Rear Brake for a Trike 1

[zaraf1]

Dear Community,

I am designing a loader-trike. The full laden weight will be around 800Kgs.
For rear axle, I am using some off-the-shelf parts from a small car with disc brake systems. These parts very basically placed in front-axle in that car. So I am just reversing the axles. For front I am using motorcycle fork also with a disc brake.
What I know is that in a braking system around 70 percent is provided by front braking due to load transfer. Now my question is if I could use just the rear-axle braking to completely brake the trike by not relying on the front motorcycle fork. I am not sure if the motorcycle fork will be able to handle that force at all?

 

[gruntguru]

Not so. A car that locks the front wheels first is safer and more stable than one that locks the rears.

Very easy to demonstrate in a car with adjustable brake bias. Apply the brakes firmly while rounding a gentle turn.

je suis charlie

 

[Compositepro]

Do you care to explain why that would be, grunguru?

 

[3DDave]

Composite - locking the fronts generally means a loss of directional influence from the tires; they become direction independent in terms of drag. The same is the case for the front wheels of a shopping cart where the front wheel drag is the same as the rear wheel drag and the only difference is the directionality. Try shoving a shopping cart backwards to see what eliminating direction control for trailing wheels is like.

 

[Compositepro]

3DDave I fully agree with your first sentence but that is true for front or rear tires. As for shopping carts, many carts have the casters in the back and that often makes them easier to control because steering is simply push left or right rather than torquing the cart around the rear axles. I see no relevance stability with regard to wheel locking.

 

[3DDave]

Composite - My fault; the 99.999% of shopping carts have non-steering rear wheels, but where ever you live can be different.

I meant exactly this particular style of shopping cart:

https://shopcarriage-trade.com/large-plastic-grocery-shopping-cart

where, as I specified, the rear wheels do not align with the direction of push but the front wheels do.

Find one and give it a big shove backwards and tell me what you observe it doing.

 

[Compositepro]

If all wheels lock at speed the car will continue straight while slowing, due to momentum. The car will spin slowly and randomly. If the rear wheels lock, same thing happens except you can control the spin with the steering wheels.
If the front wheels lock the car will rapidly veer to the side because the back end momentum will spin around the skidding front end, and the rotating rear tires will steer the car sideways.

On bicycles and motorcycles skidding the rear wheel is a very common, and fun, thing to do. If you lock the front wheel you will immediately crash.

 

[Compositepro]

I edited my post of 12:33 to remove a clause that somehow got pasted in the wrong spot, creating non-sense. Sorry. Dave, from the picture in the link I cannot tell which wheels are castered but I think the issue that you are talking about is trying to push a wheel against its caster direction. This works fine if the caster can turn 180 and now it is correctly oriented but if the caster rotation is limited then the wheel will push the cart to the side. This, however, has nothing to do with the discussion about stability of a car in motion with locked wheels.

 

[3DDave]

Yeah, the welds in the picture are no clue.

 

[Compositepro]

Is that supposed to be sarcasm? Okay I zoomed in on your picture and it appears to have casters in front and non-castered but swept back wheels in back. A standard shopping cart in most of the US. So what is your problem with pushing one backwards? If you have a problem, it is in how your brain works and not in how the cart works. With a little practice you can probably overcome that. It is still completely irrelevant to the discussion.

 

[BrianPetersen]

Someone has not gone playing in the snow with an old skool non ABS car. Or even an ABS-equipped one, but with a mechanical hand-brake, e.g. my own little Fiat 500.

drive forward ... either let go of the steering wheel or hold it in one place ... tug the hand-brake ... car slides sideways or spins, if you started out fast enough. Any slight deviation from pointing forward, with no directional stability from the locked rear wheels, leads to a positive-feedback situation that can only end with a spin.

I back into my driveway. When it's snowing and the plows haven't come around yet, carefully timed swing left with the steering then abruptly right together with a yank on the hand-brake nicely puts the car sideways in the road just as it stops so that I can then back straight in! The ABS is powerless to do anything about the hand-brake.

It's harder nowadays due to ABS, to find out what happens when the fronts lock first but it most certainly does not result in a spin. Instead, the front stays out front. The directional stability from the still-turning rear wheels lets them follow whatever path the front does. The front stays out front no matter what the driver does with the steering wheel; the path may veer slightly left or right depending on slight slope of the road or differences in friction left to right or what-not, but the rear follows dutifully in line.

 

[3DDave]

Composite - The wheels in the back don't "sweep" as they are fixed in place and their contact point is vertically below the axle. I'm puzzled that you have seem to have never seen that configuration.

Now, to straighten out your understanding.

While the rear wheels (the non-steering ones) are turning (not locked up), they offer a restoring force along a vector at right angles to the direction they are rolling roughly proportional to the sine of the yaw angle. If the cart yaws right, the wheels push to yaw left, causing the cart to go straighter.

When wheels lock up they offer no restoring vector, only a force in line with the direction of travel at that point on the vehicle, just like the free-castering front wheels of a shopping cart offer no restoring vector by freely pivoting to align with the direction of the place on the cart they are attached to.

Running backwards, the non-steering wheels of a cart, now at the forward end based on the motion, offer a divergent force vector - if the cart yaws even slightly left the wheels push to the left, increasing the yaw, in a positive feedback loop. The free-castering (or locked-up wheels in the case of the declerating car) at the trailing end offer no restoring vector aligned with the fore-aft direction on the cart and the cart whips around, just like a car tends to do if the rear loses static friction before the front wheels do.

Thus an unstable situation exists when the rear wheels are sliding while the front wheels are turning, which most drivers end by leaving the road or taking their foot off the brake pedal.

 

[GregLocock]

Before ABS all modernish production cars were set up to lock the front wheels first in typical usage. Otherwise, as I did with the first prototype FWD Elan, when one trail brakes ineptly into a corner the back end will let go. Which is very embarrassing.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[LionelHutz]

You are confusing what happens with a 2-wheeled vehicle vs what happens with a 4-wheeled vehicle. The bike is unstable with a locked front wheel because you are basically balanced on the bike over the front wheel. Lock it up and you immediately lose the ability to balance resulting in you and the bike quickly falling over. No such thing happens with a 4-wheeled vehicle.

The turning back wheels of a car do not try to pass the front locked wheels. They certainly do not impart a turning force opposite the direction the front of the vehicle is travelling. The turning back wheels will follow the front wheels if the front locked wheels skid to the left or right.

Have you never pushed a shopping cart? To turn one around quickly give it a quick push backwards. It will spin around and head the right direction within about 4-5' of travel.

You've certainly never been around forklifts. Never seen a "death wobble" from the driver going to fast forwards and over correcting the rear steering or how certain types are meant to travel backwards fast so the steering wheel(s) are at the front.

 

[MikeHalloran]

Single wheel at the front tricycles are worse in their own special way compared with tadpole trikes as well, think Reliant Robin vs Morgan. Ultimately it all comes down to CG height and in a cargo carrying application that is almost entirely out of your hands.

IOW, it will kill you.

Make it a quad.


Mike Halloran
Corinth, NY, USA