Appropriate tightness of bolts between skateboard and trucks

[CynicusRex]

TL;DR Summary

Does bolt tightness significantly increase or decrease the strength of wood?

As you can see in the pictures below the trucks are attached to the skateboard with four screws nuts and bolts, with a 2~3mm piece of rubber between the truck and the board. I've always wondered if tightening the screws bolts harder, decreases or increases the strength of the 7-layered plywood. The same question goes for the rubber; does it decrease or increase the strength of the board? What setup will make the board last longest?

My second last board lasted a year before the wood cracked at the trucks, my previous one, one month. So I wonder if this is pure luck, or could my setup have something to do with it.

 

[phinds]

Over tightening can increase both the chance that the truck will crack and the chance that the screw will pull out. The screws appear to be countersunk on top so you would REALLY have to over tighten, and the screw would have to be very strong steel, to pull the screw through the board, so that's actually unlikely but you could certainly weaken the truck.

 

[CynicusRex]

@phinds I think I have made the mistake of assuming everyone knows about skateboards, my bad. So, my worry is not the trucks; I've never seen the baseplate (that part which is mounted to the skateboard with 4 screws nuts and bolts) crack. It's always the wood that cracks (or the truck's axle, but that's irrelevant here). Also, the holes in the wood are pre-drilled, so the bolts aren't actually 'screwed' in the wood, but only held by the nuts on the other end. The countersinking you can see is mainly the grip tape on top of the wood that has rotated with the bolt.

In skateboarding it's predominantly the wood (the 'deck') that bears the brunt of the beating. Especially the tail because that's where your back foot lands. Ideally, you land with both feet on the bolts, but most often during practice you land with a foot too far on one end creating some sort of lever between your foot and the wood, the baseplate, the bolts. This is where my question lies. How tight do I have to screw the bolts in order for the wood to endure the most stress? And does the rubber sheet help with that, or not.

I haven't a clue about the tightness, but I have a mixed idea about the rubber. It helps with shock absorption, but by increasing the height of the baseplate, it also increases the 'strength of the lever' of the truck, increasing the force of impacts, making the wood more susceptible to cracks. Anyway, I'm just guessing here.

 

[phinds]

I'd say the rubber does more good than harm. As for tightening, I'd say just beyond finger tight --- just tight enough to not come loose, but I don't know skateboards

 

[berkeman]

Summary: Does bolt tightness significantly increase or decrease the strength of wood?

I've always wondered if tightening the screws bolts harder, decreases or increases the strength of the 7-layered plywood.

Since you can buy the parts separately and assemble the trucks onto your board, does the truck or board manufacturer list a recommended torque for the nuts? In motorcycle and car repair manuals, torques are listed for every single step of the work.

 

[jim mcnamara]

Caveat: None of anyone's suggestions will ever help the cracking problem if your current setup is "perfect" for you.

If you are active in a skatepark, the impulse on landing can be large. The forces are borne by the junction of the truck and board itself. Impacts are proportional to how much you + board weigh (mass) and how much elevation (potential energy) you have gained on a jump, for example. Hitting an uneven sidewalk crack applies impulse in a somewhat different direction from jumps and flips, and stresses the truck/board interface in interesting ways.

Your weight: Are you near the weight limit for your current board? Is there an acceptable board meant for heavier users?

Short answer:
Use the manufacturers torque recommendations for the bolts. I am assuming the truck manufacturer gave you what they think are the best bolts. Could be a bad assumption. Get a torque wrench if you need one.
See the chart in the link. Below.

Bolt torque is not your main problem. IMO.

If longevity is top priority, you may need to consider other types of board material. You may also want shock pads for the trucks. Lining of the bolt holes is another place to start. Since pads can change the behavior of the board, you may consider them a negative. Boards and separate trucks are very like buying shoes, one size does not fit all.

I cannot see the kingpin clearly in your pictures, guessing that is your board in the photos. Kingpin structure may reduce cracking as well by changing impulse on the board.

You can set and forget bolt torque. Do not over-torque. If you remove the truck and then put it back, consider buying a supply of new nuts - don't reuse nuts. Especially if they are nylon lock nuts. You do understand what those numbers and markings associated with bolts are?

Example:
https://www.boltdepot.com/fastener-information/materials-and-grades/bolt-grade-chart.aspx
MTBF (mean time between failures a product) data does not seem to exist for anything in the skateboard world. So from my perch it seems like a crapshoot to get what you need without spending a lot of money on failures.

 

[CynicusRex]

I'd say the rubber does more good than harm. As for tightening, I'd say just beyond finger tight --- just tight enough to not come loose, but I don't know skateboards

I'm going with this advice.

MTBF (mean time between failures a product) data does not seem to exist for anything in the skateboard world. So from my perch it seems like a crapshoot to get what you need without spending a lot of money on failures.

& @berkeman

Exactly; hence me posting this here. Skaters are left in the dark when it comes to hardware information. Most people just go by brand experience and word-of-mouth.

 

[gmax137]

They look like locknuts in the picture. As @jim mcnamara noted, these should not be re-used.

I would hold the bolt head stationary and tighten the nuts until the rubber just starts to squish (and depending on how that feels, maybe another half-turn). Too much torque would risk crushing the wooden deck below the bolt head, which would probably lead to cracking failure sooner.

Is the hole in the deck pretty close to the bolt diameter? I would think holes too big would allow movement; a close fit would be best.

 

[CynicusRex]

They look like locknuts in the picture. As @jim mcnamara noted, these should not be re-used.

I would hold the bolt head stationary and tighten the nuts until the rubber just starts to squish (and depending on how that feels, maybe another half-turn). Too much torque would risk crushing the wooden deck below the bolt head, which would probably lead to cracking failure sooner.

Is the hole in the deck pretty close to the bolt diameter? I would think holes too big would allow movement; a close fit would be best.

According to the markings my bolts are: Grade 8 - Medium carbon alloy steel, quenched and tempered. (And the holes are a tight fit.)
Why shouldn't these be re-used? As far as I knew, as long as they hold tightly, it's fine? Does it influence the integrity of the board or something?

Also, I might have been applying too much torque indeed. I relaxed them already; hopefully I haven't done too much damage yet. All that being said, I might have the perfect board setup and still crack the board tomorrow after one unlucky landing. Regardless, all this information is definitely helpful and interesting.

 

[gmax137]

The bolts can be reused. The locknuts are really only good for locking once. If you unthread them they won't necessarily lock properly the next time.

That said, you wouldn't be the first person to reuse a lock nut. Taking a good look at the board and hardware before use (loose nuts, starting cracks, etc) is cheap insurance.

 

[Dullard]

I think that it is possible to over-tighten the screws. More importantly (maybe), the presence of the rubber pad between the truck base and the board maximizes the chance that the wedge-shaped screw head will do exactly what wedges do when a large force is applied. The lack of solid support on the bottom of the board (around the screw holes, due to the rubber) will result in displacement of the wood, rather than simple compression. The head will 'sink' deeper into the wood and a crack is the ultimate result. I don't think this is about the screws - it's about the wood.

There are 2 things that I might do to prevent this problem:

Make a '4-hole nut plate' for the top side of the board, and attach the trucks with screws (from the bottom). Studs and nuts would also work. The plate could be 'sunk' into the top surface of the board. This will spread the load and completely eliminate the 'wedging' effect.

If that's too much trouble, I would add a metal plate (drilled to the pattern of the truck) between the board and the rubber pad. This isn't as good as the top plate, but could reduce the magnitude of the 'wedging.'

 

[CynicusRex]

Make a '4-hole nut plate' for the top side of the board, and attach the trucks with screws (from the bottom). Studs and nuts would also work. The plate could be 'sunk' into the top surface of the board. This will spread the load and completely eliminate the 'wedging' effect.

If that's too much trouble, I would add a metal plate (drilled to the pattern of the truck) between the board and the rubber pad. This isn't as good as the top plate, but could reduce the magnitude of the 'wedging.'

Some companies have already implemented such solutions with carbon fiber in the area you describe. I might buy one sooner or later. The second solution might be interesting to try myself, but I think a 1mm piece of rubber isn't going to make any noticeable difference in shock absorption. Or will it?

 

[Tom.G]

but I think a 1mm piece of rubber isn't going to make any noticeable difference in shock absorption. Or will it?

Probably little effect in shock absorption but does work to help spread the forces at the interface of truck and board.

The major design problem I see in the photos is the truck is mounted on a curved section of the board. This concentrates the forces on a line across the board where the truck is in good contact with the wood.

This could be by design to act as a shock absorber using the wood as a spring. But wood, like all solid materials, will fracture under repeated flexing. If the board springiness is not needed, a shim shaped to match the board curve to the truck mounting surface should greatly improve reliability.

Since the truck mounting bolts don't see much variation in tension force during use, this part may not apply for skateboards. For general-use assembly the maximum effctiveness of joining fasteners is when the fastener tension applies 1/2 the yield or deforming force of the weakest material. There are tables (probably in MACHINERY'S HANDBOOK) for finding bolt tension versus tightening torque. They are valid for "new, clean, lightly lubricated threads."

If you are using self-locking nuts (i.e. Nylok or similiar), remember to account for their locking torque when using a torque wrench.

Cheers,
Tom

 

[256bits]

Summary: Does bolt tightness significantly increase or decrease the strength of wood?
My second last board lasted a year before the wood cracked at the trucks

It would be interesting to know where the board cracked - towards rear end / forward end of the truck.

The bolts have to be tight enough to minimize movement of the truck in the forward/backward direction, or sideways and minimal rotation of the truck forward backward or sideways when subjected to landing forces.

Maybe your bolts aren't tight enough.
And the truck edge is the stress contact line cutting into the wood causing failure over repeated landings.

Perhaps they are too tight.
And the pre-stress compression of the wood leads to over-compression fatigue failure.
I would consider that the least likely due to the lack of a top plate.

The rubber acts as a point stress reliever and attempts to spread the forces onto a much larger area.
Some movement of the truck does occur due the elasticity - a hard rubber I would think best.
A thin steel plate a bit larger than the truck could spread the contact line over a wider area beyond the truck edges.

 

[CynicusRex]

They look like locknuts in the picture. As @jim mcnamara noted, these should not be re-used.

I would hold the bolt head stationary and tighten the nuts until the rubber just starts to squish (and depending on how that feels, maybe another half-turn). Too much torque would risk crushing the wooden deck below the bolt head, which would probably lead to cracking failure sooner.

Is the hole in the deck pretty close to the bolt diameter? I would think holes too big would allow movement; a close fit would be best.

I've had two new decks so far, on one I tightened the bolts hard and noticed pressure cracks after a couple of days. The one I'm using now is tightened just hard enough to keep the trucks in place without crushing the wood below, and I haven't seen any pressure cracks yet.

Thank you.

 

[JBA]

Placing a stiff plate that extends completely across the width of the board between the mounting bracket and the board and bit of a distance toward the center of the board may help because right now all of the boards bending stress is being concentrated on the small area and narrow width of the truck mounting base.

 

[gmax137]

haven't seen any pressure cracks yet.

 

[evancris2087]

Although rigid trucks are stable but that does not always mean they’re the best skateboard configuration. we should loosen them to turn on a skateboard easier. I think so. Other ideas??