Author Topic: Fastback vs Hatchback - tyre noise level  (Read 812 times)

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Offline philly

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Fastback vs Hatchback - tyre noise level
« on: July 05, 2018, 07:19:38 PM »
Is BM petrol 2.0 120 hp Fastback much quieter/comfier/refined than the hatchback? Considering it comes with 17" wheels, it should be but in practical terms, driving at motorway speeds of 70-80 mph - which one would be a better choice?

Offline moozmooz

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Re: Fastback vs Hatchback - tyre noise level
« Reply #1 on: July 06, 2018, 12:31:04 AM »
"Considering it comes with 17" wheels, it should be " Why?
Tyre noise is generated by the tyre and the road surface. How much gets in depends on insulation, which is tuned to the car. I'd imagine there'd be very little difference.

Offline Engineer Andy

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Re: Fastback vs Hatchback - tyre noise level
« Reply #2 on: July 31, 2018, 01:39:29 PM »
Is BM petrol 2.0 120 hp Fastback much quieter/comfier/refined than the hatchback? Considering it comes with 17" wheels, it should be but in practical terms, driving at motorway speeds of 70-80 mph - which one would be a better choice?

Like the gen1 and 2 Mazda3s, the gen3 (its a saloon even if its called a fastback) has a stiffer bodyshell than the hatchback because of the way the boot/back window are designed/constructed.  As such, the handling will be slightly better, though I doubt if most people will notice unless they drove the cars to the limit.  The suspension set up again may slightly differ in terms of where things go, but any differences will be minor, as will anything to do with noise transmitted through to the cabin: at most, the thicker back shelf of the fastback may give slightly better noise dampening to those passengers in the back seats, but gain, its probably marginal at best.

I bought my gen1 saloon because it looked far better than the hatch (the differences in that department have significantly lessened in the gen2 and current gen3), and the boot was larger by 25% (though is less user friendly because the opening [same on the current fastback] is far smaller than on the hatch).  Other than that, the only other differences (other than price at dealerships) was that you get slightly less headroom in the back of the saloon/fastback becaused of the slope of the back window, but you get the 'amazing' benefit of the car being 0.1 sec quicker to 60mph (wooo!).

If you want to keep the car long term and want a comfortable ride, go for the SE-L/SE-L Nav (or below) models shod on 205/60 R16 tyres rather than the Sport models (or any ones upgraded on wheels/tyres) shod on 18in rims.  I found the ride quality on the lower profile tyres to be fine when new and on well-surfaced roads, but noticably rougher (more than I would accept) on either poorly-surfaced/potholed roads (worse than my old car on either its OEM 16in set of my new [more comfortable] 15in higher profile tyres) on cars that were only a couple of years old.

The choice of tyre fitted does make quite a bit of difference as well - the Toyo 16in OEMs are only regarded as fair by owners from what I've read; the 18in Dunlops fare better but like all low profile tyre, don't last as long in comparison and are quite hard to get hold of, and thus are expensive.  The unusual tyre size combos for both the latest 16in and 18in versions mean that both have far less available from other makes (worse for the 18in ones) and are about 20-35% more expensive than more 'standard' 16 and 18in tyres.

Offline Tall333

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Re: Fastback vs Hatchback - tyre noise level
« Reply #3 on: July 31, 2018, 03:37:57 PM »
The road noise in our 2015 SE-L fastback with the smaller wheels is ok but not great.  BTW I changed 19" to 17" wheels on my CX-5 after running it a while, then changed them back after another while.  There's pretty much no difference in road noise.  Bump absorption yes, but not noise.

Offline moozmooz

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Re: Fastback vs Hatchback - tyre noise level
« Reply #4 on: August 01, 2018, 12:42:45 AM »
" has a stiffer bodyshell than the hatchback because of the way the boot/back window are designed/constructed". Any proof of this or are you just speculating based on the bigger aperture in a hatchback?

Offline Engineer Andy

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Re: Fastback vs Hatchback - tyre noise level
« Reply #5 on: August 01, 2018, 06:41:48 PM »
" has a stiffer bodyshell than the hatchback because of the way the boot/back window are designed/constructed". Any proof of this or are you just speculating based on the bigger aperture in a hatchback?

Basic engineering mate - the saloons and fastback model has a structural beam going across the bottom of the rear window to support it and the boot lid/mechanism, whereas the hatch doesn't - it relies on contact along between the boot lid and the rest of the car to add some structural stiffness.  Its the reason why in the Nissan 350Z that its got a honking great big beam crossing the boot taking up valuable space - to give the car extra stiffness so it doesn't flex/move about as much when throwing it into corners at high speed.  A saloon does that by default.

This gives such cars a more settled ride, but as I said before, for non-performance cars like the Mazda3, its less of an issue now that the technology has moved on since the mk1 I've got (reviewers could tell the difference between the handling of the hatch and saloon).  It would be very noticeable for a high performance car, or if Mazda had (unlikely, admitedly) still kept an MPS model and offered it in both hatch and fastback form.

Offline moozmooz

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Re: Fastback vs Hatchback - tyre noise level
« Reply #6 on: August 01, 2018, 07:27:46 PM »
Ok. So speculation. Basic engineering, as well as being dangerous, also tells you there's more than one way to skin a cat, and to stiffen a structure. :)

Offline Engineer Andy

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Re: Fastback vs Hatchback - tyre noise level
« Reply #7 on: August 04, 2018, 01:26:23 PM »
Ok. So speculation. Basic engineering, as well as being dangerous, also tells you there's more than one way to skin a cat, and to stiffen a structure. :)

Other than the shape of the boot/rear window area, there's no difference to the structure of the mk3 between the the hatch and the fastback.  I actually READ this in an article several years ago about the similar differences in the mk1 (mine), and I see no reason why it would be any different now.  I'm also an engineer myself, so am inclined to believe what it said as I've seen what permanently bracing a structure does to its stiffness.  Whether you believe that is entirely up to you.  As I said, it makes A difference, but not a HUGE one, and likely only to be noticeable when driving hard, nor am I saying the hatch versions are rubbish at handling, etc, etc, just in case you were wondering.

Offline moozmooz

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Re: Fastback vs Hatchback - tyre noise level
« Reply #8 on: August 05, 2018, 02:49:57 AM »
In the absence of any hard evidence to the contrary. I'd find it hard to believe that there would be any measurable difference in stiffness/performance. It's more than likely that Mazda will have designed both body shapes to the same performance criteria.

The structures are different, both in the shape/size of the hole in the back of the shell and in the location of the tailgate/boot lid hinge supports, but as an engineer, you should be aware of the techniques used to stiffen structures against torsional and buckling loads. It's simple stuff that's been on the go for decades, so I'm confident that Mazda, are perfectly capable of designing/manufacturing a body shell that isn't compromised by having a hatch rather than a boot.

"it relies on contact along between the boot lid and the rest of the car to add some structural stiffness"
If that were true, the paint/rubber seals along the "contact" area wouldn't last long and I've seen no evidence of that on my Gen 1, my Gen 3, or any of my previous hatched cars. If you check. you'll find there's a considerable clearance in the area you refer to. You're confusing how a car structure behaves under impact and how it behaves normally.

Your reference to the Nissan 350Z is irrelevant. It's a 300bhp rear wheel drive designed for performance, not boot capacity.

"A saloon does that by default" Not true. There's absolutely nothing to say a saloon will be stiffer than a hatch by default, more likely the opposite without stiffening, as hanging a boot on the back extends the length of the overhang, resulting in a softer rear end.

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Re: Fastback vs Hatchback - tyre noise level
« Reply #9 on: August 05, 2018, 02:32:33 PM »
In the absence of any hard evidence to the contrary. I'd find it hard to believe that there would be any measurable difference in stiffness/performance. It's more than likely that Mazda will have designed both body shapes to the same performance criteria.

The structures are different, both in the shape/size of the hole in the back of the shell and in the location of the tailgate/boot lid hinge supports, but as an engineer, you should be aware of the techniques used to stiffen structures against torsional and buckling loads. It's simple stuff that's been on the go for decades, so I'm confident that Mazda, are perfectly capable of designing/manufacturing a body shell that isn't compromised by having a hatch rather than a boot.

"it relies on contact along between the boot lid and the rest of the car to add some structural stiffness"
If that were true, the paint/rubber seals along the "contact" area wouldn't last long and I've seen no evidence of that on my Gen 1, my Gen 3, or any of my previous hatched cars. If you check. you'll find there's a considerable clearance in the area you refer to. You're confusing how a car structure behaves under impact and how it behaves normally.

Your reference to the Nissan 350Z is irrelevant. It's a 300bhp rear wheel drive designed for performance, not boot capacity.

"A saloon does that by default" Not true. There's absolutely nothing to say a saloon will be stiffer than a hatch by default, more likely the opposite without stiffening, as hanging a boot on the back extends the length of the overhang, resulting in a softer rear end.

So why does the Nissan have a big beam acorss the boot, significantly reducing its capacity?  Styling?  Surely all those 'brilliant' Nissan engineers (who designed the fantastic handling Skyline, a saloon) could find a way to make it stiff enough with a such a large hatch opening without it so that the handling of the car wouldn't be affected?

As regards the 'contact' between the surfaces, you know very well I'm referring to permantly contact surfaces, e.g. welded or suchlike, not the modicum provided by the hatch door when it's shut.  If you think you're right and that Mazda's hatch is equally as stiff as the saloons, go and ask them and report back (with credible evidence).  Frankly you appear to be taking offence (e.g. that your 3 hatch isn't as good handling as my saloon [if yours is a mk2, then its probably better than my mk1 in that regard]) where there never was any.

Offline StevenRB45

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Re: Fastback vs Hatchback - tyre noise level
« Reply #10 on: August 05, 2018, 04:41:34 PM »


From the Gen 2 brochure.

The red strengthening under the bonnet is present on my hatch..clearly the piece at the back of the cabin isn't.

Theoretically it would make a difference to torsional strength similar to a rear strut brace assuming the floor pan is identical.


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Re: Fastback vs Hatchback - tyre noise level
« Reply #11 on: August 06, 2018, 01:02:02 AM »
"So why does the Nissan have a big beam acorss the boot"

I suggest you re-read what I wrote "Your reference to the Nissan 350Z is irrelevant". The reason it's irrelevant is because we're discussing the Mazda 3. The Nissan 350Z is an entirely different car so has no relevance to a discussion about the structural differences between a Mazda3 hatch and fastback. You'd be as well asking why F1 cars have DRS.

"As regards the 'contact' between the surfaces, you know very well I'm referring to permantly contact surfaces"

I certainly don't. I'll remind you what you said. "....it relies on contact along between the boot lid and the rest of the car to add some structural stiffness." It seems pretty clear to me what you're saying.

"If you think you're right and that Mazda's hatch is equally as stiff as the saloons, go and ask them and report back (with credible evidence)."

You made the statement "Like the gen1 and 2 Mazda3s, the gen3 (its a saloon even if its called a fastback) has a stiffer bodyshell than the hatchback because of the way the boot/back window are designed/constructed.". I asked you for proof, and I'm still waiting.

"Frankly you appear to be taking offence (e.g. that your 3 hatch isn't as good handling as my saloon [if yours is a mk2, then its probably better than my mk1 in that regard]) where there never was any."

Nothing could be further from the truth. Firstly, I don't think anybody would bother to attempt to argue that my Gen 3 hatch doesn't handle better than your Gen 1 saloon. If it didn't, the Mazda design team would be looking for other jobs. Whether my Gen1 hatch handles bettter than your Gen1 saloon doesn't bother me either. I don't drive like a moron trying to find the limit of adhesion, so I'm unlikely to find out.

My interest is purely in establishing whether you have anything factual to back up the statement you made or not. Call it an engineer's curiosity, nothing more.

Finally, going back to "So why does the Nissan have a big beam acorss the boot, significantly reducing its capacity?"

Nowhere in the design brief of a car like the Nissan 350Z is there a call for it to be able to carry home the weekly shop from Tesco (other supermarkets are available). The "capacity" hasn't been reduced. The "capacity" was never there. It's a bit of available space within the structure that has been utilised as a second glove box - you could never call it a boot. Nissan don't even quote a capacity for the space, just approximate dimensions.
The structure has been specifically designed around a relatively small, fat wheeled, 300bhp rear wheel drive sports car. Given all that, a clunky beam is the cheapest and least subtle way of stiffening the body. Given the limited space and surface area in the body shell, any other solution would increase the cost of the car dramatically.
« Last Edit: August 06, 2018, 01:33:50 AM by moozmooz »

Offline moozmooz

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Re: Fastback vs Hatchback - tyre noise level
« Reply #12 on: August 06, 2018, 01:31:19 AM »
"Hidden strengths:...…………...is standard on all Mazda 3 models."

"Theoretically it would make a difference to torsional strength similar to a rear strut brace assuming the floor pan is identical."

Strut braces don't contribute a lot if anything to body torsional strength. All they do is ensure that the tops of the struts are tied, so remain equidistant, so that the "box" with corners at the top and bottom of the struts looking from the front of the car remains a trapezium, at all times. Without a brace, that same box is free to become a quadrilateral - the tops of the struts moving independently of one another.
Any increase in the ability to resist a torsional load will be limited to the suspension mounting area. The torsional load will be dissipated elsewhere in the car body, most likely in the cabin section, but will vary dependent on specifics of the car design.

Many after market strut braces are little more than posing bars - a furry dice for under the bonnet - and would likely collapse if subjected to a compressive load.

EDIT. The dangers of multitasking in the early hours. I've corrected my geometry, but on reflection, it's possibly clearer to think of the suspension box as an exhaust clamp, with the suspension components as the U bolt and the strut brace as the tie plate. Without the tie plate, you can wiggle the ends of the u bolt.
« Last Edit: August 06, 2018, 10:27:14 AM by moozmooz »

Offline Willpower

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Re: Fastback vs Hatchback - tyre noise level
« Reply #13 on: August 06, 2018, 08:45:27 AM »
Whilst the flow of this thread may be interesting to many people, I think we are straying far away from the original question posed by the OP which was not concerning structural strength.

I'm sure that such a discussion could continue in the Bodywork section of the forum if needs be.



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Re: Fastback vs Hatchback - tyre noise level
« Reply #13 on: August 06, 2018, 08:45:27 AM »