Need advice on cargo bike steering geometry

[JuanGrande]

So I have manly lurked on this site for the last couple of years... reading lots of posts and taking things in. I need some advice on building the front end for a cargo bike. In the past when I have built a cargo trike (see bikes below) I have always kept the head tube at 90° and the wheel axles in-line with the pivot point for the front. It was suggested to me that adding 5 - 10° of tilt to the headtube would help with handling through turns. If I did add some tilt to the head tube, I was debating about whether to add rake to the wheel setup and try and establish a desirable trail...

Due to the design of this type of cargo bike, not sure adding rake gets me much and I am having trouble grasping in my mind how tilt in the headtube will help handling (I may end up mocking up a protype to try things out). Anyway, I thought I would post to the forum and see if anyone had thoughts, advice, or experience with this style of bike/design. Thanks.

Below are some pics of versions 1 and 3 of the cargo/mobile bar bike and an action shot...

Jonathan

[steve garro]

Hey, Juanathan - check this out, there are a ton of steeering/ackerman vids on youtube & this one is very, very helpful - I think "Ackerman" is what your are pondering.
Check it out & let me know if you have any more Q's.
I ride trikes allot.....
- Garro.

[JuanGrande]

Thanks Steve. Great link... Lots to digest... Some great videos like off this one too. I will ponder this and if there are further questions I will come back and post more questions. Thanks again!

Jonathan

[bellman]

Quick question Jonathan, do you use the brakes seperately to aid in turning like a tractor?

I would think that a set of thrust bearings at the pivot would work wonders.

[JuanGrande]

Nope, the brakes are simply for their intended purpose... stop this beast. We had roughly 500lb of ice, booze, batteries and other stuff on this bike. If you notice in the first pic (version 1 of the bike), we originally relied on just the rear brake... let's just say that we didn't stop very quickly. The dual disc brakes up front worked excellent. I used a single lever to pull two cables and it took less than 10min to adjust the pull so both brakes grabbed equally.

Not sure what you mean by "thrust bearings at the pivot"... would a different pivot mechanism help with handling? The current pivot used for the front end is a modified trailer axel rotating on needle bearings. Got the piece from a supplier for building your own bumper/tire carrier for off-road vehicles. Only cost $40, smooth as silk, and can handle a huge weight load. One idea I had played with was adding a steering dampener to help with bump-steering issues. The design works well until you try and go faster than 10-15mph. When I did the original research for which design approach to use, I choose the single pivot because of the ability to carry large and heavy loads, easy to maneuver at low speeds, and simplicity to build/reliability. The downside was the inherent instability at faster speeds with the design.

Jonathan

[JuanGrande]

Steve, thanks again for the link. A quick question - in you experience, have you built a single pivot trike (i.e. instead of the trike front wheels both rotating, the entire front end assembly, wheels included, rotate around a single pivot point)? The reason I ask is that I am struggling with how to apply the "Ackerman" principle or some of the other aspects discussed in the video when the wheels don't turn individually, the entire front assembly turns. That's why I was wondering if aspects like headtube angle, and fork rake might apply more to this design. In doing my initial research on which approach to use, I knew that corning and higher speed stability were issues. Intuitively, the only thing that I can think of to help this would be some sort of dampener to address bump steer issues.

That said, I think for a cargo bike that will be used for city streets, I will try and adopt to a different front end design to allow for better handling and try and reconfigure how the cargo is carried.

Jonathan

[bellman]

That sounds like a bomber pivot joint, perfect solution that axle.

I guess I misunderstood handling for hard to manuever.

Does the front end start to shake at higher speeds? maybe toeing the wheels in a little would help. You know, make them sort of crosseyed.

It might be interesting to see how they are aligned now. It's possible that they are turned out and are both pulling away from the center line.

One more thing, after looking at the picture again, what if you move the pivot behind the center line of the axles? It would be like rake but in a different plane.

Cool project, I hope you get it dialed

[steve garro]

Steve, thanks again for the link. A quick question - in you experience, have you built a single pivot trike (i.e. instead of the trike front wheels both rotating, the entire front end assembly, wheels included, rotate around a single pivot point)? The reason I ask is that I am struggling with how to apply the "Ackerman" principle or some of the other aspects discussed in the video when the wheels don't turn individually, the entire front assembly turns. That's why I was wondering if aspects like headtube angle, and fork rake might apply more to this design. In doing my initial research on which approach to use, I knew that corning and higher speed stability were issues. Intuitively, the only thing that I can think of to help this would be some sort of dampener to address bump steer issues.

That said, I think for a cargo bike that will be used for city streets, I will try and adopt to a different front end design to allow for better handling and try and reconfigure how the cargo is carried.

Jonathan

no, I have not. Man - mine slides turns FAST! Keeping the load low seems to be key.
As you pivot your Ackerman will change, causing the wheels to scrub even more, I would think - the inside wheel would be on a much tighter track......
I think you need to think more about KING PIN INCLINATION instead of rake/trail........as it is your current design your contact patch of the tire is right at the bottom {or, just in front of it, actually} so it's inclination to ride over a bump would be comprimised...
Great topic! - Garro.

[Oldbikerider]

Hi Jonathan, I think I can help here.

Akerman steering, as the link posted by Steve points out, refers to a steering linkage geometry that during turns makes the inside wheel steer more than the outside wheel. This is needed so that the tires don't scrub during turns. But the beam front axle system that you are using in the photographed cargo bikes, where the whole front end including the wheels pivots as one around the steering pivot, naturally puts the two front wheels at the correct steering angle. You don't need any special linkages to produce Ackerman steering. Think of imaginary lines going directly through all three axles on the machine intersecting at a point at the centre of the turn you are making on the road. These three lines all intersect at the same point, so the steering angles are all OK and the tires will not scrub.

Actually, the interesting thing about Ackerman steering (no matter how it is achieved) is that it only works perfectly at one speed. As soon as a dual track vehicle is cornering, weight transfers from the inside to the outside wheels and this will produce different slip angles on the inside and outside wheels, which means that for zero scrub, they now want to turn at different "Ackerman" angles. On a low speed vehicle like yours you can ignore this, as most of the time the two front wheels will have similar loads on them, meaning that they'll have similar slip angles, so the "pure" geometry of your beam axle will give the correct result. The linkage described in the video above, where the steering arms point to the rear wheel line also gives the correct steering angles at low speed. Race cars, which spend most of their time at high speed need less Ackerman, sometimes even having anti-Ackerman linkages, where the outside wheel turns more that the inside because it has most if not all of the load on it. And of course, once a vehicle starts skidding things get even more complicated.

The beam axle you're using, also means that king pin inclination and conventional castor angle have no meaning. These angles are relevant and very important on trikes that pivot the front wheels out near the hub, but your beam system is different. One problem with the beam axle is that steering and stability can get tricky when one wheel hits a bump. You may have noticed this? This is due to the large lateral offset from the wheels to the steering pivot, which produces a large moment back through the steering when one wheel gets knocked by a bump or by uneven braking. This is the problem that is addressed by king pin inclination in a conventional hub steered system, but cannot be addressed in the same way with a beam axle. You are on the right track fitting a steering damper, as this will minimize instantaneous movements back through the steering.

Making the "head angle" other than 90 degrees is probably not a good idea on your trike. If you tilted it backwards, like on a bicycle, this would make the rear section of the trike tilt sideways the "wrong" way in turns. This would be disconcerting, as it would feel like it was tipping you out of the saddle. It would also tilt the load box fore and aft, which would also not be a good thing. So I'd suggest that you should leave the steering pivot head angle at 90 degrees, or maybe tilt it forward slightly to give some minimum lean of the rear section of the trike into turns. (But on second thoughts, I'd leave it at 90 degrees).

BUT, you can give the vehicle some trail, by moving the front axle lines behind the steering pivot. This will give some self centering to the steering at speed which will help with stability. You'll need to experiment with the correct offset, but I'd think that what works on bicycles, maybe 65mm or so, would be a good starting point. The good thing with your trike, if you keep the head angle at 90 degrees, is that there will be no wheel flop to confuse the handling at low speed, so offsetting the front axles rearward is one simple variable that you can observe on it's own.

Good luck, and feel free to ask any questions if you think I can help.

Graham.

[vulture]

Zero Ackerman works on F-1 and rock crawlers. Funny.

[Oldbikerider]

Zero Ackerman works on F-1 and rock crawlers. Funny.

Makes sense for the F1 car, the two front wheels will be cornering at vastly different slip angles, so the inside one doesn't need to be steering tighter than the outside (it's got less load on it). Zero Ackerman gives that result.

Rock crawlers; I've got no idea what their design parameters are. They are slow speed, but I'd think are spinning all their wheels a lot?

Graham.

[JuanGrande]

Graham, thanks for the great info! In doing some more research on the design, I stumbled across this on a website for someone building similar cargo bikes which jibes with the advice I was given;

' By cambering the “kingpin” 10 degrees to the rear, the trike frame leans into turns, thereby dramatically reducing the “tip” inherent in tricycles. ' ... wheelburro.com

This is contrary to what I think you said above, which is that a head tube tilt similar to a bike, rearward, would tilt the frame the wrong direction... this has been the part that I have been struggling with visualizing. Lean into turns hasn't been the problem for me, my desire has been for better stability and handling at higher speeds. I like the idea of playing with different trail amounts. At this point I am going to mock-up some different designs and see what works.

Jonathan

[Oldbikerider]

' By cambering the “kingpin” 10 degrees to the rear, the trike frame leans into turns, thereby dramatically reducing the “tip” inherent in tricycles. ' ... wheelburro.com

Jonathan,
The trikes on that website drop the steering pin downwards from the load carrying frame, so "cambering 10 degrees to the rear" might actually mean the bottom is more rearward than the top, ie the opposite to a bicycle. This would indeed tilt the rear section into turns.

Graham.