "Restoration of Everything" which is a nice soothing YouTube channel if you want to put your brain into neutral

My subconscious has a tendency to percolate ideas over a long period and it keeps noticing the same thing...

Let's take a generic bike, for example: standard steel tube frame, it can only be of roughly one size for a moderate spectrum of human sizes. Add to that all the legal requirements: lights, indicators, speedo, etc. and all the stuff to support them: battery, regulator, wiring loom. Comfort things like a seat. Standard controls and so on.

Obviously wheels would be nice along with brakes. What else?

Oh yeah! The engine

So what I'm saying is: there's a certain amount of "base weight" you have to have just to get started regardless of engine size. This is particularly acute for some models of 50cc road bikes and quite a lot of 125cc. You see a standard bike but one component sticks out as being undersized: the barrel. What drives it home is the fact the gearbox is almost always a "base size" across quite a range of engine capacities. (As a side note this seems a modern trend. Way back there was no problem making small barrel bikes with tiny 3-gear boxes.)

Going the other way, e.g. litre bikes, no problem - you just build bigger.

So the big question: what is the optimum before you have to build bigger? How powerful can you make a bike before you start making a seriously stronger gearbox, reinforce the frame, wack on 4 piston brake calipers?

tl;dr ^^This is what happens when you have a Saturday night without beer
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[pepperami]

You need to talk to Alan Milyard, what he doesn’t know isn’t worth knowing.
You must of heard of the Flying Milyard?



10,000 bhp, 20 litres , 0-100 in a second, or your bike’s shit!

Everybody wants a bit MOAR POWA!.
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I am the sum total of my own existence, what went before makes me who I am now!

[Easy-X]

That's not quite the direction I was thinking (but an amusing bike nonetheless!) Let's loot at something like dirt bikes...

50cc is more a sop to regulatory pressures, it's probably not what you'd buy new if you didn't have to (assuming adult with full licence.) On the other hand 125s (2-stroke obviously) are a perfectly valid proposition for power to weight but you could do better.

The market seems to have coalesced around 250 & 450 (4-strokes) but from what I've seen - bear in mind I hardly know anything - is a 250 2-stroke rebarrelled to 300/320cc seems like the thing to do these days so just for the narrow subset of dirt bikes that might be the answer to my question.
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[Teflon-Mike]

[wr6133]

Not an answer but musings ( or maybe just total crap), to add to musings.....

Bikes are not (Outside of sport)built for optimal power to weight. Gimping the bike to fit a licence or price point is probably higher on the design list. If power to weight were a consideration steel frames would be a memory.

Some are built almost in opposition to power to weight. Yamaha r125 is a good example, the bike has been made chunkier to look bigger (and appeal to the morons).... pretty sure if it lost the bloat and the fat wheels it would perform better by virtue of weighing less. However then chavvy mcjizzwaste couldn't peel off the 25 part of r125 and fool the local cum bucket he has a superbike.

Many bikes are overbraked, do you really need 2 discs and a total of 8 pistons on the front end of a 60bhp twin? Probably not and a single disk setup would shed weight. I suspect this is because journalists on review in absence of real faults will pick at shit so bikes end up over specc'd in some regard.

I suppose to answer your question I'm going to say around 170-180kg that's the ballpark figure on my GSXR 1000. It has no modern electrobollocks, just the basic bare minimum to be road legal added to an ali framed bike with a 1000cc il4 motor. For a big il4 i don't see how any significant further weight could be lost without resorting to spending very silly money (carbon arm and wheels).

My Enfield again no electroshite beyond efi. 184kg (wet), this bike is overweight. The 535 aircooled single weighs bugger all, the frame and swingarm are steel. Making them in ali though would have made a very light bike with a pricetag nobody would pay.

[JackButler]

I'm not sure what the question is either, but here goes anyway.

The whole power train is engineered to cope with the power put through it, or at least it should be, everything has its weak spot. Engineering power delivery is about much more than just desigining parts to fit.

If you wish to understand what engineering, say a complete motorcycle, is all about then there are several thousands of books you need to read along with the thousands of hours of practical learning first.

A bloke who's opinion on such matters is to be highly respected reckons the average to good rider doesn't use much more than a CBR600 can give, so maybe that's your benchmark for power to weight.
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I clean my guns with the tears of the snivelling, left wing scum.

[Easy-X]

Sometimes you wonder whether to be silent and be thought a fool or hit "Submit" and remove all doubt

I think I probably had a mad idea that there was a "sweet spot" everyone was missing out. Obviously the guys at Honda, BMW, Yamaha, et al know a thing or two but laws and marketing trump all that.

Very interesting though, thanks guys
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[trevor saxe-coburg-gotha]

yeah interesting - i want to ask a related question but don't really know how to phrase it. It's about *where* is the best place for the weight to be. Take something basic like the now proverbial sv650. Next imagine that all the mass that isn't the frame and wheels + handle bars and forks, etc., could somehow be condensed down into a tiny volume about the size of a bike battery - where is the best place for that to be? Is it just above ground level at a point halfway between the front and back wheels? In other words, if it were possible to compress the engine and other sundry components into e.g. a cubic foot of mass, where should it be placed so that the bike can corner most efficiently i.e. with max stability?

Manufacturers talk about centralisation of mass and such like, but apparently it's important to distinguish between that and centre of gravity. I just found an interesting discussion about this, but it's mainly in relation to off-road bikes. Presumably different emphases apply to different uses a bike is designed for.

https://transmoto.com.au/explained-mass-centralisation/
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"Life is a sexually transmitted disease and the mortality rate is one hundred percent."

Mobylette Type 50 ---> Raleigh Grifter ---> Neval Minsk 125

[Easy-X]

Worth considering the rider can be counted as "movable ballast" Both for race bikes (leaning and getting the knee down) and off-road (standing on the pegs.)

<addendum> Superbowl tonight so I've got plenty of beer in. Currently sampling Camden's "Off Menu" IPA
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[Pete.]

[trevor saxe-coburg-gotha]

as i said - just above ground level
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"Life is a sexually transmitted disease and the mortality rate is one hundred percent."

Mobylette Type 50 ---> Raleigh Grifter ---> Neval Minsk 125

[Pete.]

[Easy-X]

[JackButler]

Where's the "centre of thrust" on a motorcycle?

Would that be the rear tyre's contact patch?

Interestingly, advanced racing motorcycle design is only just now recognising that they need to be designing the bike for where the race is won, & that would be somewhere in the corners.
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I clean my guns with the tears of the snivelling, left wing scum.

[stevo as b4]

Post by OP is pointless, because of course there's no ideal power to weight ratio to generalise across all people, bikes, preferences and requirements. It's pure Autism talk that's all!

I like these days enduro and supermoto bikes, but also light naked roadsters or dinky 125 race replicas.

For me these days for road legal bikes, 90-130kg bikes are what I'm after or would prefer for what I'd like to use them for. Power well 12-50bhp is around the region Iike too. But I wouldn't buy a bike just for the weight figure or the claimed bhp figure, and neither would any of you lot!

[Pete.]

[JackButler]

[Pete.]

[Easy-X]

[trevor saxe-coburg-gotha]

[wr6133]

[DJP]

I read it thus: What would an engineer build with a clean sheet, minimalist design unencumbered by regulatory considerations?

Plenty of research (by car & bike manufacturers) suggests that the optimum cylinder size for power/economy trade off is 400-500cc.

So how about a 400cc single: Lightweight, compact, economical. BUT... Probably not very powerful.

So how about an 800cc twin? Now we're talking: Nice compact engine, whole bike not much bigger or heavier than a 125, but with about 5 or 6 times the power, and still with decent economy.

Triples and fours? Fine engines for sure, but we're starting to lose the compact dimensions and economy.

So I'm going to go with an 800cc (ish) lightweight twin. And they already exist: Think Yamaha MT07 or KTM 790.

Both decently quick. Both capable of impressive economy. Neither much bigger or heavier than a 125.

Sorted!
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Suzuki Bandit 1250
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[trevor saxe-coburg-gotha]

[MarJay]

[trevor saxe-coburg-gotha]