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Old 23rd May 2011, 10:37   #1
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Default Why have a transmission tunnel in FWD cars vs a flat floor at the rear?

Some cars have flat flooring (no hump) or some have it huge like Vento.

Are there any design considerations, pros and cons for car designer ? Because from buyer perspective, flat is best...so its surprising why all the new cars don't come with flat floor.
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Old 23rd May 2011, 11:31   #2
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Default re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear?

It usually accomodates the propellor shaft for front engined and rear wheel driven vehicles and in other cars it is used for exhaust routing. So for most of the modern cars (front egined and front wheel driven), if they can plan the exhaust routing to be least intrusive into the rear cabin floor, we would get a flat floor. May be underside pictures of cars with flat rear cabin floor will give an insight into this.
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Old 23rd May 2011, 11:54   #3
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Default re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear?

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Originally Posted by Revention View Post
Some cars have flat flooring (no hump) .
Civic is one car which has a flat floor at the rear.
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Old 23rd May 2011, 12:19   #4
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Default re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear?

I was more curious about the fact that though its possible to have flat floor now the newer cars like Vento still did not opt for it ...
So is it case of some trade off in design : safety vs flat floor, ride quality vs flat foor or something like that......
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Old 23rd May 2011, 13:00   #5
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Default re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear?

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Originally Posted by Revention View Post
....
So is it case of some trade off in design : safety vs flat floor, ride quality vs flat foor or something like that......
May be in Indian conditions, Volkswagen designers found that between Ground Clearance Vs Flat Floor, GC had to be given priority. Sometimes sales are put off just because of low GC, and little because of not having a flat floor.
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Old 23rd May 2011, 13:48   #6
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Default re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear?

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Originally Posted by dhanushs View Post
May be in Indian conditions, Volkswagen designers found that between Ground Clearance Vs Flat Floor, GC had to be given priority. Sometimes sales are put off just because of low GC, and little because of not having a flat floor.
That makes perfect sense.
But do all cars with flat or almost flat rear floors have GC issues ?
I think Hondas have, but Etios doesn't have.
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Old 23rd May 2011, 14:12   #7
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Default re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear?

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Originally Posted by pjbiju View Post
... in other cars it is used for exhaust routing...
... and, more importantly, for safe routing of the brake lines.
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Old 24th May 2011, 00:02   #8
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Default re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear?

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... and, more importantly, for safe routing of the brake lines.
Just curious, the rear brakes are supposed to be drum type in most cars and connected to the hand brake only. correct?

OT: when we use the brake pedal does it activate the Front disk brakes only or is it front disk + rear drum brake together?

And if it's just the exhaust routing and brake supply lines then why do they need such a big hump. There has to be something else too.

What about tall boys like Wagon-R / Santro / Ritz. They should have smaller humps compared to low stance cars. do they?
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Old 24th May 2011, 00:26   #9
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Default

Quote:
Originally Posted by Chewbacca

Just curious, the rear brakes are supposed to be drum type in most cars and connected to the hand brake only. correct?

OT: when we use the brake pedal does it activate the Front disk brakes only or is it front disk + rear drum brake together?

And if it's just the exhaust routing and brake supply lines then why do they need such a big hump. There has to be something else too.

What about tall boys like Wagon-R / Santro / Ritz. They should have smaller humps compared to low stance cars. do they?
When you brake both front and rear brakes applied in a car, no matter if they are drum or disk.
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Old 24th May 2011, 00:46   #10
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Default re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear?

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Originally Posted by Chewbacca View Post
What about tall boys like Wagon-R / Santro / Ritz. They should have smaller humps compared to low stance cars. do they?
What does stance have to do with the hump?
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Old 24th May 2011, 01:11   #11
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Default re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear?

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Originally Posted by dhanushs View Post
What does stance have to do with the hump?
Tall cars have more usable vertical space allowing designers to raise the floor height by 1 or 2 inches and then spread out the exhaust / brake lines laterally to reduce the hump. The middle seat passenger will be much more comfy and the car will sell as a true 5-seater.
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Old 24th May 2011, 10:23   #12
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Default re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear?

Toyota Corolla Altis also had a flat floor (almost) in the rear. I saw this in 2009 when I went in toyota innova. The sales person informed me that it was kept it this way to ensure better comfort for persons seated in the rear and the propeller shaft was shifted slight down to accomodate flat floor in the rear. I didnt see any ground clearance problem in corolla altis as a result of this.
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Old 24th May 2011, 10:40   #13
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Default re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear?

Apart from the reasons cited above, another reason for not providing flat floors is body stiffness. The bump/kink helps make the body more rigid.
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Old 26th May 2011, 11:12   #14
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Default re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear?

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Originally Posted by sgiitk View Post
Apart from the reasons cited above, another reason for not providing flat floors is body stiffness. The bump/kink helps make the body more rigid.
Perfect! To understand what sgiitk meant, try out this simple exercise.

Name:  FOLD.jpg
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Old 26th May 2011, 11:29   #15
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Default re: Why have a transmission tunnel in FWD cars vs a flat floor at the rear?

Quote:
Originally Posted by sgiitk View Post
Apart from the reasons cited above, another reason for not providing flat floors is body stiffness. The bump/kink helps make the body more rigid.
That's quite nice - thanks professor
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