Why have a transmission tunnel in FWD cars vs a flat floor at the rear?

Klub Class · 3rd June 2011, 23:57

Please excuse me for this

post!

Quote:

Originally Posted by dhanushs

huh?. where does SX4 have and AWD?.

The Suzuki SX4 hatchback sold elsewhere is equipped with 'intelligent All Wheel Drive' (iAWD).

DicKy · 4th June 2011, 10:09

For the hatch/crossover version in the international markets if am right.

dezrskb · 23rd July 2012, 00:14

Hi guys,
I was checking out cars today in 15-20L range (intend to buy within 6mths). Checked Skoda Laura today. But astonishingly the floor hump at back (/transmission hump) seems to be quite high making this car worth 21L (OTR) suitable for only 4. I found it more difficult to sit in the middle than in my Swift Dzire. I'm yet to check out VW Jetta, Renault Fluence etc which are in this range. But if its to improve rigidity and strength as somebody has mentioned in this thread, then I guess its going to be like that in all others. If so I think I will start looking for an SUV / cross overs like Yeti which seem to be having a smaller hump. Anybody can suggest sedans with smaller hump?

ajmat · 23rd July 2012, 08:38

The hump has several purposes

1: To let the exhaust, brake and fuel lines to be protected by recessing the above

2: TO improve the aerodynamics and allow smoother airflow hence better fuel consumption

3: To allow for 4wd variants

anandpadhye · 23rd July 2012, 11:08

If the central hump is intended for strengthning, why not have many smaller ones - corrugated floor like the corrugated boxes. Will do the job and save space as well.

May be they do not want to maintain separate designs for FWD and RWD models to save cost. Hats off to Honda who created a separate design for the Civic. Makes life a lot easier for the middle passenger.

TheLizardKing · 21st May 2014, 15:57

Quote:

Originally Posted by ajmat

2: TO improve the aerodynamics and allow smoother airflow hence better fuel consumption

I know this is an old thread (I got directed here from a newer thread), but I was very interested and read it through. I have a Polo, and have always wondered about the hump.

Regarding your point no. 2, how can a hump INSIDE the car contribute to aerodynamics? Surely you are not talking about the airflow UNDER the car?

Zappo · 21st May 2014, 16:42

Quote:

Originally Posted by TheLizardKing

Regarding your point no. 2, how can a hump INSIDE the car contribute to aerodynamics? Surely you are not talking about the airflow UNDER the car?

Welll... actually he is!

TheLizardKing · 21st May 2014, 16:49

Quote:

Originally Posted by Zappo

Welll... actually he is!

Really?

Wow. Would the improvement in aerodynamics really matter in a city car? I am going to read up a bit more on this. Will update the thread if I find some useful information to share.

rav11stars · 8th January 2015, 12:01

I did what I'm best at. Goooogling

The posts that I read reiterated what Rehaan said. It's left to reinforce the strength of the floorpan. I think it's okay posting the references from which I gathered this info.

http://myfiatworld.com/showthread.php?t=839
"The transmission tunnel serves dual purpose - 1] The central hump adds tremendous stiffening of the floor pan in a monocoque chassis and that helps in the chassis dynamics; and 2] the fuel lines, exhaust, brake lines and other wiring are routed through the transmission tunnel, thereby helping in the safety quotient of these plumbings, wires, etc and also helps maintaining appropriate ground clearance of the car.

sit in the back of a Civic and you can actually feel the rear of the car sway from side to side even while going in a straight line! That is not to say that this might be the only reason for the Civic's flawed dynamics, but maybe the lack of a transmission tunnel does have a role to play here."

Found the below one to be agreeable and sensible.

http://www.honestjohn.co.uk/forum/po...ex.htm?t=59300
"Longitudinally, a transmission tunnel acts as a crush tube, which can initially be rigid and act as a safety cell, but once deformed absorb significant amounts of crash energy.

In a side impact situation, a transmission tunnel offers somewhere for the impacted seat to move into, away from the crash site.

In bending of the floorpan, the tunnel effectively replaces one large plate like structure with two half sized ones - this raises the first natural frequency of the floorpan significantly. As bending vibrations are the ones which couple most effectively with sound, this is also a good thing for NVH.

Going further, a stiff floorpan also reduces the problem of scuttle shake, and contributes to overall body stiffness. Where the ends of the transmission tunnel are effectivey built in to the remaining structure, there is an enhancement of torsional rigidity - if the ends of the transmission tunnel terminate in unsupported panels, there's no torsional rigidity gain."

Some nice pics of flat floorpans here.
http://oppositelock.jalopnik.com/why...els-1593184088

batterylow · 8th January 2015, 13:49

Found this simple video on the benefits of a corrugated sheet.

http://www.youtube.com/watch?v=M7QpukKWsOs

saket77 · 8th January 2015, 16:10

The discussion so far has made us believe in 2 things, which seem very logical:

1. The hump is used as a safe passage for the exhaust lines/ brake linings and other such linkages.

2. Also, the hump makes the floor pan more rigid.

However, many cars, noticeably like the Honda Civic has been sold with a flat rear floor pan. Any ideas about the negatives of such arrangement? I am sure that Honda engineers would not mess up safety for a global model like Civic just for the purpose of attaining a flat floor. So, how do some cars achieve a flat floor without messing with things such as safety, dynamics, and design?

ManojVM · 22nd April 2015, 15:18

Many cars come in both all wheel drive as well as front wheel drive versions. Since the all wheel drive version needs a transmission tunnel, I guess it would be more economical to use the same body on the front wheel drive version as well.

Normally the all wheel drive versions are the more premium ones. I think by having no transmission tunnel in the lower , FWD models, it would discourage customers from buying the higher end ones.

GTO · 8th May 2015, 09:15

Quote:

Originally Posted by ManojVM

Normally the all wheel drive versions are the more premium ones. I think by having no transmission tunnel in the lower , FWD models, it would discourage customers from buying the higher end ones.

I don't know of any car sold in India that has a floor hump only for the AWD version. Removing the hump for the lower FWD variant requires structural changes to the floorpan. No manufacturer would be willing to do this.

Also note that many FWD-only cars have a floor hump too. The floor hump can be used to route exhaust & brake plumbing, and it also helps to make the floor more rigid. Related post.

girimajiananth · 8th May 2015, 12:40

Quoting a reply from a friend

Quote:

The "transmission hump" in a FWD car is used to route only the exhaust. Brake and fuel lines need to be kept away from the heat of the exhaust. A quick peek under the car can confirm the same. Brake lines might be exposed on the underside, but fuel line would get more protection.

Why have a transmission tunnel in FWD cars vs a flat floor at the rear?-1.jpg

Khushrav · 17th March 2017, 10:45

Through various forums and multiple reviews, we have been acquainted with the dreaded hump in the back seat which eats up the leg space for the 5th passenger substantially. While some attribute it to the transmission tunnel, others to the HVAC and mechanical architecture one theme is consistent. The hump is very pronounced on all European cars.

I'm not able to understand if it has any reasons like better safety, better underbody protection or some EURO regulations etc. But if almost all Japanese and Indian cars can do without it, why does it become a mandatory sore feature in all other cars?

I'm sure it is not a lack of engineering prowess with the Europeans. So then why cant they get this right? Is it a case of low priority due to smaller sales in Asia or something else?

4th June 2011, 10:09	#32
DicKy Distinguished - BHPian Join Date: Jan 2010 Location: TVPM Posts: 3,827 Thanked: 11,788 Times	re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear? For the hatch/crossover version in the international markets if am right.
	() Thanks

23rd July 2012, 00:14	#33
dezrskb BHPian Join Date: Jul 2011 Location: Udupi Posts: 232 Thanked: 140 Times	Re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear? Hi guys, I was checking out cars today in 15-20L range (intend to buy within 6mths). Checked Skoda Laura today. But astonishingly the floor hump at back (/transmission hump) seems to be quite high making this car worth 21L (OTR) suitable for only 4. I found it more difficult to sit in the middle than in my Swift Dzire. I'm yet to check out VW Jetta, Renault Fluence etc which are in this range. But if its to improve rigidity and strength as somebody has mentioned in this thread, then I guess its going to be like that in all others. If so I think I will start looking for an SUV / cross overs like Yeti which seem to be having a smaller hump. Anybody can suggest sedans with smaller hump?
	() Thanks

23rd July 2012, 08:38	#34
ajmat Team-BHP Support Join Date: Feb 2004 Location: Bangalore Posts: 14,862 Thanked: 27,966 Times View My Garage	Re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear? The hump has several purposes 1: To let the exhaust, brake and fuel lines to be protected by recessing the above 2: TO improve the aerodynamics and allow smoother airflow hence better fuel consumption 3: To allow for 4wd variants
	() Thanks

23rd July 2012, 11:08	#35
anandpadhye Distinguished - BHPian Join Date: Apr 2007 Location: Pune Posts: 3,054 Thanked: 3,313 Times	Re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear? If the central hump is intended for strengthning, why not have many smaller ones - corrugated floor like the corrugated boxes. Will do the job and save space as well. May be they do not want to maintain separate designs for FWD and RWD models to save cost. Hats off to Honda who created a separate design for the Civic. Makes life a lot easier for the middle passenger. Last edited by anandpadhye : 23rd July 2012 at 11:26.
	(1) Thanks

8th January 2015, 12:01	#39
rav11stars BHPian Join Date: Sep 2014 Location: RJY->PUN->DXB Posts: 356 Thanked: 824 Times View My Garage	Re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear? I did what I'm best at. Goooogling The posts that I read reiterated what Rehaan said. It's left to reinforce the strength of the floorpan. I think it's okay posting the references from which I gathered this info. http://myfiatworld.com/showthread.php?t=839 "The transmission tunnel serves dual purpose - 1] The central hump adds tremendous stiffening of the floor pan in a monocoque chassis and that helps in the chassis dynamics; and 2] the fuel lines, exhaust, brake lines and other wiring are routed through the transmission tunnel, thereby helping in the safety quotient of these plumbings, wires, etc and also helps maintaining appropriate ground clearance of the car. sit in the back of a Civic and you can actually feel the rear of the car sway from side to side even while going in a straight line! That is not to say that this might be the only reason for the Civic's flawed dynamics, but maybe the lack of a transmission tunnel does have a role to play here." Found the below one to be agreeable and sensible. http://www.honestjohn.co.uk/forum/po...ex.htm?t=59300 "Longitudinally, a transmission tunnel acts as a crush tube, which can initially be rigid and act as a safety cell, but once deformed absorb significant amounts of crash energy. In a side impact situation, a transmission tunnel offers somewhere for the impacted seat to move into, away from the crash site. In bending of the floorpan, the tunnel effectively replaces one large plate like structure with two half sized ones - this raises the first natural frequency of the floorpan significantly. As bending vibrations are the ones which couple most effectively with sound, this is also a good thing for NVH. Going further, a stiff floorpan also reduces the problem of scuttle shake, and contributes to overall body stiffness. Where the ends of the transmission tunnel are effectivey built in to the remaining structure, there is an enhancement of torsional rigidity - if the ends of the transmission tunnel terminate in unsupported panels, there's no torsional rigidity gain." Some nice pics of flat floorpans here. http://oppositelock.jalopnik.com/why...els-1593184088
	(2) Thanks

8th January 2015, 16:10	#41
saket77 Distinguished - BHPian Join Date: Dec 2012 Location: Ranchi Posts: 4,395 Thanked: 12,023 Times	Re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear? The discussion so far has made us believe in 2 things, which seem very logical: 1. The hump is used as a safe passage for the exhaust lines/ brake linings and other such linkages. 2. Also, the hump makes the floor pan more rigid. However, many cars, noticeably like the Honda Civic has been sold with a flat rear floor pan. Any ideas about the negatives of such arrangement? I am sure that Honda engineers would not mess up safety for a global model like Civic just for the purpose of attaining a flat floor. So, how do some cars achieve a flat floor without messing with things such as safety, dynamics, and design?
	() Thanks

22nd April 2015, 15:18	#42
ManojVM Newbie Join Date: Apr 2015 Location: Bangalore Posts: 8 Thanked: 6 Times	Re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear? Many cars come in both all wheel drive as well as front wheel drive versions. Since the all wheel drive version needs a transmission tunnel, I guess it would be more economical to use the same body on the front wheel drive version as well. Normally the all wheel drive versions are the more premium ones. I think by having no transmission tunnel in the lower , FWD models, it would discourage customers from buying the higher end ones.
	() Thanks

17th March 2017, 10:45	#45
Khushrav BHPian Join Date: Sep 2014 Location: Mumbai Posts: 98 Thanked: 160 Times	There's a Hump in my Car Through various forums and multiple reviews, we have been acquainted with the dreaded hump in the back seat which eats up the leg space for the 5th passenger substantially. While some attribute it to the transmission tunnel, others to the HVAC and mechanical architecture one theme is consistent. The hump is very pronounced on all European cars. I'm not able to understand if it has any reasons like better safety, better underbody protection or some EURO regulations etc. But if almost all Japanese and Indian cars can do without it, why does it become a mandatory sore feature in all other cars? I'm sure it is not a lack of engineering prowess with the Europeans. So then why cant they get this right? Is it a case of low priority due to smaller sales in Asia or something else?
	(1) Thanks