which metal for Velomobile front axle

[memeticmusic]

I am building a tilting E-velomobile (around 130kg including me), 1 hub motor in the back , 2 24" front wheels with suspension. I have 2 hope tech pro 5 20mmx110mm hubs. I will have the axles custom machined. I was thinking of using titanium for the axles, is that overkill? I don't trust aluminum, is mild steel enough? tube or bar? will be used on rough roads and gravel. Looking for suggestions.

Thank you

 

[Chalo]

I am building a tilting E-velomobile (around 130kg including me), 1 hub motor in the back , 2 24" front wheels with suspension. I have 2 hope tech pro 5 20mmx110mm hubs. I will have the axles custom machined. I was thinking of using titanium for the axles, is that overkill? I don't trust aluminum, is mild steel enough? tube or bar? will be used on rough roads and gravel. Looking for suggestions.

Titanium is weaker and less stiff than relevant steel alternatives. There are some very strong aluminum alloys you could use, but they are all about 1/3 as stiff as a steel part the same size.

Since the hubs constrain the diameter of your axles, this is one of those places where steel's benefits complement the requirements for the part. If the axles are welded into an assembly, I recommend seamless 4130 chromoly tubing or cold drawn round bar. If they're mechanically connected without welding, you could consider using a high strength stainless steel like 17-4 PH.

 

[PaPaSteve]

These are 2 examples of vehicles I made using 20mm thru axle bicycle hubs.
Being human powered they are low speed, low weight so this knowledge may not apply to your application.
Axles and front steering knuckles for the orange quad are machined from 7075 aluminum and hard anodized.
Steering knuckles on the red trike are formed mild steel and the axles are a good quality steel bolt.
For a front axle application your stress analysis should be calculated for sheer and bending.
Titanium alloys 6al4V and 3al2.5V are commonly found and would offer good strength if the cost is in your budget.
17-4 stainless is roughly the same strength as 6al4V Ti

Keep in mind the 20mm bearings commonly used in bicycle hubs are not expected to see high weights or high speeds.

 

[Chalo]

Axles and front steering knuckles for the orange quad are machined from 7075 aluminum and hard anodized.

That's one of those "very strong aluminum alloys" I was talking about. Also see 7168 and 7078. I like 7040 and 7050 for their machinability and lack of internal stresses, even though they're not quite as strong.

17-4 stainless is roughly the same strength as 6al4V Ti

But it is 66% more rigid for the same dimensions.

 

[PaPaSteve]

Having done a bit of prototyping on small, light weight vehicles like these I've found that to reduce any "bump steer" feed back into the steering controls it's a good idea to point the steering axis to the center of the tire contact patch or there about.

When doing a first run prototype I tend to use the lowest cost solutions.

7075 is a commonly sourced alloy so a small piece is more likely found in the local machine shop.

Back of the envelope numbers to rough calculate with.

Grade 8 bolt Yield 130,000
4130 cro/mo Yield 75,000
17-4 Stainless Yield 145,000
6al4V Titanium Yield 160,000

But then I'm still learning how to make things
THX for keeping me up to date

 

[Chalo]

Because 4130 and other conventional steels are conditioned by heat treatment, there's no accurate way to characterize them except in the raw state or as heat treated. Chromoly is easy to harden and temper if more strength is required.

Correspondingly, other metals that may not be heat treatable become annealed when welded and can't recover their pre-welding strength except by cold work.

Because of the difficulty machining or welding Ti, plus its tendency to gall or seize, and its inferior stiffness-to-weight versus steel or aluminum, and its extreme cost, I think it's usually not the right material for the (almost any) job. It makes pretty great spy planes, though.

 

[PaPaSteve]

Other than cost I've found titanium to make a really excellent bike frame material.
For small run prototype machining consider contacting Paragon Machine Works

 

[Chalo]

Other than cost I've found titanium to make a really excellent bike frame material.

Agreed. It's even worth a significant cost premium because of its specific benefits for frame construction. It's a less clear case for components, especially when their size is fixed and traditionally assumes the use of steel.

When you can bulk up the dimensions of the part, aluminum often becomes the clear winner. When you can't, steel really shines. Ti is a good choice when its corrosion resistance, resilience, and fatigue resistance work for you.

Paradoxically, the high cost of titanium parts is likely why it's hung around in the bicycle industry, to meet the demand for fancy stuff that costs a lot. It's very good at many things but not best at anything that matters to cyclists.

 

[The Toecutter]

Chalo nailed it. I will add that if you're looking to save money, try to find scrap aluminum from decommissioned airplanes and their components.

The McPherson struts and steering plates on my Milan SL velomobile are made of aircraft grade aluminum, and are surprisingly strong. An A4 70 steel bolt broke off inside as I was loosening it to do some brake work. I had to have a friend weld a nut to the broken bolt to remove it, and the threading inside appears to have no damage.

 

[PaPaSteve]

For a large(ish) company looking to deliver a light welded frame at a low price point, aluminum makes sense.
Aluminum is low cost, fast and easy to machine, easy to weld but requires the extra step of post treatment.
Taiwan factories I've toured would do everything in house.
For a small shop like mine the treatment was shipped out increasing the risk of loss or damage ... it happened ... too much.
Steel and Ti frames can be put in service with no further steps other than paint or polish.
For a equal cross section aluminum tolerates the least amount of flexing cycles.
The reason behind oversized tubes, it solves the flexing cycle problem with geometry rather than material strength.

For Memeticmusic's first prototype a good quality bolt like this would be a good start for holding the hub to the steering knuckle.

 

[PaPaSteve]

When we say A4-70 it generally means that the bolt / screw is from SS 316 grade and has an ultimate tensile strength as 70 kg or 700 n/mm2.
316 stainless is actually low strength

 

[memeticmusic]

Thanks for the help, with this info I will go for 4130 bar that I will have threaded at both ends so that I can bolt the shaft on the kingpin at one end and the wheel will be held in place with one bolt at the other .

 

[PaPaSteve]

Your welcome

If your idea is a stud that is to stay permantly fastened in the steering knuckle the consequence will be : when the wheel is removed the disc brake caliper will need to be removed also.
My tactic is to use a partially threaded bolt with the un-threaded part is long enough to nearly reach the knuckle.
the threaded part will be too long . . . cut the excess off.
This strategy lowers the cost and increases the strength.

18.8 stainless = 100,000 psi tensile
10.9 steel = 150,000 psi tensile

Thanks for the help, with this info I will go for 4130 bar that I will have threaded at both ends so that I can bolt the shaft on the kingpin at one end and the wheel will be held in place with one bolt at the other .

 

[memeticmusic]

Your welcome

If your idea is a stud that is to stay permantly fastened in the steering knuckle the consequence will be : when the wheel is removed the disc brake caliper will need to be removed also.
My tactic is to use a partially threaded bolt with the un-threaded part is long enough to nearly reach the knuckle.
the threaded part will be too long . . . cut the excess off.
This strategy lowers the cost and increases the strength.

18.8 stainless = 100,000 psi tensile
10.9 steel = 150,000 psi tensile

I was thinking that if the shaft is bolted on the knuckle there would be less bending stress on the hub. With a bolt the hub is holding the wheel as well as the axle. In example pics the wheel may be easily removed with the axle.

 

[PaPaSteve]

OK . . . so the axle bolt goes through a hole in the steering knuckle and the assembly is held in place by the nut on the back side of the knuckle ?
Have you tried this strategy with the brake caliper in place ?

For the red trike I've shown in a previous picture the steering knuckle was formed from 1/4" (6.2mm) mild steel with a nut welded to the back side.
Remove the bolt and the brake disc and wheel slides out of the caliper.
For the quad the knuckle is designed thicker to accommodate a suspension strut.
Then the bottom side of the strut is designed with an open slot so when the quick release is open the assembly drops out of the caliper in the same manner as a bicycle dropout.
The aluminum knuckle is machined with a "lawyer lip" so if the quick release is accidentally knocked open the wheel can't fall out.

 

[Chalo]

OK . . . so the axle bolt goes through a hole in the steering knuckle and the assembly is held in place by the nut on the back side of the knuckle ?
Have you tried this strategy with the brake caliper in place ?

How we resolved this issue at the pedicab manufacturer I used to work for, was to mount the brake rotor separately from the wheel, with both of them keyed to the axle. The wheel comes off first, but the brake stays behind. The axle (front or rear) runs on its own bearings, with the hub only having a blank bore with a keyway.

 

[PaPaSteve]

How we resolved this issue at the pedicab manufacturer I used to work for, was to mount the brake rotor separately from the wheel, with both of them keyed to the axle. The wheel comes off first, but the brake stays behind. The axle (front or rear) runs on its own bearings, with the hub only having a blank bore with a keyway.

Yep . . . like a typical automotive application, entire brake assembly stays on the car when the wheel is removed.
Most all motorcycles, bicycles have one half of the disk brake assembly on the wheel and the caliper half stays on the vehicle.
Wheel examples presented earlier in this thread have 20mm thru axle bicycle hubs with the disc mounted to the hub.
It's a matter of convenience to be able to remove the wheel without unbolting the caliper.
One bolt rather than several bolts.

 

[memeticmusic]

OK . . . so the axle bolt goes through a hole in the steering knuckle and the assembly is held in place by the nut on the back side of the knuckle ?
Have you tried this strategy with the brake caliper in place ?

For the red trike I've shown in a previous picture the steering knuckle was formed from 1/4" (6.2mm) mild steel with a nut welded to the back side.
Remove the bolt and the brake disc and wheel slides out of the caliper.
For the quad the knuckle is designed thicker to accommodate a suspension strut.
Then the bottom side of the strut is designed with an open slot so when the quick release is open the assembly drops out of the caliper in the same manner as a bicycle dropout.
The aluminum knuckle is machined with a "lawyer lip" so if the quick release is accidentally knocked open the wheel can't fall out.

OK . . . so the axle bolt goes through a hole in the steering knuckle and the assembly is held in place by the nut on the back side of the knuckle ? Yes

I have not tried it, my thought was it would be easier to replace parts. I don't mind removing the caliper, it's only 2 bolt.
Now I just looked at the 17-4 stainless looks like a good idea indeed.

 

[PaPaSteve]

When the trike and quad were designed they logical choice was hydraulic calipers.
The method for centering the friction pads to the disc was with a stack of shim washers.
Removing the calipers it was easy to drop or mix up the washer stack.
What I was doing was for production, quick release wheels allowed a smaller package for shipping and fitting into a trunk or such.
Simple reassembly after shipping was an important feature.

 

[memeticmusic]

When the trike and quad were designed they logical choice was hydraulic calipers.
The method for centering the friction pads to the disc was with a stack of shim washers.
Removing the calipers it was easy to drop or mix up the washer stack.
What I was doing was for production, quick release wheels allowed a smaller package for shipping and fitting into a trunk or such.
Simple reassembly after shipping was an important feature.

Do you have any videos, the quad looks nice?

 

[PaPaSteve]

Sorry . . . no videos I can think of.