Full Suspension Quad Bike Build

Hi All,

I am starting to design this 4 wheel, full suspension, fully electric bike and there is a whole lot I have questions about. First off the bike is primarily designed for disabled people. You will notice the left lever will be pull to go, push to brake. The right lever is pull to go right, push to go left. The reason for doing it this way is because those are movements I have most strength in. (I am a c6 quad SCI) I have a bunch of questions Ill just list bullet style:

-Swingarm or independent suspension? Ive been leaning towards a swingarm for stability and simplicity but the benefits of independent suspension I think would be that I could have one motor for each rear wheel which would provide propulsion redundancy in case of motor failure and also provide sort of a limited slip differential effect, but this is at the cost of simplicity and I suppose body roll in turns... What would you do?

-Motor. How much power would you give this thing? I am thinking 30mph would be plenty fast and maybe 10hp? I need a very reliable power plant. I don't want to get stranded! Please send me motor links! The buggy chassis before a motor and batteries should be under 100 lbs. Is there a way to do regenerative braking fairly easily?

Caster angle? This is a steering geometry question for the front suspension. How much caster angle should I give it?

I know I have a bunch more questions but this should get us started. Thanks for you help in advance!

Skye Parker

skyeg3 said:

-Motor. How much power would you give this thing? I am thinking 30mph would be plenty fast and maybe 10hp?

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Sounds like this is more like a scooter or motorcycle class vehicle than a bicycle-class. Would you like me to move it to that subforum? (it doesn't have to be moved, but you might find more info there on higher-powered stuff in that section, though there are certainly some high-powered builds here in the ebike sections).


To figure out what power level you'll actually need, we need to know your terrain, weather (winds/etc), usage, speeds, desired acceleration rate (how fast you need to reach your cruisng speed from a stop, for instance), weight, etc. Then to help find out what battery you need, we also need to know how far you need to go at those kinds of power levels.



As for reliability, hubmotors are probably your best bet, as they can be run for a wide range of speeds and power levels, with only moving wear parts that might need to be eventually replaced being the bearings.

They have disadvantages, such as weight, specifically unsprung weight, but they are much simpler to deal with than other options.

I switched to hubs from chain drives for that simpliicity, and so far they[ve proven more reliable for longer than the other homemade drives I've used--but I've never had any production middrives/chaindrives/etc., to compare with production hubmotors, so there may well be reliable drives (liek Stokemonkey, etc) that equal hubs for your purposes. (there is a new thread going on now about comparisons of those over in the non-hub-motor-drives subforum).

My SB Cruiser trike uses two hubmotor wheels in the rear, and will probably have a front hub too after I build a new fork for the purpose.


I don't know enough about suspension to help you with the rest of that.


A side note for you, if legality matters to you, regarding usability of this in public places/roads. should you encounter problems with law enforcement for any reason: many (probably most) places in the USA define a bicycle as something with 3 wheels or less (occasionally with "in contact with the ground" added in there somwehre). Usually four wheels are only allowed on larger motor vehicles, like cars and trucks, that are registered/licensed/insured, so you'd probably want ot check with your local laws on this before proceeding if you think you might have any law enforcement that gets picky about stuff like that.

Speed is another issue, variable by state; here in AZ it's <20MPH max, other places it is less or more (some up to 30MPH).

Anyway, that's all your business, but I wanted to bring it up in case you didn't know about that stuff.

If you're not using pedals its easier to suspend the seat then the rear wheels.

Balloon tires like Schwalbe Big Apples or even fat tires are pretty good at smoothing the ride. With fats on all 4 you probably don't need front suspension to keep from lifting a tire over most bumps.

I would consider 20" front and 24 or 26" rear tires. Smaller front tires allow a tighter turn radius without rider intrusion.

If you ever want to get the quad into the house it will need to fit through the door. Most doors are about 32" wide. The quad is more stable then a trike and most trikes will fit through doors.

Caster angle predominantly controls return-to-center; take hand off and how well it returns to going straight. That's a tricky one as any torque difference left-right will encourage a turn. 10° negative should be about right. Caster also affects camber change through the turn, and 10° will also be within reason there too.

Samagaga makes a decent differential if you want to try a single motor to drive both rear wheels.



Best of luck on the project.

Welcome to the forum.
Sounds like a fun project. I've wanted to build a Quad for a long time. Sadly they aren't legal in Texas, even if only pedal powered.

Your target weight and power profiles are probably going to be incompatible. A good 2 wheeled 2 horsepower Ebike with no suspension will often come in at 65lbs. add suspension and strong off road frame, plus a motor and batteries capable of 10hp, you're going over 100 pounds. A full suspension off road quad frame and wheels and devices to accommodate someone who can't pedal will add a lot more weight to that.
With a huge budget, 150lbs sounds reasonable. with a smaller budget, you're looking at 200+ pounds.
You can shave some weight and possibly clear up some legal issues by going with a Delta trike configuration: a single rear wheel.

But then Quads are more awesome. We will need some more info about how you plan to use this. A bike built for light trails and high speeds on road is going to have totally different requirements from something built to climb Pikes peak, ford streams, and blast down ski trails in the off season.

independent rear suspension would give superior performance, but a solid rear axle of various designs would be simpler to build, and have fewer possible points of failure. A Differential like Triketech posted, commonly used for go-carts, would work well. Actually most of the rear axle brakes and drive parts are probably available as off the shelf go-cart parts.

Another factor is going to be the steering. a trike or a quad is going to need a fair amount of strength to steer. even a conventional mountain bike can leave an athlete with tired arms after a fast ride. trying to do so with 1 arm in a side position that isn't optimal for good leverage is going to likely need power assist. There are servos like the Seiko PS-050 that might work, but how that was implemented would depend a lot on your own needs and the builder's skills as designing control boards and fabricating parts.

I am doing similar build. Not much progress yet. You can have a looksy here:
https://endless-sphere.com/forums/viewtopic.php?f=2&t=73001&start=0

I too thougt of going double a arm suspension but then i thought it will add more weight. Then mcpherson, but its fairly complicated and the i decided to try sort of double a arm concept using 2 pultruded fibre glass flat bars. It gives more aero, easy to make. Bad thing is i dont know how it will turn out
I will use samagaga differentil and 2-3kw 2 speed motor plus 3 speed cassette. I dont use hub motors because to alter speed, you need more battery, then all components must be swapped, controller, bms, charger etc instead of just a sprocket.
It does not have to be heavy, it all depends on application. I will use plywood and glue&stitch technique for the body and aluminum chassis.

P.S. Oh and mine is planned for two and a child, tandem with 29" wheels

How to get one classified as an electric wheelchair? Maybe large powered rear wheels and small front steering only fat cart tires? Some of the guys with souped up electric wheelchairs go pretty fast and have quite a long range, almost nerve-wracking to watch. They must know what they're doing though. Maybe base it along the lines of a racing manual W/C but with twin rear hubmotors and four wheels. :| Geared hubs so you can freewheel and make it manually operable to some extent.
Now I want to try that!

Drunkskunk said:

Delta trike configuration: a single rear wheel.

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FWIW, Delta is two rear, one front. Tadpole is one rear, two front.

Aside from that...trikes do introduce other problems, no matter which version you pick (I'd rather do a quad for my purposes, too, but only cars and up are allowed four wheels here in AZ). So it's all a compromise.

amberwolf said:

Sounds like this is more like a scooter or motorcycle class vehicle than a bicycle-class. Would you like me to move it to that subforum?

They have disadvantages, such as weight, specifically unsprung weight, but they are much simpler to deal with than other options.

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Thanks Amberwolf. Yes please move it. Yes the motor decision is forthe unsprung weight. As for police enforcement and legality... Its an electric wheelchair.. Ill hang a blue placard from the rollbar.

Triketech said:

I would consider 20" front and 24 or 26" rear tires. Smaller front tires allow a tighter turn radius without rider intrusion.

If you ever want to get the quad into the house it will need to fit through the door. Most doors are about 32" wide. The quad is more stable then a trike and most trikes will fit through doors.

Caster angle predominantly controls return-to-center; take hand off and how well it returns to going straight. That's a tricky one as any torque difference left-right will encourage a turn. 10° negative should be about right. Caster also affects camber change through the turn, and 10° will also be within reason there too.

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I like the smaller bmx size as I want to be able to slide this thing through corners. I just dont see the 26" wheels handling that lateral load. Theres no way Im making it narrow enough to fit through a doorway. This is for use OUTSIDE 10 degrees - noted.

Maybe an alternative idea for the rear is to use two independent suspended moped or MTB rear swingarms/forks with a hub motor in each (2wd). I'm not sure what speeds the Magic Pie in the cast 20" wheel (with sticky 16" moped tire) is capable of. But that way you would have serious acceleration and reliable back up when one motor or controller fails. As the controllers are built in it would make for a very clean and simple built. And apparently it has a reverse capability but not sure if that is "on the fly".

SlowCo said:

Maybe an alternative idea for the rear is to use two independent suspended moped or MTB rear swingarms/forks with a hub motor in each (2wd). I'm not sure what speeds the Magic Pie in the cast 20" wheel (with sticky 16" moped tire) is capable of. But that way you would have serious acceleration and reliable back up when one motor or controller fails. As the controllers are built in it would make for a very clean and simple built. And apparently it has a reverse capability but not sure if that is "on the fly".

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Im actually really wanting 2wd with two independent systems. I would like to stay away from in hub motors as I want to do some rough terrain and I dont like all that unsprung weight. So Ill need some inboard motors.. still need to source one.

Yes, it's probably much wiser to build with a lot more sprung weight in the form of a reduction, differential, driveshafts and cv joints...
Because then when the single controller or motor or diff or reduction etc. fail you strand immediately :lol:

If it's not for competition i wouldn't be hung up on the unsprung weight so much.

SlowCo said:

Yes, it's probably much wiser to build with a lot more sprung weight in the form of a reduction, differential, driveshafts and cv joints...
Because then when the single controller or motor or diff or reduction etc. fail you strand immediately :lol:

If it's not for competition i wouldn't be hung up on the unsprung weight so much.

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Thats why Im using two independent systems. Never be stranded anywhere. Yea I am not 100% set on the current setup for the reasons you mention. Using hub motors would be good also bc I could put the batteries where the motors are now... Id have to figure out how to mount a hub motor from one side with a disc brake... I'll keep thinking on it.

Not ready to give up on the in-board drive just yet. Does anyone know of a motor that would be good for this? To go 30mph using the config shown the motor would be spinning 3000 rpm. Don't need any faster than that. Just estimating I think 2000w sounds about rtight. (There will be an identical setup on the other side).

CLICK THE PIC---->

http://www.goldenmotor.com/

klick at the goldenmotor website on the BLDC Motor button and scroll down to the BLDC-108 1500W motor and controller. If it needs to be even cheaper you can scroll down to the Electric Tricycle Motors.

SlowCo said:

http://www.goldenmotor.com/

klick at the goldenmotor website on the BLDC Motor button and scroll down to the BLDC-108 1500W motor and controller. If it needs to be even cheaper you can scroll down to the Electric Tricycle Motors.

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Thanks Slowco those look perfect actually. I was also thinking a great solution could be a inboard mounted hub motor. Just need to find one with a 8" or less diameter with enough power.

This may help...i'm trying to something like this with a powerful mid drive...
http://www.contesengineering.com/athos.html

Stoked to see how similar projects go...folowing

ContesEng. has a heap of youtube vids...and some nice technical drawings of suspension design on that site^.

Leebolectric said:

This may help...i'm trying to something like this with a powerful mid drive...
http://www.contesengineering.com/athos.html

Stoked to see how similar projects go...folowing

ContesEng. has a heap of youtube vids...and some nice technical drawings of suspension design on that site^.

Click to expand...

Hey Leebolectric. Yea that bike is sweet and I've watched the youtubes but no contact on the site unfortunately. I'd check yours out as well do you have a thread?

I have no progress yet...only a wish so far...but looking into frame builders...or maybe just go to UBI.
When I get it rolling..i'll post for sure

Just an update on current things I am researching...

I was almost persuaded to use hub motors for this build but after seeing that the mxus 3000 weighs in at 20lbs.. I just dont see one sided mounting working well. So I am back to the inboard motor design for now. (It's much sexier anyway)

So my current hurdle is finding the perfect motor. The wheel drive shaft only needs to spin about 400rpms to get me up to 30mph and I want to get there with one belt stage where I think I can do about a 6:1 reduction. So I need something that spins about 3000rpm. Any ideas?