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Old 02-23-2020, 01:27 AM   #1
Rock Crawler
 
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Default Suspension Shocks Dynamics Question

What is it about a "big bore" shock that makes them perform better than a smaller diameter shock body?

I understand that it's the increased fluid volume in the larger diameter shock body. My question is why does a larger fluid volume benefit performance?

Whether your shock body is wide or narrow, your piston is still up against the shock body inner walls, with no fluid going around the sides. So how does increasing the fluid volume in the body help the shock perform better? Or is it just marketing speak?

Thanks for any light you hydraulic engineers can shed on the topic for me.
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Old 02-23-2020, 07:16 AM   #2
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Default Re: Suspension Shocks Dynamics Question

The larger the bore, the more it can lift or push,
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Old 02-27-2020, 05:42 PM   #3
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Default Re: Suspension Shocks Dynamics Question

So let me see if I'm understanding you.

Take two different shocks, one big bore and the other a smaller diameter shock body both with 30wt oil in them. The big bore shock will be more damped (greater resistance to motion) than the smaller diameter shock body.

This is because of a larger diameter piston has more surface area. More surface area = more force acting on it.

Sound right?
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Old 02-27-2020, 05:59 PM   #4
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Default Re: Suspension Shocks Dynamics Question

One of the differences comes from the shaft size and how much it can "pack". As the shock shaft extends into the shock body it displaces oil the same volume as the shaft. simply put a smaller shaft will pack or displace less oil. The pistons themselves dont displace any extra oil as they never leave the shock body. This isn't to say the piston doesn't have an effect but a big bore will stiffen up and dampen more because of its ability to "pack" more.

I've yet to feel a small bore shock match the feel of a big bore. Though I have wondered if these small bore shocks just need a much thicker oil to feel closer to the big bores.
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Old 02-27-2020, 06:09 PM   #5
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Default Re: Suspension Shocks Dynamics Question

Quote:
Originally Posted by HumboldtEF View Post
One of the differences comes from the shaft size and how much it can "pack". As the shock shaft extends into the shock body it displaces oil the same volume as the shaft. simply put a smaller shaft will pack or displace less oil. The pistons themselves dont displace any extra oil as they never leave the shock body. This isn't to say the piston doesn't have an effect but a big bore will stiffen up and dampen more because of its ability to "pack" more.

I've yet to feel a small bore shock match the feel of a big bore. Though I have wondered if these small bore shocks just need a much thicker oil to feel closer to the big bores.
Perfect explanation. Just as Humboldt stated, the shocks dampening characteristics are very consistent throughout the entire motion. Where a small bore will travel faster extended and slower compressed. Of course the shaft diameter increases usually but the ratio is still much less. You also get a stronger shaft. There are some other benefits but mostly related to full size racing.

Last edited by Joshs4x4toyz; 02-27-2020 at 06:52 PM.
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Old 02-27-2020, 08:58 PM   #6
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Default Re: Suspension Shocks Dynamics Question

Ok, I think I'm starting to pick up on what I have been missing.

It has more to to with shock shaft size than with fluid volume in the shock body. So "big bore" is referring to the shock shaft, not the shock body.

And, since it's about the shaft, fluid displacement is the characteristic that's important, not surface area of the piston.

Helpful. Thanks guys.
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Old 02-27-2020, 11:12 PM   #7
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Default Re: Suspension Shocks Dynamics Question

Well maybe I was not getting things out correctly.

The 'big bore' term does actually refer to the body itself. Shaft is not always bigger, but if youre making a big bore shock you might as well increase the shaft diameter, but not all companies do. So lets look at Proline standard vs big bore both having a 3.5mm shaft and considering bodies to be 40mm long from a 90mm shock configuration.


Big bore (10.8mm diameter) has a cubic volume of 3664mm

Standard (7.5mm diameter) has a cubic volume of 1767mm

The shaft is 3.5mm so at 40mm it will want to displace a volume of 385mm

That is 10.5% of the big bore total volume and 22% of the Standard.

Now shock oil does not compress, so if you try to displace the full 385 in the standard it would get tight messing up the dampening that you want throughout the movement. With the big bore, you would literally be able to fit twice as much shaft in before reaching the same compression problem. Of course there are different ways to build these shock with different caps, bladders etc. But that is the basic theory and why Big bores are so much smoother throughout the entire cycle. Of course if the manufacturer takes advantage of the big bore and increases shaft diameter they are closer to the same ratio, but a much much stronger shaft.

Hopefully that makes more sense and others chime in as well that might make more sense.
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Old 02-28-2020, 07:05 AM   #8
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Default Re: Suspension Shocks Dynamics Question

Thanks man, it does make sense now. Due to a larger shock body, the big bore shock has a lower ratio of fluid displacement to available space for said displacement.

A non big bore has less room for said displacement, which negatively effects performance by ramping up dampening further into the stroke due to the decreased room for displacement.

I guess it wasn't just marketing speak.
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