Last post on Dec 20, 2008 at 8:51 AM
You are in the Mazda Mazda5
What is this discussion about?
Mazda MAZDA5, Van
#9 of 12 Re: Roof rack and kayaks is not a weight issue [watkinst]
Dec 19, 2008 (6:32 pm)
Well, good summary, but still the question is about the Mazda5 being underpowered, so everything counts and has an impact, including the weight, especially as the drag coefficient with the kayaks will increase *almost* equally regardless of the vehicle.
Crazy example, but I'm sure driving a 4dr Toyota Echo will feel worse and with less power (especially with crosswinds) than a Mazda5 with exactly the same kayaks on the roof, so weight/engine power should still be relevant
My 2 cents
#10 of 12 Re: Roof rack and kayaks is not a weight issue
Dec 19, 2008 (8:31 pm)
So here are the quick back of the envelope numbers:
... To drive 75 MPH with two kayaks, you'd be using about 60% of the available engine power, on flat roads with no wind.
... On a flat road, you could drive 75 MPH with two kayaks in to ~15 MPH head wind, before running out of engine power in 5th gear.
... With no wind, you could drive 75 MPH up a ~3% slope, with 600 lbs + the car's weight, before running out of engine power in 5th gear.
The Mazda5 has a frontal area that is less than 2.86 m^2.
At 75 mph, less than 27 HP is needed to maintain speed.
Two kayaks would be on the order of 0.5 m^2.
They'll be pretty aerodynamic, but assume they're not with a Cd of 0.50.
At 75 mph, an additional 8 HP would be needed.
So 2 kayaks + a Mazda 5 need roughly 35 HP to maintain 75 mph.
Assume that the drive train is 80% efficient.
You then need 44 "engine" HP to maintain 75 mph.
Off the top of my head, I do not remember the exact RPM at 75 mph in 5th, and it does vary from auto to stick, but it's in the vicinity of 3100 RPM.
Peak torque is 143 lb ft at 4500 RPM, but torque curves are pretty flat. Assume 85% of that at 3100 RPM, or 122 lb ft.
Available (full throttle) torque of 122 lb ft 3100 RPM is somewhere around 72 HP.
*** all numbers approximate, and a sea level
#11 of 12 Re: Roof rack and kayaks is not a weight issue [coolmazda5]
Dec 19, 2008 (8:38 pm)
Thank you watkinst. That is the answer I was really looking for. It is not the weight of the kayaks that are a concern. Combined they weight slightly more than 100 pounds. That is not a great deal of weight. It is more the drag than the weight that concerns me. Another concern is cross winds. I would still appreciate it if anyone has any real experience with kayaks on top of their Mazda 5, especially so if you have the kayaks in "saddle roof racks" and could tell me how it has affected the handling and power of this vehicle.
I'm pretty sure we are going to get this vehicle no matter what the answer, as hauling kayaks is not the only or even the major thing we will use this vehicle for. I do like the Mazda 5, I would just like to know what we are in for if we hit a cross wind or even a head wind with a couple of kayaks on top. I think your answer watkinst came closest to the information I was looking for, but I do thank all for the replies.
#12 of 12 Re: Roof rack and kayaks is not a weight issue [Quickbeam]
Dec 20, 2008 (8:51 am)
I'm pretty sure we are going to get this vehicle no matter what the answer
Anyway, in summary, it will haul you around safely and it won't flip