Mounting your battery, Center of Gravity.

[Tiberius]

Just had an epiphany on this.....

I've gone over to rear hub motors for my new builds. I've got a commercially built bike with a front hub motor and I'm not entirely happy with the handling - I've been putting it down to the weight of the motor on the front wheel affecting the steering

But if we consider this idea of the roll axis. It passes very close to the rear hub, but well above the front hub. That alone explains the difference between the rear hub and front hub motors. Its not that the front hub weight is on the part that steers, its not that its at the front rather than the back. The important factor is that its well off the roll axis.

Nick

PS. There are corollaries to this. If you can only afford one lightweight rim/tyre, put it on the front. Use a rim rather than a disc brake on the front.

 

[Fumesucker]

Okay, so I have a thick skull sometimes. I had to take my bike out for a ride and really analyze what has come naturally since about a week after I got my first bike quite long ago.

Eh, we all have our blind spots. It's interesting that it's not what we don't know that is most dangerous to us but rather what we do know that isn't so.

Thanks for an enjoyable chat, I often solidify my own understanding of things when I try to explain them to others.

Your observations make it fairly clear why riding someone on the handlebars doesn't really louse up the handling as much as you might think it would.

I'm pretty sure someone on this thread said they mounted their batteries on the front fork and liked the way that made the bike feel.

 

[Fumesucker]

Just had an epiphany on this.....

I've gone over to rear hub motors for my new builds. I've got a commercially built bike with a front hub motor and I'm not entirely happy with the handling - I've been putting it down to the weight of the motor on the front wheel affecting the steering

But if we consider this idea of the roll axis. It passes very close to the rear hub, but well above the front hub. That alone explains the difference between the rear hub and front hub motors. Its not that the front hub weight is on the part that steers, its not that its at the front rather than the back. The important factor is that its well off the roll axis.

Nick

PS. There are corollaries to this. If you can only afford one lightweight rim/tyre, put it on the front. Use a rim rather than a disc brake on the front.

Interesting, I hadn't thought of that.

I think you are correct.

 

[safe]

Its not that the front hub weight is on the part that steers, its not that its at the front rather than the back. The important factor is that its well off the roll axis.

I agree. That's how it seems to work.

 

[John in CR]

Its not that the front hub weight is on the part that steers, its not that its at the front rather than the back. The important factor is that its well off the roll axis.

I agree. That's how it seems to work.

I think it's a combination of the gyroscopic effect of that extra weight spinning, and the extra weight on the wheel. I think everyone should take a hard look at the animation previously posted. The rotation around an axis in front is minimal while it's up above the ground, and then the axis moves rapidly down to the ground where it remains for most of the roll during a turn. The only issue I notice with my front motor, is when I do something quick and forcefully like dodge a pothole I hadn't noticed. At low speed, I can feel that extra weight change direction quickly, and at higher speeds I think I can feel the gyroscopic effect. If anything, with a lower speed pothole dodge, the front hub motor weight seems to lower the roll axis, resulting in a greater motion at the handlebars, more like they are moving themselves than more force required to make them move. Some of that is probably the pull of the motor too. It's really difficult to pin down, and even more difficult to put into words.

John

 

[safe]

Can we agree that no matter the effect it's not good?

Ideally we want zero weight on the wheels and all the weight of the bike concentrated into an infinitely small (like a "Black Hole") point that happens to coincide with the center of mass of the rider and all this is also centered along the rotational axis that the gerometry defines.

That's a lot of "perfects" to deal with. :lol:

So we "muddle through" with many compromises. There is no such thing as "perfect" in actual practice and many adaptive compromises seem to work out okay.

As a rule though... weight should be concentrated along an axis that roughly follows a line from the axle of the rear wheel forward to near the top of the front wheel.

Ebikes suffer from the problem that the best place to put the bikes weight is also the place where your legs need room to pedal. So there is a serious conflict of interests with ebikes and it will be interesting to see how people choose to resolve this in the future. I'm moving to either a frame centered battery location (like my existing bike) or a "radiator mount" location while stretching the wheelbase out long enough to make it possible. With enough wheelbase you can place the batteries in front of the pedals like the radiators on road racer motorcycles.

 

[mikereidis]

I'm pretty sure someone on this thread said they mounted their batteries on the front fork and liked the way that made the bike feel.

I don't recall anyone else, but I said I would CONSIDER mounting my batts high (somehow) on/over the front boom. Maybe when the snow flies I'll try rebuilding this way. I'd probably have to put them on a pole or something, if I want to maintain my front pedals. Would need some serious building to make sure they are on good up there. I don't think duct tape alone would cut it. Don't want to see my batts flying into the river on a cliff corner.

As a test, I've tried riding at low speed with my left foot as low above the ground as possible and as high as possible. I believe I noted that my roll rate was slower, and low speed stability higher, with my foot up high.

I'm REALLY noticing lately how my recumbent low speed handling sucks, perhaps because I have 30 pounds of lithium now and bike weighs about 80 pounds (!). Lately I need a right foot kick AND a left foot kick to get moving at minimum stable speed (and start my pedal first controller) instead of just one right kick, and I think this is due to the weight. When tired, it can take me several clumsy looking attempts to get going sometimes, with me falling to one side or the other and USUALLY catching myself with a foot.

So yeah, if mounting batts at front of boom as high as possible helps me noticably at slow speed, I will do it. Will also help solve my under-seat steering to battery interference issues which prevent me from taking tighter turns at present.

 

[safe]

If the wheelbase is long enough so that your pedals can clear in the front you can use a radiator mount location. This would require a custom frame though.

 

[John in CR]

I'm REALLY noticing lately how my recumbent low speed handling sucks, perhaps because I have 30 pounds of lithium now and bike weighs about 80 pounds (!). Lately I need a right foot kick AND a left foot kick to get moving at minimum stable speed (and start my pedal first controller) instead of just one right kick, and I think this is due to the weight. When tired, it can take me several clumsy looking attempts to get going sometimes, with me falling to one side or the other and USUALLY catching myself with a foot.

So yeah, if mounting batts at front of boom as high as possible helps me noticably at slow speed, I will do it. Will also help solve my under-seat steering to battery interference issues which prevent me from taking tighter turns at present.

Hey Mike,
Batteries out on a pole sounds problematic to me. What about away from center to both sides in order to slow the roll rate, like walking a tightrope with a pole to help stability by reducing the roll rate. Do you squeeze through tight enough places that some extra width would be harmful, other than aerodynamically? It would be easy to experiment with using a stick or tube with some weight at the ends. A small change in your backrest toward a more upright position might work wonders too, since it's the vertical length of a rider on a upright bike the creates the stability, not the absolute height of their center of gravity. eg A low riding recumbent doesn't look significantly less stable than the high rider on the SWB recumbent. Those high ones look pretty shaky too.

Hey Safe,
Time to rethink those lines and edit. The rear definitely rolls around the point where the rear tire contacts the ground, and the front end of the roll axis moves up and down, not based on weight, but based on the stage of the curve and how sharp a turn you make.

John

 

[safe]

Hey Safe,
Time to rethink those lines and edit. The rear definitely rolls around the point where the rear tire contacts the ground, and the front end of the roll axis moves up and down, not based on weight, but based on the stage of the curve and how sharp a turn you make.

Going to have to disagree on this...

First let's agree that the true dynamics are complex and to be "perfect" you can't use a single rotational axis to describe everything that is going on with countersteer. Let me instead sort of walk you through a "thought experiment" about it:

So you start out with the bike upright with no lean and following a perfectly straight line forward. You want to turn RIGHT so in order to make this happen you need to countersteer and slightly turn the handlebars LEFT. Now that the handlebars and front wheel are pointing off of center and in the direction LEFT the tire initiates a torque on the forks that twists the frame at an angle. Now here's the "catch"... the front wheel has actually moved off of that original straight line path and is now a few inches to the LEFT of the original path. The rear wheel is now TURNED relative to the original path. So the rear wheel is actually now going to follow the front and start to diverge to the LEFT.

Okay...

Now at somepoint you are going to want to stabilize the lean angle or else you will continue to fall into the turn all the way to the ground so you have to limit your countersteer to no more than the lean allows. So you have to now begin to reverse the handlebars and steer inwards to the RIGHT so that the front end of the bike starts to head towards that original path that was a straight line. Finally... finally... the front tire crosses that original straight line path and starts to actually head to the right... but where is the rear wheel?

The rear wheel is starting from slightly to the LEFT of the original path and only follows the front wheel over the line afterwards.

The crazy thing is that when you want to go in one direction you have to have ROOM in the other (a few inches) or you can't initiate a lean.

I was forced off the road by a big truck and got out onto the lip of the road and once my tires started off the edge there was nothing I could do... no room left to countersteer.

Try it... try riding on a straight line and watch where your tire goes... it will cross the line in one direction first and then come back over it. Puddles are good for this... get the tires wet and then ride on dry pavement and then you can see the tracks. Many people never really study what is going on.

You could actually get a tape measure and find out the difference of deviation from the straight path of the front verses the rear wheel and then be able to mathematically determine (approximately) your steering geometry. Maybe for a simple turn the front moves 4 inches to the left and the rear only moves 2 inches... or something like that. From those numbers you could figure the abstract "rotational axis based on geometry alone".

 

[huskydave]

Every bike is different riders weight are different, weights of the bike type of suspension and their are many variables. I think either in the frame near the center of the bike or make baracket or a few so your batteries hang down near the headtub are the best bet but it depends on what you want to do with the bike and what type of bike you choose for conversion. If you plan on pedaling that changes things as well you need room for you legs to move and for you to be comfortable in different positions. Tell us you game plan choose a bike with steel dropouts and we wil help you decide what you need. Give as much feedback as possible. I have used a lot of veterans ideas and they worked out great.

 

[John in CR]

Safe,

Disagreement is fine and can be productive, but your paragraphs of typing support only agreement. It's common sense that at the rear, unless the tire slides, the axis of any roll is at the ground. We agree that the front is different due to the counter steer that sets up the proper weight distribution for a turn. What this means in terms of the effects of battery location on handling is unclear to me, other than to support my initial opinion that inside the triangle or as low as possible near the rear axle will leave your bike handling most like it does with no batteries. My experience, though somewhat limited, is that battery placement is far more important when you are off your bike moving it around. Then high battery placement is much more noticeable and problematic than while riding.

I want to build a better frame that incorporates a place to enclose batteries, so I believe experimentation is warranted using significant weight placed in different locations to evaluate the effects on handling. Since a vertical orientation of the rider leads to greater stability, it's likely that the bikes with the vertical battery boxes behind the seat post are already using the ideal. It provides minimal impact on aerodynamics, they're out of the way on or off the bike, and it keeps the front end light, making it easier to pick up and manhandle in tight quarters. Full suspension is a roadblock though, and cushy comfort cruisers are probably mandatory for widespread use by people spoiled by the comfort of cars.

John

 

[mikereidis]

My experience, though somewhat limited, is that battery placement is far more important when you are off your bike moving it around. Then high battery placement is much more noticeable and problematic than while riding.

John

Good point, and as always, testing/experience trumps theory.

Yes, it will be harder to walk a bike and keep it vertical with weight high versus low, regardless of front or back. ALTHOUGH, bikes ARE steered when walking them.

I think I will try putting half of my batteries, and my controller on front boom to test this weekend. Other half of batteries will be removed, since I have a modular battery system now (I'll be at 40v, 12ah, instead of 80v, 12ah). All those batteries under my seat interfering with my range of steering, REALLY makes this bike unsafe when turning at lower speeds (needed for tight corners especially, but even wide roads are giving me some issue).


Hmmm, if weight high at the front improves low speed handling, why would it make walking the bike (extra low speed) harder ?

 

[SteveMush]

However, with the 25lbs up there, including controller, etc, how much of a differance would I feel while handling and riding the bike with this much weight mounted up so high? Of course I will be going with a rear hub, which is 25lbs down low, but not sure what this will do to the overall stability of the bike.

Yes, it will be harder to walk a bike and keep it vertical with weight high versus low, regardless of front or back. ALTHOUGH, bikes ARE steered when walking them.

I just completed a "stealth" converssion of a $40 old Royce Union 10-speed road bike to an electric commuter bike for my 22 mile daily round trip to work & back.

What was a light, nimble bike now has about 9 pounds of motor/drive unit hung under the top horizontal frome tube (clamped to the slanted members at each end & covered w/alum sheet & finished to make a plain looking black "box" between my legs, with a single drive chain down to the crank sprockets underneath).

I put four 12V 12AH batteries to make up 24V, 24AH into a fabric "tackle box" bag, strapped atop a carrier platform above the rear wheel. That put another 35 pounds at that location. Oh yes - I'm a quite capable long distance road bike rider, but at 280 lbs, I am by no means a light component of the system, myself...

As per the quotes above from others, this nimble bike is now very top-heavy and unstable when walking it, but quite rideable on the road, traveling. It is entirely a different vehicle, but I fully expected it to be so.

One note: All that extra weight makes for good brakes being critical, and any injuries that might occur from dumping the bike would be much worse. I found this out when I got on it in the dark at night, went to the end of my driveway, and dumped it when it didn'turn, but the fromt wheel skidded and slid with all that inertia behind it. I had a front dflat tire and didn't know it in the dark until I skidded on the rim. This makes me fear a real skid at speed on a wet road, for instance, as what would have been a minor knee-scrape on a light roady was actually a quite bad foot-sprain as 60 pounds or so of heavy bike fell on my foot...

From my short experience so far with this commuter conversion, my ideal ebike just might be an elongated frame with batteries in a row along a horizontal lower frame tube. A long wheelbase recumbent type might be ideal, I suppose.

Power details on my commuter roadie:

Kollmorgen 'HiKol" CCW version (cheap, about 400 watts, nominally, higher under load), Lashout planetary gearbox on motor (4.75:1), chained (#25) to 72T spur on 5/8" alum jackshaft with go-cart ball bearings, 16T AC Claw freewheel sprockt on freewhel adaptor, driving a bike chain to a 44t on freewheeling sprocket set at the bottom bracket. I can use 5 speeds. Lots of torque from a small motor with 5 speeds!. Top speed (level grount) presently at 17mph. Going to a 17T AC sprocket will give me close to 20mph so I can get a boost pedaling a bit faster dowhills. Speed up moderate grades is about 10mph. In comparison, I used to do my commuter trip in an avg of 14mph. Now I get 16mph avg at present gearing without arriving at work in ball-of-sweat conditionj Not bad for hauling my fat *** around!

 

[safe]

It's common sense that at the rear, unless the tire slides, the axis of any roll is at the ground.

You need to go and do that puddle test I mentioned... get your tires wet and ride on a straight line, then make a turn to one side. If you then go back and look at the track marks of your tire you will see the front tire move first in the opposite direction and then back again over the line and then off to the turn. What will shock you is that the rear tire moves in the opposite direction slightly too as it follows the front wheel before it also follows back over the line. The frame veers to the opposite side and leans at the same time. (the torque on the frame is both rotational and also a pivot to the countersteer path at the same time)

Another factor is the shape and width of the tire. If you have a very wide and flat road racer tire it changes things a great deal compared to a tire that narrows to a point. (like a typical street bicycle tire... 1" wide) You end up having to rethink everything with wide tires verses narrow. (it seems that a steeper steering geometry and less offset works better with wide tires)

The fact of the matter is that it's complicated... all these factors contribute to handling... but I'm sure when you think about it longer you will realize that it's pretty much impossible for the front tire to change direction without it effecting the rear... the rear does not hold a straight line... and in fact "wiggles" during countersteer and lean preparation.

Below are the tracks for a right turn...

 

[safe]

I put four 12V 12AH batteries to make up 24V, 24AH into a fabric "tackle box" bag, strapped atop a carrier platform above the rear wheel. That put another 35 pounds at that location.

I've got four 18Ah SLA's in the middle (as you can see) and the handling is superb. My only advice is that if there's no way to mount batteries in the middle (which complicates pedaling) then you might try a saddlebag location on the rear. Many of the people on this forum have rear suspension and saddlebacks are impossible for them, but if you have a street bike you could lower the weight a great deal and it would handle a heck of a lot better.

 

[SteveMush]

Handling with the large pack over the rear wheel is quite adequate for commuting. It's only a bear when 'walking' it as it is considearbly top-heavy. That is due to the height of the mass, not it's front/back distribution. When riding, it's mass is not a problem at all - just not as "nimble" as my light non-electric roady is (as too be expected).

What is an issue is the need to be cognizent of the safety aspects of all that extra weight - Good brakes needed, spills will be harder & likely more injurious. That's a perscription for any heavy ebike of any configuration

I have an idea of what my ideal ebike configuration would be, but I have to learn to weld aluminum tube, first In the meantime, I'm thoroughly enjoying my daily commute these days, gas-free!

 

[safe]

Handling During Braking

Things get really screwy when you add braking to turning.

If you try to countersteer while under heavy breaking you are likely to lose the front of the bike and go down. You have to get on the brakes hard when you are upright and then practically yank the handlebars in the countersteer direction to initiate the lean. The rear has to drift out a little and this is where I heard the guy who is a professional road racer (Jay64) talk about "backing it into the turn" by using some rear brake to loosen the rear end and get it to come around a little faster. The rear has to drift out... not as much as the front, but it does have to deviate from the initial path before the bike is leaned over fully.

Anyway... there's a transitional area where using the front brakes is bad news, but a little rear braking is okay. I've found myself doing that a lot because it seems to steady the rear end more... it actually makes the bike want to stand up a little and that has the effect of keeping the front end down better. Just a trace of rear brake... don't grap a handful of it. It's hard to describe, but it has to do with all the geometry issues we've been discussing.

 

[safe]

Handling with the large pack over the rear wheel is quite adequate for commuting.

If you are "done" fiddling with the bike then I understand that, but it would be a good test (people here would be interested on your feedback) if you tried a saddleback configuration for your batteries.

You never know...

The improvement might be so significant that you will give it rave reviews and it will spawn a minor trend in bike building.

 

[SteveMush]

I might fabricate a couple of carrier trays to place the batteries at the side and below the rear axle (low).

The "stealth" aspect of the bike is paramount, though, because I live in New York State, where ebikes are specifically not legal to ride at all.

(There's been a bill proposed in each year to legalize them, but it has not passed yet)

The way I have it set up, all one sees is a neat "black box" below the frame's top bar, between my legs, and just one chain from below the "box" to the bottom bracket sprockets - again, between my feet & legs. The battery pack is in a "normal looking" bag on the back rack.

Except for the whine from the planetary gearbox on the motor, one wouldn't know it was an ebike. Certainly not when i turn the motor off and pedal past the police

Hanging four 12v, 12AH batteries "in the breeze" at the sides of the back wheel might cause me some problems there....

 

[steveCA]

I posted this on another thread, but you might enjoy looking at my "ugly" but (what IMO is a) very effective solution to the battery mounting "center of gravity" problem...

I have one fairly close photo of my battery box assembly. Its really very simple. I needed a stonger front axle, so I purchased a new front hub with a solid front axle, same size as the axle used in BMX. I then extended that axle on each side using some titanium "buddy pegs" made for BMX bikes. I then cut a second BMX axle in half and screwed the halves into the outer threaded bores of the titanium buddy pegs.

I then measured my batteries and made a trip to my local surplus metal supplier. I found a length of rectangular aluminum tubing that had a wall thickness of about 3/16, and was long enough to cut in half. Internal dimensions were not perfect, but I used the extra space to provide some cushion between box and duct tape battery. I removed about 40% of the weight of the boxes by milling and drilling. I machined out the four steel "hangers" that connect the boxes to the axle. It turned out to be solid as can be. I have one thousand miles of rough commuting miles on it now, and there are no rattles, nothing has come loose. If I happen to hit something with one of the boxes, it will actually give a little by turning a bit on the axle, no harm done. Balance is excellent, with some adjustment needed for leaning into turns. This rig requires a small additional weight shift to make low turns at speed. I much prefer making this adjustment to having a high weight load when the battery is mounted above the rear axle, or worse, on a back rack. Bike will stand on the stock stand with no danger of falling over.

Hope that helps... let me know if you need more photos... I'l break out the camera again!

Thanks,
SteveCA

 

[safe]

Just as long as you realize that mounting weight low at the front wheel is violating every rule in the book I'm okay with it... gosh... that looks to my mind to be a disaster waiting to happen.

My advice... go sloooooooowwwwwww....

If you plan to mount your batteries low they need to be at the rear and not the front. This design is clearly incorrect.

 

[SteveMush]

I was thinking to create a similar "off the axle" hanging battery system, but off the rear axle, with pannier-type bags to either side of the rear wheel. I was thinking to mount two battery trays at about axle height and enclose them with the panniers, perhaps cutting out the back and bottom of the panniers to maintain the "stealth" goal I had to begin with.

 

[John in CR]

Safe,
Adding a correct graphic to an incorrect one doesn't make it any better. Just because the rear wheel doesn't go straight, doesn't move the rear end roll axis off the ground other than the tiny amount resulting from tire flex, but that's deep in the curve anyway and has nothing to do with the effects weight has on countersteer. I'm starting to understand that violating the Rules According to Safe is a good thing.


SteveCA,
I find it very interesting that you're not experiencing clearance issues with corners so low and wide in the front. I'd be terrified of those bottom corners getting caught on something solid resulting in some kind of cartwheel wipeout. If it's really not an issue where and how you ride my only suggestion would be to make them more aerodynamic with fat rounded fronts and longish tails. Not only could it look cool with a retro/futuristic feel, but also add some crash protection for your batteries. Plus it could be done for negligible weight penalty more than offset by gains related to wind resistance.

I'm about to split one of my Pings in half to mount saddlebag style near the headset, since only one fits in the triangle. It's not going there because I believe it's the best place in terms of handling, just convenient and safe for my batts. If it wasn't so wet here during part of the year and the roads too bumpy, I'd consider turning them into a couple of axle hanging blunt torpedoes.

John

 

[steveCA]

I'll admit that my biggest concern when I started my battery mounting project was clearance... 1. Clearance between the front spokes and the battery boxes... and 2. Clearance between the battery boxes and the ground while turning.

It turns out that careful selection of the right materials that have the correct stiffness and rigidity was key. The bike is powered by the Phoenix Cruiser, and 30ah of LIFEPO4, and will easily hit 30mph on the flat. My commute is 20 miles round trip and it is NOT smooth and straight. After 1000 miles without problems, I can say that the proof is in the use. Could there still be some catastrophic failure? Absolutely! But the highest risk IMO is getting run over by some crazy pilot of one of the 3K lbs behemoths I share the road with!

On the clearance issues... I check the rigidity of the battery boxes each morning by giving them a good up and down yank! No movement so far, and like I said above, the road is full of high speed bumps, cracks and potholes.

The ground clearance issue is something I keep in mind while riding. I find that in a tight right turn for instance, if I have my right foot in the down pedal position the pedal hits the ground long before the bottom of the battery box hits. Still, the limit is there... I have never actually hit the boxes in a tight turn so far. (I've never hit the pedals on the bike, either.) It is also true that I have to keep in mind the width restriction. This turns out to be easier than you might think, though, since the assembly sticks right out there in front of you, and not behind, where I can't see how close I might be to scraping against something.

The safety test for me has been in miles put on the bike... and so far, I'm feeling very comfortable with the design.

SteveCA