Torque specs for 6,000lb party bike

[Electro Dave]

Hi,

I'm trying to figure out the minimum specs of each components of the new party bike I'll be building to make sure nothing fails prematurely, but I also don't want to over do it because of cost and weight.

This is our current bike, for reference. We will use a similar design as a starting point, but it will be greatly improved. You can see there is seating for 7 passengers on each side (only 6 pedals because of the rear axle), 2 on the rear bench, one driver plus up to 4 standing up in the middle (dance floor), and yes they dance and party and yes we did rides with 20 passengers plus the driver, yes it was a pretty wild summer.

The new design will use gearbox instead of open spur and sprocket gearing. We will also use bigger shafting and stronger frame construction, along with better quality components.

Here's what I need to know for now :

The force required to break the inertia of the vehicle from stop to forward;
The energy required to accelerate the vehicle from 0 to 10km/h;
The force required to keep the vehicle momentum at 10km/h on a uphill direction of given angle;
The maximum torque that could be inflicted onto the components when the wheels are locked;
The minimum torque or strength rating of each components for shock load and at max rpm.

The party bike consist of a total of 12 people facing each other on each side, so 6 people on the left and 6 on the right. Each person can pedal at their own pace due to an individual freewheel for each rider. The crankset is linked to a shaft running the length of the side, so all 6 people power the same shaft on their side. Then, using a "T" 3-way right angle bevel gearbox located at midpoint on the shaft, the output of said shaft is being transferred perpendicularly towards the other side of the bike, using a 1:1 ratio. The output of the gearbox from each side then give into a third "T" gearbox that direct the power toward the front of the bike to a front wheel drive axle. There is no clutch or over-load protection at all. The rotation of the front wheels is directly linked to the rotation of the shafts and when in reverse, also rotates the pedals.

Since the driver can only steer and brake, people try to stump on the pedals as hard as they can when we're about to let go the brake at an intersection or when we're stopped, or when we start to brake, because the pedals only move forward if the wheels are moving too. With up to 12 people weighting down or pushing down the pedals and the brakes locking the wheels, there is a lot of torque applied to every components of the bike, as well as when we're going uphill, or trying to keep up a top speed on flat or uphill ground.

Considering the bike itself weight about 1 700lbs and we can have up to 20 people on the bike (but only a max of 12 to pedal) plus the driver, plus backpacks, boom box, etc, a worst case total weight could be of :

1 700lbs + (21*200lbs) + (21*5lbs) = 6 005lbs

Least worst case weight with 12 riders plus driver without gear would be of :

1 700lbs + (13*120lbs) = 3260lbs

So we are looking at minimum specs for a load of 6 000lbs, or 2 723Kg, for a front wheel drive vehicle with no suspension (the pneumatic of the tires is enough by itself for the comfort), powered by up to 12 (200lbs) riders who could put out easily 1340lbs/inch of torque each when wheels are locked, or roughly 1,8 to 2 HP each when going uphill and spinning cranks at 90 to 120 rpm while standing on the pedals. Each gearbox would collect 6 riders and the central gearbox would collect the whole output of the 12 riders and give it directly to the front axle, then to the wheels (hopefully I'll be able to find a front axle differential at below 1:3.5 ratio). With our current bike, we found a sweet spot of a 0,75:1 ratio between one rotation of the crank and 3/4 rotation of the wheel. Assuming a similar bare bike weight and tire outer diameter, we would aim for such final ratio. If my understanding of power is correct, this means the HP needed to move the bike from inertia to forward and the HP needed to have to bike moving at 10km/h in flat ground with only the normal air and tire friction, plus gravity, or to move it at 10km/h uphill, will directly tell me how much HP each rider would need to produce if they are all in sync or how many riders minimum would be needed to pedal the bike without discomfort (going at a slow pace).

The crank arms are about 170mm long (6,7"), the ratio between crank and freewheel would be of 1.6:1, the steel shaft would be of a minimum 7/8" diameter, of 5' long sections spaced at 2' between each rider (so each freewheel would be 2' apart, next to a billow block bearing) and a "T" right angle gearbox with 1:1 ratio would be in the middle of each side, collecting a 5' shaft of each side (from the front of the bike and the back of the bike) and then to output to a center "T" gearbox that could be of increaser ratio to achieve the proper crank to wheel ratio.

Wheels would be 15" rims, let's say 20" diameter tire.

What minimum and recommended ratings would my gearbox need to be ?

I was looking at the Hub City model 66 gearbox, that gives me a max of 2 330 lbs/inch of torque with 3,7 HP at 100rpm, but I don't know if it would be able to withstand 6 people stomping on the pedals (do torque adds up or is it non-linear like decibels are ?), knowing that a single rider could give 1340/1.6=838lbs/inch of torque right away ?If I go with 838*6=5028lbs/inch, I'm looking at a heck of a gearbox to withstand that initial shock load torque when we let go the brake and people are eager to start moving. It uses a 1,25" shafting. The model 65 is the same, except with dual hollow shaft and one solid shaft.

I was also thing about using a 4 way gearbox with two hollow shaft and two solid, in the middle, so I could also install a DC motor on the 4th shaft that would be the same physical shaft as the one going to the front wheel drive axle, so I can use it as a generator and electrical brake/load when going downhill and as an assistance when moving the bike alone and up/down the trailer, etc.

Hope I didn't made it too complicated !

David

 

[jedishrfu]

While I can't comment on your design, I wanted to ask if you've considered using a brass gear anywhere. The idea is that if the torque gets too great then the gear will shear and the rest of your mechanical parts will be rescued.

I don't know where the best place to put it would be but perhaps on the drive shaft of your motor.

Also your estimate of 6000 lbs is what a truck might carry and so your frame would need to be able to handle that without breaking under the weight. Similarly for your tire ratings.

Other folks here may be able to help with the physics.

 

[Electro Dave]

While I can't comment on your design, I wanted to ask if you've considered using a brass gear anywhere. The idea is that if the torque gets too great then the gear will shear and the rest of your mechanical parts will be rescued.

I don't know where the best place to put it would be but perhaps on the drive shaft of your motor.

Also your estimate of 6000 lbs is what a truck might carry and so your frame would need to be able to handle that without breaking under the weight. Similarly for your tire ratings.

Other folks here may be able to help with the physics.

Good point about the overload or shear gear, I was actually looking up these products today. I was a few company that makes overload safety couplings such as the Dalton Gear "ROSDC" Rigid Overload Safety Device Coupling, and I was thinking of installing this kind of coupling on the input of the middle gearbox so if riders on one side pedals more heavy than the other side, the bike won't come to a complete stop due to a simple super performance of a few riders, but then I would also need to add some friction device at the end of the left and right shaft so if the overload is triggered, the affected riders won't bust their chops all in a sudden from no more resistance on the pedals.

 

[cjl]

You're definitely not going to get 1.8 or 2 horsepower from each rider unless you've got a professional group of sprint cyclists on your vehicle. Half a horsepower is probably a more reasonable typical number, though it wouldn't hurt to have a decent amount of margin in your calculations.

 

[Electro Dave]

You're definitely not going to get 1.8 or 2 horsepower from each rider unless you've got a professional group of sprint cyclists on your vehicle. Half a horsepower is probably a more reasonable typical number, though it wouldn't hurt to have a decent amount of margin in your calculations.

You're right, as these calculations do include a worst case scenario margin, as I already know what size of gearbox and shafting I need to be able to successfully move the bike. However, real world use show that it won't make it for even one mile before complete destruction of drivetrain. It's a party bike and anything goes. From small group of 100lbs ladies who can't pedal it very fast to the wild Aussie group of 20 who rock the bike upside down and jam it like they were wrestling crocodiles all night long, and of course music bands playing live on the bike rolling around downtown with all their crew, speakers, sound console, dj table, etc etc

We can't afford downtime due to mechanical failure, but we also can't afford to overdo the gearing because of the cost, so I want to choose something big enough without going overboard on budget and size. I know 1" shafting is more than enough, even 7/8" is fine, but when I look at specs of 1:1 gearbox that uses a 1" shaft, I see numbers below what a single rider could break during continuous duty and shock load. So I go with a bit bigger, but I still feel like I'm on the borderline of rated duty for our needs. I'm really looking at how much torque the gears can handle without premature wear or excessive stress, more than the HP being generated, because most of the load is happening between 0 and 50 rpm, 0 being when the wheels are locked and 50 when we're racing on a slight to moderate uphill. I'm really considering to add safety overload coupling devices to the output of the gearbox of each side, because if I put just one at the output of the central gearbox, the gearbox on the side will strip before the overload trips. The thing is people will try to pedal as hard as they can until they reach a certain rpm where they have to be seated and in good shape to keep pushing hard. 40rpm is about the max before they sit down and 90rpm is where they start to lack peak power, but they'll spin easily up to 120rpm if the bike is on a flat or downhill path and sometimes up to 150rpm for a few seconds when they feel like hitting their knees on the frame.

 

[CWatters]

I think it's quite reasonable to assume that Max torque will occur with everyone jumping on the pedals and yes torque adds. The question is do you need the gearbox to handle that continuously or just survive peak loads of that magnitude. Got to be a question for the gearbox manufacturer.

 

[Electro Dave]

I'll need the gearbox to endure such abuse quite often actually.

 

[Electro Dave]

However if I do add a safety overload coupling that slips when preset torque is reached, a bit like a fishing reel drag, people won't put the hurt on the gearbox too much, granted I set the preload torque to be equal to the nominal torque rating of the gearbox output for a given rpm. The gearbox will then spin at that max torque preload to the max rpm people can put out at that torque. Otherwise the gearbox won't bulge, but won't be damaged because the torque applied will be within the rated range and not damageable.

Instead, by using an overload coupling on the output of the side gearbox to the central gearbox (so a total of two coupling for the vehicle), I can select a gearbox without adding a costly safety factor on top of the heavy duty factor, but a 350lbs/ft max torque spec device is $600, making it as pricey as the gearbox alone. So I'll have to do some maths to see what is the most cost-effective option, regarding gearbox size, overload device size, and where I install the overload coupling. Let's say my gearbox can take up to 210lbs/ft input from 6 riders, and is located in the middle, so 3 riders from each side. Assuming each rider can give me 100lbs/ft at the crank when going uphill (gotta make sure the overload doesn't trip when we are trying to move) and have a 2.6:1 ratio between the crank and shaft, so that gives me 38,46lbs/ft at the shaft, times 3, so about 115lbs/ft of torque that I want to give on the input of each side of the gearbox, for a max of 230lbs/ft output. Then if I place a overload coupling with a max rating of 175lbs/ft that cost $266 USD on each side of the gearbox, for a total of 4 overload couplings for the whole vehicle, that adds up to $1 064 vs $1 200 by using only two bigger couplings. Since I already have to buy "L" type jaw couplings to connect the shafts to the gearbox, with an average cost of $50 each, then I reduce the cost of each overload coupling by $50.

Using 4 OSDC of 175lbs/ft and only two jaw coupling, cost it about 4*$266+2*$50 = $1 164
Using 2 OSDC of 360lbs/ft and 4 jaw couplings, cost is about 2*$600+4*$50 = $1 400

Then since the OSDC use a friction disc clutch-like design, I will need to buy spare discs to have readily since they'll wear eventually. Each OSDC uses 2 discs and each disc is about $20. Final cost for each setup goes :

$1 164 + 4*$20*2 = $1 324
$1 400 + 2*$20*2 = $1 480

Not a huge price difference in the end, but if I add a spare OSDC and spare jaw coupling for each, to keep as backup for worst-case scenario, the bill goes flying to $1 640 vs $2 130.