Recently I have been thinking about building a new car and I want to do something a little weird/different.
I want to do 3 wheel steerings because I think that it would be nice because it could reduce tire scrub thereby making the car more efficient (and more importantly save money on tires) in turns. I did 2 other models, one with a pivot arm, and the other with a rack and pinion and both of these steering systems still create tire scrub even with correct ackerman angles I can include these other models if someone wants me to. But in my model with 3 wheel steering you can see all 3 wheels are on the same turning circle throughout most of it's turning range.
So I have a few questions:
Has anyone every tried anything like this before? And was it anywhere near successful?
How would I reinforce the rear wheel linkage since it will have to pivot?
Are there any disadvantages to this besides the added weight and complexity?
Would I still incorporate caster into the rear wheel?
Thanks for the help!
-- Edited by Nitoragro on Saturday 9th of February 2019 01:36:11 AM
-- Edited by Nitoragro on Saturday 9th of February 2019 02:21:55 AM
It seems to me that you can't have the rear pivot ahead of the wheel like that.
It needs to be above the contact patch of the rear tire otherwise you are trying to drag the wheel sideways to make it turn. Or I guess it could be seen as effectively shortening the length of the car by dragging the rear wheels backwards as you turn. Like an articulated loader.
This is something I have always wanted to experiment with. We scrub a lot of energy going through a turn. I would like to go whole hog and try a tilting 'free to castor' 3 wheeler. Here is a good place to read about the concept: tiltingvehicle.blogspot.com
I would suggest you build in a way that allows you to try different concepts. For example, allow the rear wheel to castor freely like a shopping kart with the steering controlled by the front wheels, trying steering just the rear wheel with the fronts locked, or with the fronts free to castor, try steering all three wheels together.
Everything needs to be adjustable. The rear wheel might need less steering angle than the front wheel. The castor on the wheels might be a little or a lot.
Archer321 said
Feb 9, 2019
I'm concerned there could be the danger of cornering TOO hard if the rear wheel turns as sharp as the rear. At least if you still have to 20 degrees or so of front wheel turn that most cars today seem to have.
I guess if you reduce that and start with what you want your turning radius to be and then calculate what sort of swing you need each wheel to have to accomplish that you would probably be ok.
I can just imagine what accidentally taking a 180 degree turn in a 6 ft radius at 30 mph would look like....
Violent.
Nitoragro said
Feb 11, 2019
I just found a small error in my design. I made the spindle connection on the rear wheel 4.25" instead of 4" as it should be. If i'm lucky I might be able to make this even closer to "perfect"
Cliff,
I fully agree with making everything adjustable so I can test everything. As for the idea about castors on the rear wheel I think it would be really similar to the Razor "crazy cart". So I can see a castors working to completely eliminate tire scrub however I don't know if it will allow the car to corner faster... I guess i'll have to test that, or just see what I would rather have.
Archer,
On your first post, I don't think I would have thought about that until it was to late. So I think a good way I can look at this is like a tricycle. I would need to get the kingpin inclination correct then it would fix the problem because then the wheel will rotate along the contact patch of the tire. But a problem I think I would have to overcome would be to make it so I don't have a ridiculous amount of castor on the rear wheel, so i think I will just have to find the best place to put the pivot.
As for your second post, I assume you meant "if the rear wheel turns as much as the front" and I don't see how this would happen if everything is connected with steering linkages. If you look at the 1st and 3rd pictures I uploaded you can see the rear wheel turns less then the front two (it actually turns half of the average of the front two). I can see if this was backwards and the rear wheel turns twice as much as the front then there would be a major problem.
Thanks for the help, I will keep this post updated with my updated models, which hopefully I can get done in the next few days. Maybe Ill even be able to make some little prototypes to test this all with.
Nitoragro said
Feb 13, 2019
So I didn't think 3 wheel steering was complicated enough... so i added another challenge for myself, I decided to add rear suspension. I have a rough setup of the suspension kinematics, however they will need some fine tuning...
I plan to use an air shock that I am going to take off of my mountain bike (so I have an excuse to upgrade my bike) and I even have the model set to only compress as much as the real shock will let it.
I know one problem I will run into is the tie rod that will connect the rear wheel to the front pivot arm will need to be modified. Since the rear axle path shorten the car's wheel base it will force the rear wheel to turn when it hits a bump. However I think I know how to fix this, I plan to have 2 tie rods that connect to the rear wheel. This will allow the tie rods to adjust to the length of the rear wheel. Kind of like in a F1 car where the tie rod pivots at the same place as the suspension to everything moves the same amount together.
For building this setup I want to use some bike parts. I have found a head tube and steerer tube from a bike. The nice thing about this is I would be able to use the normal bike headset bearings and properly set the bearing pre-load on the steering mechanism
-- Edited by Nitoragro on Wednesday 13th of February 2019 04:39:51 AM
Ambitious!
I like it.
Just a thought, if you used a springer front fork on your rear wheel it should simplify your steering linkage as it would keep all of the changing suspension geometry limited to that fork.
ProEV said
Feb 13, 2019
Is that plate with the dots to give adjustment choices for the shock?
It is important to have that flexibility. You will probably find that you need to tune motion ratio. In other words, if the wheel moves up 1 inch, how far will the shock compress?
The second adjustment the shock will control is rear ride height. We aim for flat bottom at speed which requires a little static rake.
Nitoragro said
Feb 13, 2019
Archer,
I didn't even know what a springer fork was... but I don't think I understand what you mean when you say that the fork would limit the changing geometry to the fork. Do you just mean that would make everything more compact? But 1 large problem I have with the ones I can afford is they use 1" threaded steerer tubes. And these are the types of steerer tubes from cheap bikes so I just wouldn't feel safe on it.
Cliff,
Yeah that plate was just so I could mess with shock placement on the model. My goal is for the car to have 1" of suspension travel and the shock I plan to use has 1.5" of travel. But for me to use all the shock and only use 1 inch of travel it would need to be very large linkage, so I might only be using about 1/3 of the shock travel
Nitoragro said
Feb 16, 2019
I spent WAY to long doing this and I wasn't able to hook up the rear steering yet... using this software for such a weird type of motion the tie rod will need to go will be incredibly difficult.
The rear suspension is just about where I want it and it still has the 1.5" of rollover height it needs.
I was able to get a good chunk of the frame made, ill just need to add a 2nd rollbar, some flooring, and some more triangular supports but I have the rough shape of what I want done. Soon i'll set all the materials of all the parts to what they really are instead of the "generic" material so I can get a rough weight and center of mass.
Recently I have been thinking about building a new car and I want to do something a little weird/different.
I want to do 3 wheel steerings because I think that it would be nice because it could reduce tire scrub thereby making the car more efficient (and more importantly save money on tires) in turns. I did 2 other models, one with a pivot arm, and the other with a rack and pinion and both of these steering systems still create tire scrub even with correct ackerman angles I can include these other models if someone wants me to. But in my model with 3 wheel steering you can see all 3 wheels are on the same turning circle throughout most of it's turning range.
So I have a few questions:
Has anyone every tried anything like this before? And was it anywhere near successful?
How would I reinforce the rear wheel linkage since it will have to pivot?
Are there any disadvantages to this besides the added weight and complexity?
Would I still incorporate caster into the rear wheel?
Thanks for the help!
-- Edited by Nitoragro on Saturday 9th of February 2019 01:36:11 AM
-- Edited by Nitoragro on Saturday 9th of February 2019 02:21:55 AM
It seems to me that you can't have the rear pivot ahead of the wheel like that.
It needs to be above the contact patch of the rear tire otherwise you are trying to drag the wheel sideways to make it turn. Or I guess it could be seen as effectively shortening the length of the car by dragging the rear wheels backwards as you turn. Like an articulated loader.
I would suggest you build in a way that allows you to try different concepts. For example, allow the rear wheel to castor freely like a shopping kart with the steering controlled by the front wheels, trying steering just the rear wheel with the fronts locked, or with the fronts free to castor, try steering all three wheels together.
Everything needs to be adjustable. The rear wheel might need less steering angle than the front wheel. The castor on the wheels might be a little or a lot.
Cliff,
I fully agree with making everything adjustable so I can test everything. As for the idea about castors on the rear wheel I think it would be really similar to the Razor "crazy cart". So I can see a castors working to completely eliminate tire scrub however I don't know if it will allow the car to corner faster... I guess i'll have to test that, or just see what I would rather have.
Archer,
On your first post, I don't think I would have thought about that until it was to late. So I think a good way I can look at this is like a tricycle. I would need to get the kingpin inclination correct then it would fix the problem because then the wheel will rotate along the contact patch of the tire. But a problem I think I would have to overcome would be to make it so I don't have a ridiculous amount of castor on the rear wheel, so i think I will just have to find the best place to put the pivot.
As for your second post, I assume you meant "if the rear wheel turns as much as the front" and I don't see how this would happen if everything is connected with steering linkages. If you look at the 1st and 3rd pictures I uploaded you can see the rear wheel turns less then the front two (it actually turns half of the average of the front two). I can see if this was backwards and the rear wheel turns twice as much as the front then there would be a major problem.
Thanks for the help, I will keep this post updated with my updated models, which hopefully I can get done in the next few days. Maybe Ill even be able to make some little prototypes to test this all with.
So I didn't think 3 wheel steering was complicated enough... so i added another challenge for myself, I decided to add rear suspension. I have a rough setup of the suspension kinematics, however they will need some fine tuning...
I plan to use an air shock that I am going to take off of my mountain bike (so I have an excuse to upgrade my bike) and I even have the model set to only compress as much as the real shock will let it.
I know one problem I will run into is the tie rod that will connect the rear wheel to the front pivot arm will need to be modified. Since the rear axle path shorten the car's wheel base it will force the rear wheel to turn when it hits a bump. However I think I know how to fix this, I plan to have 2 tie rods that connect to the rear wheel. This will allow the tie rods to adjust to the length of the rear wheel. Kind of like in a F1 car where the tie rod pivots at the same place as the suspension to everything moves the same amount together.
For building this setup I want to use some bike parts. I have found a head tube and steerer tube from a bike. The nice thing about this is I would be able to use the normal bike headset bearings and properly set the bearing pre-load on the steering mechanism
-- Edited by Nitoragro on Wednesday 13th of February 2019 04:39:51 AM
It is important to have that flexibility. You will probably find that you need to tune motion ratio. In other words, if the wheel moves up 1 inch, how far will the shock compress?
The second adjustment the shock will control is rear ride height. We aim for flat bottom at speed which requires a little static rake.
I didn't even know what a springer fork was... but I don't think I understand what you mean when you say that the fork would limit the changing geometry to the fork. Do you just mean that would make everything more compact? But 1 large problem I have with the ones I can afford is they use 1" threaded steerer tubes. And these are the types of steerer tubes from cheap bikes so I just wouldn't feel safe on it.
Cliff,
Yeah that plate was just so I could mess with shock placement on the model. My goal is for the car to have 1" of suspension travel and the shock I plan to use has 1.5" of travel. But for me to use all the shock and only use 1 inch of travel it would need to be very large linkage, so I might only be using about 1/3 of the shock travel
I spent WAY to long doing this and I wasn't able to hook up the rear steering yet... using this software for such a weird type of motion the tie rod will need to go will be incredibly difficult.
The rear suspension is just about where I want it and it still has the 1.5" of rollover height it needs.
I was able to get a good chunk of the frame made, ill just need to add a 2nd rollbar, some flooring, and some more triangular supports but I have the rough shape of what I want done. Soon i'll set all the materials of all the parts to what they really are instead of the "generic" material so I can get a rough weight and center of mass.