Gearing and Fuel Economy

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Hexafluoride

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Mar 15, 2015
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Wisconsin
Hi everyone! I've had an interest in motorized bicycles for a long time now and I'm finally getting around to building my first one. I'm looking at putting a 4hp engine I have laying around on a Schwinn Meridian tricycle.

From what I've gathered, bikes and trikes aren't very stable at speeds greater than 30-40 mph and a 4hp engine should provide enough power to hit upwards of 50 mph if geared to do so. I want to build this trike for maximum fuel economy and I was wondering how to go about the gearing. Acceleration isn't much of a concern, but the trike has to eventually hit 35ish mph.

I was thinking of gearing the engine so that the trike is going around 50 mph at 3600 rpm. I would never go 50, but if I'm cruising at 35 mph, my thought is that the lower rpms would allow for better fuel economy.

What do you guys think? The big questions I have are: Would the gearing make acceleration impractically slow? Will running the engine at lower rpms result in a non-negligible increase in fuel economy?
 

2door

Moderator
Staff member
Sep 15, 2008
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Littleton, Colorado
Welcome to the forum, Hexa. Thanks for joining us.

You're going to hear this from others so I'll be the first to tell you that trikes are inherently unstable. 35 isn't an unreasonable figure and is completely attainable with the right engine and gearing but, if you have no prior experience controlling a three wheeled machine at that speed you need to be very careful initially. They can be a hand full.

As for 50mph? You wouldn't get me to ride it at that speed. Be careful, please.

Tom
 

Davezilla

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Mar 15, 2014
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Your goal is a bit far fetched at best... there's more than rpm that'll determine mileage and in some cases... especially in strikes with small engines, the way your going to get Max efficiency is by gearing it down not up.
If you're after the best fuel efficiency you need to also consider engine load, if you gear it up so high that it requires 3600 rpm to push it at 50 mph you're going to need a ton of torque to do so... a lot more torque than a 4hp engine can produce. You need to reduce engine load as much as possible thennrunnyour gearing so it'll cruise at the lowest possible rpm with the highest vacuum reading... the higher the vacuum the lower the load placed on the engine is. This is why motorcycles typically run at much higher rpms than a car. If you tried to gear a motorcycle to cruise at 3500rpm to do 70mph you would want that engine to be at or close to its torque peak so you would need roughly around 1500 cc's for the engine to run comfortably at 65 at that low of an rpm. That's basically about 80 ft/lb torque or so, but since it only requires about 20 ft/lb torque to maintain this speed, it can do it very efficiently and use less gas.
now lets say we have a smaller engine capable of cranking out 20 lb/ft torque at this rpm, it takes this much torque to maintain that speed at that rpm, but the engine is maxxed out doing so so the vacuum at this speed is zero and its gas mileage will suffer badly even tho its a much smaller engine and has enough torque tonmaintain that speed at that rpm.
Now what we need to do is multiply this torque thrum gearing so the engine only needs to produce lets say 7 lb/ft torque to move the bike at this speed but at a higher rpm like lets say 7000 rpm. The engine can maintainnthe speed but this time the throttle is barely open and the rpms are a lot higher and if a vacuum gauge was installed you would see about 1/2 to 3/4 the amount of vacuum you would read at this rpm but in neutral... now we're able to get better mileage because were not overloading the engine by gearing it too high for its output.
all my numbers in this are just figuratively speaking but if you get what I'm talking about you'll understand how to set up the engine and gearing so thebike can cruise at the desired speed with the least amount of load on the engine and at the lowest rpm possible. The easiest way to measure load is with a vacuum gauge. High vacuum = low load so you always want to gear for the lowest rpm that will give you the highest vacuum reading, this is the formula for getting the best fuel mileage.
We cant just gear something super high to get it to run a certain speed because there's always a point where the engine wont be able to reach or maintain that speed and the harder the engine has to work, the more fuel it needs to burn. Think of it like a 18 speed mountainbike... you can pit it in the highest gear at 10 mph but its not going to be that easy to pedal, but at 20 its not that hard... until you reach a slight hill or have a slight head wind, now you have to really pedal hard and its a lot of work, but you can down shift. Once you downshift you can.maintain that speed fairly easy, but if you shift down too far you're pedaling too quicknand working too much for the speed you're going.... your engine feels the same way so there's that happy meduim you need to find so it can maintain the speed you want but still have some power to spare.
 

Hexafluoride

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Mar 15, 2015
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Wisconsin
Thank you for the information! I just want to clarify, I don't intend to go 50mph, probably 30 or 40 tops. Based on your response Dave, I believe I would be best off gearing it so that I have lower rpms at my desired cruising speed.
 

Davezilla

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Mar 15, 2014
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Exactly... you want to set your cruise rpm so the engine has as little load on it as possible but at the lowest rpm you can run it without it loading up.
going fast on a trike can be dangerous because of their inherent tendency to roll over in a turn if you try to turn going too fast. You can do things to lower your center of gravity to help as well as widening the track on the rear wheels but this will just raise the speed in which it'll lose stability. Try to mount your engine as low in the frame as possible and set the seat bight as low as you can ride it comfortably... this will help some, and every little bit helps here.
Personally I wouldn't ride on a trike faster than 25 or 30 mph unless on a long straightaway and slow way down at the turns until you really get used to how it rides and where its going to start to tip over. You can do this in an empty parking lot or on a wide enough road to find what speed one of the rear wheels will lift off the road and get a feel for it so you can recover before it tips over in case you get in a situation where you may need to swerve to avoid a crash.

Back to the gearing, if running a China girl engine you can reach up to about 40 on a trike with 26" wheels with a strong engine and a 36 tooth rear sprocket but acceleration might be weak. You can hit 40 with a 44 tooth but you need to port the cylinder so it can hit 9500+ rpm. A 40 or 42 tooth rear would probably give the best all around performance with a decent cruise speed at around 30 to 35 mph.

If you're going with a 4 stroke or andifferent 2 stroke let us know which engine you have in mind so we can figure the best gearing. My opinion gonna 4 stroke that would do good in a trike would be a.predator 212 with a Comet CVT and a 50 tooth rear, the cvt would let the bike accelerate very well but the rpms will drop considerably at cruise since these will sense engine load and rpm to set the ratio right where its needed at any rpm or speed. I had a go mart with one of these torque converters whenni was younger and it accelerated really well and topped out around 60 mph but would cruise all day at 30 to 40 without feeling likenit was over reving all the time, then when ya gunned the throttle it would take off good.
 

mapbike

Well-Known Member
Mar 14, 2010
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Central Area of Texas
I agree that a ttike at high speed is a big risk, one little bobble and you'll be skipping and rolling across the road.

3600 rpm with a 4hp engine wont even get you to 30mph with a gear ratio that will have enough grunt to get you over a speed bump.

30-35 mph is possible with a 4hp engine thats set up right, but the rpms will need to be above 3600 to attain it and to get those rpms the governor needs to be bypassed.

Best wishes on the build and personally I dont recommend doing 25-30mph on a trike unless its a Harley Trike...lol
 

BarelyAWake

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Jul 21, 2009
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Jus' for some "stability" clarity, a 'Schwinn Meridian' is an upright delta tricycle & it's true this is one of the least stable configurations at speed... there are however other trike designs that while there are other considerations to be concerned with, (price, ease of build, mount/dismount, visibility, etc) turn & bump stability is less, even far less a worry.

Tadpole trikes (two in front) haven't nearly the propensity for tip-overs, even less so if it's a recumbent & even deltas can be very stable as a long wheelbase recumbent. Recumbents can be difficult to motorize, expensive, even awkward if the rider has some physical disability - but their stability is far greater due to their low center of mass. Tadpoles are far more stable until they're suddenly not whereas deltas are just somewhat unstable regardless - for example taddys are great, unless you start to sideways... then bad things happen very quickly lol, whereas they use (low) deltas for drifting.

Bumps/bump steer is a concern with all trikes, less so with what few have a suspension - but is a difficulty inherent in their design as when one side's wheel leaves the ground that leaves the other two in-line (center & side), but not the direction you're directly facing, so it becomes a bicycle in a turn rather then the tricycle you thought was going straight. Regardless of upright or recumbent generally taddys have more an issue with bumps & steering then deltas, but wheelbase also plays a factor, as does shocks & dampeners, if yer into that sort of thing.

If you wish a trike there's a bunch to choose from, it all depends on your needs & wants - if there are none other then stability at speed, a recumbent taddy is a good bet. Ease of build & budget? An upright delta like the Schwinn is a great way to get'er done. Dare to be a lil different? Recumbent deltas are fast & fun...

Your call, I jus' wanted to say not all trikes are so risky, that speed & stability are both comparative - that while all the previous advice is mostly accurate & defo well-intended, there are some oversights regarding additional suggestions, some amount of over-statement in concern for your well being... and I'm not entirely sure I'd agree an actual 4hp has such limitations.

Uprights reach 30+mph fairly easily with little more then 1.8-2.5hp with number of wheels notwithstanding, the additional drag minute comparatively. My recumbent taddy (far too) easily exceeds that with but a mere 2.5hp, but it has a low profile (wind resistance), multiple gears & a four stroke's torque ;)

Food fer thought anyway :)


edit: ...I knew I had it somewhere lol, here ya go map - a neighbor's upright delta Harley & my recumbent tadpole, his is defo faster w/the oodles of engine (also cooler & worth far more ofc) but w/o a doubt mine is more stable in the hairpin turns at least... gotta take what win ya can I figure heh;

 
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Hexafluoride

New Member
Mar 15, 2015
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Wisconsin
That tadpole looks awesome! I was considering the Meridian primarily because of cost and the ease of mounting an engine in place of the rear basket. I would love a recumbent tadpole, but the price seems to be out of my budget for now. Would it be practical to convert a recumbent bike to a tadpole trike?
 

BarelyAWake

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Jul 21, 2009
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Defo go ahead with the 'Meridian' for cost and the ease of mounting an engine - it'll make a fine ride and a great first project! I jus' wanted ta offer a bit more to help a clearer picture of the topic in general, there's a great diversity in "trikes" and not all their quirks known is all :) I'd not start a first project with a recumbent unless ya already had one, they're a bit odd & usually quite expensive.

BTW, "tadpole" just means a tricycle with two wheels in front, "recumbent" just means a more prone position then sitting bolt upright so there's no need to "convert", they can be bought that way just like an upright delta, like your 'Meridian'. A tadpole can also be upright, like fasteddy's Indian Tri-car project;



...which will look something like this, only shinier;


BTW, more info can be found in Silver's thread here if you wish;
http://motorbicycling.com/showthread.php?t=25692