Seat straddle (vs. height), and the MOI

[moto]

In the spirit of Quetelet, I offer my own measurements for motorcycles, to improve on seat height as a guide for short-seamed riders.

As is often observed, seat height is not the whole determinant of how easily one can reach the ground. Seat width, seat profile, and also the width of the motorcycle below the seat make a difference. Hence, my (ostensibly) new measurement: the seat straddle.

The straddle is measured as follows. A plumb bob is attached to a string passing over the saddle in the position the rider would occupy at a stop. The bob is allowed to hang on its string and just touch the ground, being directed in front or behind impediments like foot pegs, as needed. The taut string is then clasped between the fingers exactly at the centerline of the seat. The distance of the pinched spot from the tip of the bob is measured: this is the straddle, in inches or centimeters.

The seat straddle is always greater than the seat height. Its length approximates the combined length the inside of one's thigh, lower leg, and foot will need to extend to reach the ground.

For example, using a level placed across it, I measured the seat height of my own Griso at 31 inches, and of my Italjet at 32 inches, while both turned out to have the same straddle of 35 inches. (These were quick measurements with the wheels on the ground, bikes upright and unladen.) The same straddle measurement suggests they present about the same challenge to reaching the ground, and they do.

A more obese bike, like a Harley or a Guzzi California, needs a lower seat height to achieve the same straddle, and accordingly they do tend to have lower seat heights.

If magazines, websites or manufacturers would report the seat straddle instead of or in addition to the seat height, we'd have a better way to gauge whether bikes would fit. For example, I know a 35 inch straddle works for me, and I would be interested to know whether some other bike had a longer or shorter straddle.

The straddle is an abstract measurement. Like the body mass index (BMI) for humans, it is obviously only an approximation of a perhaps ill-defined concept. But like the BMI, I think it is good enough to be very useful.

While the straddle in inches (or centimeters) is what a person should be interested in while shopping for a bike, it also suggests a derived measure of the obesity of a bike, a number without a natural unit. This is the ratio of the straddle to the seat height, appropriately adjusted. For example the ratio is 35/31 = 1.13 for my Griso and 35/32 = 1.09 for my Buccaneer. Since these numbers are not too strikingly different, even though the Buccaneer is very noticeably skinnier than the Griso, I prefer to compare each ratio to a quite obese reference standard, namely a barrel.

The straddle of a barrel that is resting directly on the floor, divided by its diameter (the seat height) works out to 1/2 + pi/4 = 1.285. My preferred measurement of the obesity of a motorcycle is then the fractional part of the motorcycle's straddle/height ratio divided by the fractional part of the barrel's ratio. The result defines the Motorcycle Obesity Index (MOI). The MOI for my Griso is .13/.285 = .46, and for my Buccaneer is is .09/.285 = .32. A motorcycle with an MOI of 1.0 would be as fat as a barrel, and one with an MOI of near 0 would be as skinny as a bicycle. (Values greater than 1 are possible, for example a cube has an MOI of 1.17.) Perhaps standard MOI cut-points for obese and overweight motorcycles will be determined in the future.

We stand now at the beginning of a new science of motorcycle measurement! I name this science motometry (accent on the second syllable, please), and dedicate it to the memory of Adolphe Quetelet (1796-1874), the great founder of anthropometry and inventor of the BMI.

You're welcome, humanity. Comments and praise are invited.

Moto

[Tom H]

Ummm WOW. Interesting way to quantify seat height and width.

I do understand what you are saying. Seat height is not the only issue of being able to flat foot.

Width of seat, width of side covers (ala EVT), width of floorboards (ala EVT), width of foot pegs due to their position (ala R75/5 which I ball of foot).

Thank you!!!
Tom

[Scott of the Sahara]

Short guys of the world unite !
The only way that a person can know if a bike fits is to sit on it, then to ride it, then to come up to a stop sign on a canted road and put your foot down. The Norge was the only Sport Touring bike that fit my straddle requirements.

[greer]

Thank you, moto!  We took a good bit of spraddle out of the straddle by swapping out the side plates and trimming the sides of the seat pan and foam.





Got a little more reach when I removed the rubber bumpers from the seat pan, now I am just shy of flat footing.  The easy reach on the Nevada spoiled me, so I got busy making changes to the V7 II first thing.  A few little things can really add up.

The side plates would have been way out of budget, but I lucked up and found a new set on FB Marketplace for well under half price.  I am pleased as punch with them, even though they're bland as can be right now.  A kind and generous soul from Western New York just up and sent a packet of decals out of the blue; I'll be holding on to those while I ponder my paint (or not) options for the bike.

Sarah

[mtiberio]

MOI => Mothers Of Invention

[moto]

MOI => Mothers Of Invention

Right you are! I don't want to appear insensitive, so I suppose I'll rename the index the MBI, for "Motorcycle Barrel Index," which is a nice reordering of BMI too. Or, the "barrel index."

M.

[moto]

Short guys of the world unite !
The only way that a person can know if a bike fits is to sit on it, then to ride it, then to come up to a stop sign on a canted road and put your foot down. The Norge was the only Sport Touring bike that fit my straddle requirements.

There's a lot of truth to that. The straddle is only a rough guide, better than the seat height listed in published reviews. If you are near a motorcycle, by all means sit on it or take it for a ride!

M.

[moto]

Ummm WOW. Interesting way to quantify seat height and width.
...
Thank you!!!
Tom

You're welcome! Sorry my explanation ran so long.

Moto

[moto]

Thank you, moto!  We took a good bit of spraddle out of the straddle by swapping out the side plates and trimming the sides of the seat pan and foam.





Got a little more reach when I removed the rubber bumpers from the seat pan, now I am just shy of flat footing.  The easy reach on the Nevada spoiled me, so I got busy making changes to the V7 II first thing.  A few little things can really add up.

The side plates would have been way out of budget, but I lucked up and found a new set on FB Marketplace for well under half price.  I am pleased as punch with them, even though they're bland as can be right now.  A kind and generous soul from Western New York just up and sent a packet of decals out of the blue; I'll be holding on to those while I ponder my paint (or not) options for the bike.

Sarah

Sarah,

Your message is very interesting. I wonder how well the straddle measurement would reflect your subjective experiences as you compare your V7 before and after your modifications, and as you compare both to your old Nevada. Would you consider measuring the straddle and seat height on your bike (and on your Nevada if it's still around)? If so, we might be able to get someone else to do it on an unmodified V7 like yours started out.

I am not sure how replicable or reliable the measurement procedure I described would be. It would be important to be pretty precise, I think.

Using a level across the seat it is possible to get quite exact seat height measurements with a tape measure. The use of a plumb bob can also give a pretty precise straddle measurement if the centerline of the seat is carefully established before-hand. If a plumb bob is not available, a heavy fishing sinker might do just fine. (It is also necessary to have the bike vertical with both wheels on the ground.)

In any event, it was a pleasure to learn of your success in narrowing your bike. I was pleased as Punch!

Moto

[greer]

Thank you again, moto.  Shorter legs need a straight shot to the ground, for sure.  I didn't do any measuring, but I'd guess those "puffy" plastic side plates took away nearly an inch of inseam.  Doug (my husband, also 29" or so inseam) said the narrow "waist" of his Roamer was one of the first things he noticed when he test rode the bike, the reach to the ground was easy as could be.  He likes to take his younger sister along for rides, and firm footing is essential when you have a passenger climbing off and on the bike.

Sarah