Shoulder belt harness mounting

[jakks]

Trying to do some research on the benefits of mounting shoulder belts closer to the seat versus farther away.

 

It's recommended to have the seat back no more than 8" from the shoulder belt harness mounting, however multiple harness manufacturers provide charts on at what distance you have to cross them and what distance they need to be separated by.

 

Based on rough calculations I'm seeing the stress is less on the belts if the area of the belts is greater. People seem to think they stretch more if the initial length is longer. That doesn't make a lot of sense to me since the properties of the belts are assumed the same and the force, time are assumed the same. Not that it matters, but the strain on the belts is greater if the distance is shorter.

 

What's the deal?

[enginerd]

5 hours ago, jakks said:

Trying to do some research on the benefits of mounting shoulder belts closer to the seat versus farther away.

 

It's recommended to have the seat back no more than 8" from the shoulder belt harness mounting, however multiple harness manufacturers provide charts on at what distance you have to cross them and what distance they need to be separated by. The crossing requirement probably has to do with keeping the belt from sliding down off your shoulder when it stretches.

 

Based on rough calculations I'm seeing the stress is less on the belts if the area of the belts is greater. Cross sectional area would matter here, which is just width x thickness of the belt. With a given force pulling on the belt, it will feel that stress in the middle and the end and at points in between. Doesn't matter how long the belt is, the same stress will be present. 

 

People seem to think they stretch more if the initial length is longer. This is correct. No matter what the length is, the stress will be equal, therefore the strain will be equal, but strain doesn't mean equal change in length, it means equal percent change in length. So for a given impact force, a belt of length X will stretch by Y amount, and a belt of length 2X will stretch by 2Y amount (double the stretch).

 

That doesn't make a lot of sense to me since the properties of the belts are assumed the same and the force, time are assumed the same. Not that it matters, but the strain on the belts is greater if the distance is shorter.

 

What's the deal?

Off the top of my head here, from intro engineering classes ^^

I'm sure Mender will chime in and correct any inaccuracies. 

Edited March 20, 2021 by enginerd

[Rodger Coan-Burningham]

4 hours ago, jakks said:

Trying to do some research on the benefits of mounting shoulder belts closer to the seat versus farther away.

 

It's recommended to have the seat back no more than 8" from the shoulder belt harness mounting, however multiple harness manufacturers provide charts on at what distance you have to cross them and what distance they need to be separated by.

 

Based on rough calculations I'm seeing the stress is less on the belts if the area of the belts is greater. People seem to think they stretch more if the initial length is longer. That doesn't make a lot of sense to me since the properties of the belts are assumed the same and the force, time are assumed the same. Not that it matters, but the strain on the belts is greater if the distance is shorter.

 

What's the deal?

One would think the belts have a fixed elongation per length when a given force is applied, or the longer the belt the more they stretch. But, I assume the impulse force is less if the stretch is more. Impulse is force x time and equals the change in momentum. Interesting analysis that the belt manufacturers do quite often you would think? There is no information from the belt manufacturer you are dealing with to say?

[enginerd]

1 hour ago, Burningham said:

One would think the belts have a fixed elongation per length when a given force is applied, or the longer the belt the more they stretch. But, I assume the impulse force is less if the stretch is more. Impulse is force x time and equals the change in momentum. Interesting analysis that the belt manufacturers do quite often you would think? There is no information from the belt manufacturer you are dealing with to say?

Doh you're right, the harness has to dissipate the energy of the driver. Energy = work = force x displacement. Harnesses aren't subjected to a constant force / stress during a crash, they do work on the driver to bring kinetic energy to zero. So if a belt stretches more it acts over a greater distance and sees a lower stress.

Edited March 20, 2021 by enginerd

[Ian]

12 hours ago, enginerd said:

Doh you're right, the harness has to dissipate the energy of the driver. Energy = work = force x displacement. Harnesses aren't subjected to a constant force / stress during a crash, they do work on the driver to bring kinetic energy to zero. So if a belt stretches more it acts over a greater distance and sees a lower stress.

 

Correct as far as I've always understood.

 

I would assume the stress on the mount point is the weak spot the manufacturers are worried about.

 

By spreading the load over a longer section of belt, you lessen the initial shock load on whatever it's mounted to.  At the cost of potentially allowing the driver to move further.  In a single impact crash this isn't terrible, but in a multi impact crash, this stretching may mean the different between the belts keeping you all the way in, or only mostly in.  If a big impact is followed by a rollover, the last thing I want is loose should harnesses.

 

Since the majority of us are mounting shoulder straps by looping directly to the cage, there's no mounting failure to worry about, and I would say the shorter the better.

 

 

[mender]

21 hours ago, enginerd said:

Off the top of my head here, from intro engineering classes ^^

I'm sure Mender will chime in and correct any inaccuracies. 

The fun thing about physics is sorting through the real life situations to see what the critical factors could be.

 

One could argue that the belts' primary task is to match the driver's decel to the car's decel. To that end, longer belts could delay the initial restraint enough to allow a larger difference in velocity to build before the matching of decel rates is achieved. That could result in a higher level of peak force being experienced.

 

Another aspect may be to act as a buffer to reduce momentary peak loads by extending the time needed to decelerate the driver and averaging the loads felt. Longer belts would be beneficial in that case.

 

Personally, I feel that Ian's point about excess stretch allowing the driver to move about is significant enough to use the shortest belt length possible. 

Edited March 21, 2021 by mender

[Ian]

14 minutes ago, mender said:

Personally, I feel that Ian's point about excess stretch allowing the driver to move about is significant enough to use the shortest belt length possible. 

 

Thanks.

 

Coming from the stage rally world, I feel I'm a lot more conscious of how safety equipment needs to perform during a multi stage crash that may involve a large impact followed by a rollover, followed by another large impact.

[mender]

19 minutes ago, Ian said:

 

Thanks.

 

Coming from the stage rally world, I feel I'm a lot more conscious of how safety equipment needs to perform during a multi stage crash that may involve a large impact followed by a rollover, followed by another large impact.

Which brings up the topic of car construction and the importance of crush zones, a topic which could take pages by itself!

 

When I did the cage on the Civic, I considered the final weight of the car and the speeds that I expected us to see in this series. A noticable difference in both from the typical NASCAR cages that I'm used to! I had to resist the temptation of just doing what I usually did with lots of big bars and such; in essence, it would have been an overbuilt cage for the purpose. I wanted a protection zone for the driver but as much crush room as possible. 

 

The main hoop and bracing was typical but the door bars were the biggest departure from my usual.

 

Still tied into the structure of the car wherever possible but moved out to increase the buffer/crush zone. The seat ended up further back with its own structure that would crush interior panels while staying rigid to the outside of the cage if things went that far, allowing the seat to move away from the impact.

 

Better in my mind to hold the driver in place and protected by the seat while letting the rest of the car do the load averaging.

Edited March 21, 2021 by mender

[Ian]

8 hours ago, mender said:

Which brings up the topic of car construction and the importance of crush zones, a topic which could take pages by itself!

 

When I did the cage on the Civic, I considered the final weight of the car and the speeds that I expected us to see in this series. A noticable difference in both from the typical NASCAR cages that I'm used to! I had to resist the temptation of just doing what I usually did with lots of big bars and such; in essence, it would have been an overbuilt cage for the purpose. I wanted a protection zone for the driver but as much crush room as possible. 

 

The main hoop and bracing was typical but the door bars were the biggest departure from my usual.

 

Still tied into the structure of the car wherever possible but moved out to increase the buffer/crush zone. The seat ended up further back with its own structure that would crush interior panels while staying rigid to the outside of the cage if things went that far, allowing the seat to move away from the impact.

 

Better in my mind to hold the driver in place and protected by the seat while letting the rest of the car do the load averaging.

 

Very similar to our new car.

 

One thing I've always found lacking in traditional rally cages is space given to driver/co-driver.  Many rulesets require factory door structure, so the cages are often very tight to your side.  I built a rally style door bar, but freed of the requirement to keep the inner door skins intact.

 

 

[Dimsun Racing]

On 3/20/2021 at 10:28 AM, jakks said:

Trying to do some research on the benefits of mounting shoulder belts closer to the seat versus farther away.

 

It's recommended to have the seat back no more than 8" from the shoulder belt harness mounting, however multiple harness manufacturers provide charts on at what distance you have to cross them and what distance they need to be separated by. 

 

Based on rough calculations I'm seeing the stress is less on the belts if the area of the belts is greater. People seem to think they stretch more if the initial length is longer. That doesn't make a lot of sense to me since the properties of the belts are assumed the same and the force, time are assumed the same. Not that it matters, but the strain on the belts is greater if the distance is shorter.

 

What's the deal?

Seat belts are considered static as compared to dynamic as far as stretch, they stretch very little. I don't think mounting distance matters that much, look how much extra length they provide, so it must be within parameters. The more important factor is mounting angle from the horizontal. Also I don't think you are getting much more additional stretch from a multi-impact crash. Compare our level of racing to Nascar, with similar belts, it must be safe.

[Dimsun Racing]

On 3/21/2021 at 4:53 PM, Ian said:

 

 

 

You need some gusseting on the other side of the X junction in the center. If you look at the cross section right at the center you only have one tube, compare to Mender's which does not splice one of the tubes at the center, his are both continuous through the center. Although the spliced style is common I don't think it's very strong in a side impact, as the center is the weakest point in a side impact.

[mcoppola]

8 minutes ago, Dimsun Racing said:

You need some gusseting on the other side of the X junction in the center. If you look at the cross section right at the center you only have one tube, compare to Mender's which does not splice one of the tubes at the center, his are both continuous through the center. Although the spliced style is common I don't think it's very strong in a side impact, as the center is the weakest point in a side impact.

@Dimsun Racing interesting that you too noticed that.

I see so many X style door bars gusseted this way. I've posted the exact same thing you stated about increasing the cross sectional area. 

I always gusset the sides of the X that need it most, usually the 2 sides with the greatest angle differential.

 

[Chris Huggins]

This is how I do them, 2 continuous tubes with the rotation under compression trying to push them into each other and the gussets used to keep them from separating.

 

 

[Ian]

2 hours ago, Dimsun Racing said:

You need some gusseting on the other side of the X junction in the center. If you look at the cross section right at the center you only have one tube, compare to Mender's which does not splice one of the tubes at the center, his are both continuous through the center. Although the spliced style is common I don't think it's very strong in a side impact, as the center is the weakest point in a side impact.

 

That is a very old picture.  The center is gusseted, though per the rules it doesn't need to be.  Notice the sill bar.  That paired with the unbroken door bar satisfies the rules as written, the X is additional to satisfy my own personal want for safety.  Just like the FIA vertical.

[mcoppola]

33 minutes ago, Ian said:

 

That is a very old picture.  The center is gusseted, though per the rules it doesn't need to be.  Notice the sill bar.  That paired with the unbroken door bar satisfies the rules as written, the X is additional to satisfy my own personal want for safety.  Just like the FIA vertical.

Some very nice work. I recall putting a "like" on a post a few months back with some of your pics. 

The FIA A-pillar bars and gussets are a nice safe touch, along with the seat mounts, foot intrusion bars, main hoop and windshield bars corner tube gussets, A-pillar plates with dimpled holes - there's a whole lot of good stuff going on here. 

Do you have a background in Rally? This cage meets (all) of the FIA rules.

[Ian]

7 hours ago, mcoppola said:

Some very nice work. I recall putting a "like" on a post a few months back with some of your pics. 

The FIA A-pillar bars and gussets are a nice safe touch, along with the seat mounts, foot intrusion bars, main hoop and windshield bars corner tube gussets, A-pillar plates with dimpled holes - there's a whole lot of good stuff going on here. 

Do you have a background in Rally? This cage meets (all) of the FIA rules.

Thanks got my start in a rally car.  You should swing by and check it out at RA in a couple weeks.

 

And good eye, I referenced the FIA ruleset quite a bit while building the cage.

 

It did feel weird to focus only on the driver's side, the little bit of OCD in me really wanted matching door bars on the passenger side, but in reality, there's no benefit in a car thats only got 1 seat.

 

 

Quite the evolution.

 

[mcoppola]

Both cars look nicely prepped. Duh, I forgot you mentioned your Rally background. 

Is this the 2nd 240? Same color as the first?

[Ian]

Just now, mcoppola said:

Both cars look nicely prepped. Duh, I forgot you mentioned your Rally background. 

Is this the 2nd 240? Same color as the first?

 

Nope we (I) wrote that car off in 2019.  The new car is a G35.