Suzuki Burgman modification (aerodynamics + gearing mods)

[elhigh]

Originally Posted by janvos39

That would give a 325 cc with 73 bore and 77.6 stroke or should a longer stroke bring more assuming rev`s go down further. Probally a longer stroke can be accomodated in the crankcase as well. Honda uses in some models an even smaller bore to stroke ratio for the best FE.

I don't know where they got their information, but Hot VWs Magazine is doing a "mileage motor" (been doing it for two years now) and they said that for mileage, you want your engine to be as square as you can make it. So they went considerably bigger than stock, geared down a bit, added a few other changes and have gotten a reliable 38mpg in a car that used to get 29 before the engine work.

So, square it up.

[Geebee]

A big plus is that it is supposed get 83 mpg standard.

[dcb]

Originally Posted by elhigh

So, square it up.

I don't quite get why square is magic, lets see if we can figure this one out.
Lets take a hypothetical 1 cylinder engine with 6.2832 cubic inches of displacement
3 configurations:

Bore,   Stroke, Area,  Circumference
1.414,  4,       1.57, 4.44
2,      2,       3.14, 6.28 
2.828,  1,       6.28, 8.88

The first thing that jumps out is that the long stroke (4") version is rubbing twice as much piston wall as the 1" stroke version. the longer stroke piston is also travelling 4 times as fast for a given rpm.

So at first glance, longer stroke doesn't sound like a big efficiency winner, lets look at some winners:


The 470 mpg guy stroked an xl125 to 185cc
1985 Matsu talks
stock bore x stroke (mm)56.5 mm x 49.5 mm.
1985 Honda XL 125 R Motorcycle Repair, Maintenance & Service

so the bore and stroke he used was 56.5 x 73.26, which is well on the long side of square.

Even in the 1980 competition the long stroke harleys won toodling along at about 100mpg.

So something about the extra torque at lower rpms was helping both of them. It probably allowed lower cruising rpms for one thing.

[janvos39]

Recent I made a modification on the front fender to improve the flow around front wheel and scoop. At the first test ride it became clear that the engine was faster on temperature indicating that less air is captured by the scoop.
On the FE I gained approx 2 %. The graph shows a decending line with ongoing modifying. Last two readings are with the modified front fender.

[The Atomic Ass]

Hmm... Interesting touch on the front end... However, I think you might do a LOT better with a full doghouse extending from the front panel to create a shell over the wheel, essentially extending the nose down to nearly ground level.

But that's just me and I'm a rabid perfectionist arm-chair modder.

[Geebee]

And perhaps fair the rear wheel in as there is an abrupt cut off along there?

[The Atomic Ass]

Originally Posted by Geebee

And perhaps fair the rear wheel in as there is an abrupt cut off along there?

Also a good suggestion, if a bit difficult. The exhaust is fixed to the engine, which is fixed to the wheel, so it moves with the wheel, and sticks out quite a bit. The transmission likewise has a case that sticks out quite a bit on the left, and also moves with the wheel. Would be a rather wide enclosure.

[janvos39]

Mr Ass/Geebee do any of you know of available data to see the difference of the suggestions with the existing situation? I saw in the book of Hucho that BMW is doing something similar on the front fender. Should the closing of the front wheel in combination with this fender bring something additional ?
THe real improvement must come with the gearing change. At 50 MPH I only use 1/4 turn of the throttle at 75 MPH this is 3/8 turn. So I am running in an absolute bad region of the BSFC map. Also according to HUcho in some cases an improvement of Cd can lead to a lower FE.

[Geebee]

IME and research closing the hole you tear in the air neatly behind you is more important than opening it for drag.
Here is a good basic text on Aerodynamics. (same book as the .jpg )
Aerodynamics of Road Vehicles book review by Nick Demma
It will require gearing and aero to see a big change as the aero will allow taller gearing for the same power.
My experience is HPV's where the power is severley limited so you do whatever you can to get drag down as reduced drag = more speed for the same output.

[The Atomic Ass]

Originally Posted by janvos39

Mr Ass/Geebee do any of you know of available data to see the difference of the suggestions with the existing situation? I saw in the book of Hucho that BMW is doing something similar on the front fender. Should the closing of the front wheel in combination with this fender bring something additional ?
THe real improvement must come with the gearing change. At 50 MPH I only use 1/4 turn of the throttle at 75 MPH this is 3/8 turn. So I am running in an absolute bad region of the BSFC map. Also according to HUcho in some cases an improvement of Cd can lead to a lower FE.

It should bring an improvement no matter the gearing. But you should realize a larger improvement once your new transmission is setup and functioning correctly.