[WIP] Azure Racer XL

[JLiu15]

Some of you may remember my Azure Racer from a few years ago. It was a small simple buggy that was meant to be a leisurely build after my BrickFair VA projects of that summer. It had only one L motor for drive, but had decent speed. This model will be a sequel to that - it'll be larger (with Unimog tires), dark azure instead of medium azure, and powered by BuWizz and driven by 2 RC motors. In fact, this will be my first MOC to use BuWizz and RC motors - something I wanted to get for years and finally obtained two of them.

The front axle will be a double wishbone independent suspension using shocks from the 9398 set. I haven't used them in years, and it's nice to bring them back again. The rear suspension will also be independent, but it'll be the RC motors themselves that are part of the suspension. Each motor will be connected to one wheel that'll be mounted on its own pivot point and shock absorber, essentially creating an independent suspension.

I'm really looking forward to building this model. With the BuWizz on ludicrous or fast powering 2 RC motors driving a relatively lightweight model, I anticipate getting A LOT of speed out of this thing.

Photo:

Edited January 28, 2022 by JLiu15

[Jurss]

This should be fast.

[JLiu15]

Just now, Jurss said:

This should be fast.

Yup! I sure bet it will ?

[1gor]

I suppose this would be huge fast MOC 

[JLiu15]

UPDATE 11/26/20

I got some more work done on it last night after posting this topic, and now the chassis is essentially complete. I've changed the front suspension to use double shocks per wheel and the rear to use 6.5L hard shocks. Contrary to what I originally believed, the front and rear suspensions proved to be roughly equally hard and offer similar travel. The front suspension was VERY soft with just one shock per wheel.

I think this is a model I can finish in a matter of days, and get maximum fun out of driving it. Right now it's super lightweight, and I don't anticipate the bodywork and BuWizz (which should arrive tomorrow) to add too much weight.

If you have any suggestions for me, please let me know.

Photos:

Edited January 28, 2022 by JLiu15

[JintaiZ]

Looks like an excellent start! For the wheels, I think you could use 42070's.

[JLiu15]

6 minutes ago, JintaiZ said:

Looks like an excellent start! For the wheels, I think you could use 42070's.

I think those might be too big. Also not very buggy-like. I am planning to make a similar MOC using those wheels soon though.

[amorti]

Cool concept! But I do see a fairly fundamental problem.

The front wishbones are 7L, but the steering links are 6L. This means you will have bump steer. You do not want bump steer!

Try again using normal 6L wishbones, and mount a single shock inside their split. The lower mounting of the shock will reduce the leverage the wheel has over the shock, increasing the stiffness of the spring.

In general, you probably want the suspension softer than you think you do. The ideal setup uses around 30% to 50% of the available travel as static sag - exactly as in a real car. With that type of suspension, the car can float over bumps, instead of bouncing around over them, losing traction and control the whole time.

@Didumos69's greyhound demonstrates this perfectly.

 

 

Ps: It's well worth YouTube searching that model for examples of how it floats over terrain

Edited November 26, 2020 by amorti

[MikeTwo9398]

I build similar car but with a solid axel on the back. 

I used  Madoca1977’s Baja Trophy Truck  as guide line.

Here you can find what I made Baja Buggy

[JLiu15]

2 hours ago, amorti said:

Cool concept! But I do see a fairly fundamental problem.

The front wishbones are 7L, but the steering links are 6L. This means you will have bump steer. You do not want bump steer!

Try again using normal 6L wishbones, and mount a single shock inside their split. The lower mounting of the shock will reduce the leverage the wheel has over the shock, increasing the stiffness of the spring.

In general, you probably want the suspension softer than you think you do. The ideal setup uses around 30% to 50% of the available travel as static sag - exactly as in a real car. With that type of suspension, the car can float over bumps, instead of bouncing around over them, losing traction and control the whole time.

@Didumos69's greyhound demonstrates this perfectly.

 

 

Ps: It's well worth YouTube searching that model for examples of how it floats over terrain

So the steering links should ideally be the same length as the suspension arms? I haven't noticed any issues with bump steer - is this when the suspension goes up that the wheels turn themselves? I'm guessing it's probably negligible in this situation.

Also the suspension seems soft enough as it is. It's not too stiff, but also not so soft that the model will start to sag.

[amorti]

Bump steer is exactly that: as the suspension bounces, the wheels either toe in or toe out. Your chassis definitely has this, and with the speeds it will achieve, it definitely will be a problem. Honestly, now is the time to fix it.

"Not sagging" means it's too stiff. When you jack up your real car to change a tyre or brake pad, think about how much jacking it takes to get the tyre off the road.

[JLiu15]

UPDATE 11/26/20 (2)

As @amorti noted, the original front suspension design would suffer from bump steer due to the steering link being shorter than the suspension arms. I've reworked the front suspension to make the steering linkages the same length as the suspension arms (7L) by combining two of part 15459. I tried combining it with part 57515 by placing it in its bifurcated part, but that resulted in at least an 8 stud steering linkage. I'm wondering if this will be a better solution that eliminates bump steer - I think I've seen a MOC before using this solution, but I don't remember.

Photo:

Edited January 28, 2022 by JLiu15

[amorti]

In fact they're (about) √50, so just over 7, long. Marginal difference which will give slight toe out, although in an ideal world you'd want toe in on a powerful RWD chassis.

Now you have a steering link which (ball joint to ball joint) doesn't run parallel with the wishbones. It won't be perfect, but at least the link is in the same angle and of the same length as the wishbones.

You're the one holding it in your hands who can test if it'll be acceptable or not.

If it's not acceptable, would it be so bad to use 8L wishbones and links? More track width equals more stability, and you're going to need that.

As Columbo said: just one more thing.

You've fixed the rear hubs using axles into pin holes. This gives an unnecessarily wobbly connection at a point that needs all the stability it can get. Flip those 90° connectors around instead, use the pin holes for the hubs.

Edited November 26, 2020 by amorti

[JLiu15]

UPDATE 11/27/20

I've reworked the chassis a little bit. The RC motors in the previous design were incapable of moving the model as they had so little torque. I've redesigned it to be driven via the inner outputs through a 3:1 reduction in portal hubs. The motors now have a much better speed/torque balance - it has enough torque to move it without struggling in my tests with a PF AA battery box (so it should improve when I use BuWizz in fast or ludicrous) but not geared down so much that its speed is essentially that of an L motor.

The front suspension has also been modified. The suspension arms have been reduced to 6L as I really didn't like my solution for 7L tie rods. The suspension travel is comparable to before, but I did notice that the shock absorbers don't compress all the way. Honestly I'm okay with that given how much travel it has.

If you have any suggestions for me, please let me know.

Photos:

Edited January 28, 2022 by JLiu15

[Didumos69]

On 11/26/2020 at 6:02 PM, amorti said:

It's well worth YouTube searching that model for examples of how it floats over terrain

I believe I never shared this...

And I love this one...

Nice project btw! Good luck!

[Didumos69]

On 11/26/2020 at 6:02 PM, amorti said:

Try again using normal 6L wishbones, and mount a single shock inside their split. The lower mounting of the shock will reduce the leverage the wheel has over the shock, increasing the stiffness of the spring.

Alternatively, you can make the suspension arms and the steering links 9L.

Btw, there's 2 simple rules when it comes to suspension stiffness and travel.

1. The percentage by which a given weight compresses the suspension is roughly defined by the distance of the shock and the rotation point of the suspension arm to which the shock is mounted. The closer the shock to the rotation point, the less weight it can carry.

2. When extending the suspension arms while preserving the aforementioned distance (keeping the shock mounting points the same) the same weight will give the same compression percentage.

So by extending the suspension arms, you increase suspension travel without declining the weight the suspension can carry. Therefore I say, make them long if you want travel.

Courtesy by @Erik Leppen.

Edited November 28, 2020 by Didumos69

[amorti]

1 hour ago, Didumos69 said:

I believe I never shared this...

!

Not sure if you shared that here but I've seen it on YouTube and in person.

Amazing how the four wheels are glued to the ground, all the time. It's clear to see how this lets the car drive faster than if it's hop-skipping around the whole time.

Btw, it gets even better if you fit some soft 1.9"x120mm RC crawler tyres :)

[Erik Leppen]

1 hour ago, Didumos69 said:

So by extending the suspension arms, you increase suspension travel without declining the weight the suspension can carry.

No, it does not. Increasing the travel will always decrease the weight the shock can handle. This is because a shock can handle only a certain force (not weight) - namely, the force your fingers exert when compressing the shock by hand, without any levers. If you increase the travel, then the same force (or energy?) is used to push the vehicle up by a greater distance, meaning the vehicle has to be lighter.

(I'm sure there is a physicist that can explain this using the correct units.)

You can easily test this theory by using an extreme example: if you would make the suspension arms 10 times as long, you'd discover that the suspension cannot lift your vehicle 20 cm up :) (if it somehow can, add more zeroes to these example numbers until it fails :P )

 

[Didumos69]

38 minutes ago, Erik Leppen said:

No, it does not. Increasing the travel will always decrease the weight the shock can handle. This is because a shock can handle only a certain force (not weight) - namely, the force your fingers exert when compressing the shock by hand, without any levers. If you increase the travel, then the same force (or energy?) is used to push the vehicle up by a greater distance, meaning the vehicle has to be lighter.

(I'm sure there is a physicist that can explain this using the correct units.)

You can easily test this theory by using an extreme example: if you would make the suspension arms 10 times as long, you'd discover that the suspension cannot lift your vehicle 20 cm up :) (if it somehow can, add more zeroes to these example numbers until it fails :P )

You're right that the shocks can only handle a certain force. But I don't think you're right about the travel.

You have to see it this way. It's not the arm that applies force, it's the car's weight that is the source of force. How much force the car applies to the shock with it's own weight, is defined by the geometry made by the mounting points of the shock relative to the car, not relative to the outer ends of the suspension arms. With longer arms, while preserving the exact same geometry, the weight will still apply the same force on the shocks and only make the arms articulate more, but not compress the shock more. So more travel yes, more compression of the shock no.

Of course in practice the weight of the suspension arms is going to play a role too when they get very long. But when we talk about 9l suspension arms or 6l arms, that effect will be negligible. So I still arrive at the conclusion that longer arms don't affect how deep the car sinks in the springs under it's own weight, while they do increase travel.

Edited November 28, 2020 by Didumos69

[amorti]

In real vehicles, suspension springs are measured in kg/cm. Or, how much weight it takes to compress the spring by X amount. There's no other wizardry in a spring: apply a certain force and it'll compress a certain amount.

So it follows that if you get more travel, you certainly do have softer suspension as the trade off.

[Didumos69]

26 minutes ago, amorti said:

So it follows that if you get more travel, you certainly do have softer suspension as the trade off.

Yes softer, but the car's weight will compress the springs just as far as with shorter arms.

Suppose you have 6l suspension arms (5l center to center), you have travel of 2 studs and the car sinks in its suspension by 1 stud (50%). Now when you extend the suspension arms to 9l (8l center to center), while preserving the exact layout of the springs, you get roughly 8/5 times the travel (>3l) and the car still sinks in its suspension by approximately 50%.

This is of course under the assumption that the weight of the suspension arms is only a fraction of the car's complete weight.

Edited November 28, 2020 by Didumos69

[Bluehose]

On 11/26/2020 at 10:46 PM, JLiu15 said:

 

 

Hello, where are these front hub from ?

[amorti]

20 minutes ago, Didumos69 said:

Yes softer, but the car's weight will compress the springs just as far as with shorter arms.

Suppose you have 6l suspension arms (5l center to center), you have travel of 2 studs and the car sinks in its suspension by 1 stud (50%). Now when you extend the suspension arms to 9l (8l center to center), while preserving the exact layout of the springs, you get roughly 8/5 times the travel (>3l) and the car still sinks in its suspension by approximately 50%.

This is of course under the assumption that the weight of the suspension arms is only a fraction of the car's complete weight.

Yes then we do agree, because the same weight is being applied to the spring so it compresses the same amount of its travel.

Or at least it sounds right.

I feel like I'd need to build a test rig which took that to extremes, to actually prove it to myself.

[JLiu15]

@Erik Leppen that makes sense - looks like just another example of trading force for distance (e.g. levers, pulleys, gear ratios, etc.)

@Bluehose the front hubs are these and the rear hubs are these; most sets released between 2013-2015 should include a pair of both (e.g. 42000, 42029, 42037, 42039). However, I've replaced the rear ones with portal hubs in my most recent update.

[amorti]

The front hubs are much better than the current ones, as the ball joints can't pull out.