How much would roof raise affect MPG?

belfert · October 12, 2009, 09:15:07 AM

I'm thinking about doing a roof raise of 9" to 10" on my Dina. Any ideas how many MPG I would lose? The frontal area would go up by close to 10% although the front would be tapered. Seeing the roof raise Ruthi did on their Dina has inspired me to parhaps do one of my own.

My interior is pretty basic so it wouldn't be a big deal to take everything out to do the roof raise. The biggest issue in my mind would be reskinning the sides.

Jeremy · October 12, 2009, 10:37:05 AM

I suspect the answer is 'not enough to worry about, given our mileages'. But it is nontheless an interesting question. I believe you are right to think that frontal area is the biggest issue, but I'm interested myself to know how importance 'shape' is. The picture below is of the latest Plaxton - note both the roof shape and also the 'spoiler' at the back. Is this just styling, or would they have gone to this much trouble if there weren't real fuel savings to be had? I suspect the latter as a lot of truck trailers here are this shape too.

Then there is turbulence. Apologies to Sean for posting a pic below from his website. Having this much equipment and miscellaneous bracketry on the roof is probably fairly typical for bus conversions, yet must be worth many, many gallons of diesel over the years.

Jeremy

PS. Brian: How did your various new systems work out? For instance, how did you end up wiring that amplifier?

Dreamscape · October 12, 2009, 11:47:18 AM

Jeremy,

Where was the last picture taken? Sure are a bunch of Eagles in it.

Paul

Jeremy · October 12, 2009, 11:55:22 AM

Quote from: Dreamscape on October 12, 2009, 11:47:18 AM
Jeremy,

Where was the last picture taken? Sure are a bunch of Eagles in it.

Paul

Bus'n' USA 2004, apparently. Here's the page:

http://odyssey.smugmug.com/Travel/Busn-USA-2004/173090_aFeSV#6513999_gbuyE

Jeremy

John316 · October 12, 2009, 12:04:27 PM

The roof raise wasn't anything compared to what we got. We needed that extra space, and took the extra fuel in one big gulp (that keeps on going). Right now we are getting about 7 MPG (6 if we are running the generator and Oasis).

On the re panning, I don't know. We had Star Jet do ours, and they did a terrible job! We will never use them again. We could have done a better job. I would try doing it yourself. The paint job is another issue.

God bless,

John

BG6 · October 12, 2009, 12:19:03 PM

Quote from: Jeremy on October 12, 2009, 10:37:05 AMThe picture below is of the latest Plaxton - note both the roof shape and also the 'spoiler' at the back. Is this just styling, or would they have gone to this much trouble if there weren't real fuel savings to be had? I suspect the latter as a lot of truck trailers here are this shape too.

They are attempting to get a laminar flow of air from the front over the coach. At the speeds coaches go, this isn't going to make a lot of difference, and then only when there is no crosswind.

One thing that ANYONE can do to boost mileage, keep the back of the coach cleaner, and reduce wind-sway at highway speed is to install vortex generators at the back of the coach, to break up the vacuum across the rear (which Plaxton is trying to do with that spoiler). Do a web search for the companies which make them for semitrailers, look at what they make, then you can easily see how to make them yourself. This is a project I'm planning for the next weekend that I have nothing else going on.

UPDATE: Check http://www.buyairtab.com/ and you will see VGs for $2.75 each, and complete kits for a little over $200. You're not going to make them for any less than that. If you are putting on many miles, you are certain to pay them off pretty quickly.

belfert · October 12, 2009, 12:24:45 PM

I did the original skinning myself and would do it again myself. It is just a huge pain to drill the 1000+ holes and install the rivets. It cost me about $800 last time, half for materials and the other half for food and labor. I hired a high school kid for $10 an hour to help drill holes.

I've seen some skinning jobs done by "professionals" that look like crap. No skinning job should use pop rivets and be full of waves. My job wasn't 100%, but it was at least flat with no waves or ripples.

Based on my usage, I probably wouldn't spend more than $200 to $300 a year additional on fuel if my MPG dropped by .5 MPG.

John316 · October 12, 2009, 12:35:23 PM

I would say go ahead and do it. You can do it, and do it well. I don't think that you will regret it (we sure have loved our roof raise), but then again I could be wrong.

God bless,

John

ruthi · October 12, 2009, 12:51:33 PM

Hey Brian, I think the roof raise is going to be worth it, even if it means a slight increase in fuel. We have the fiberglass work done in front and back, and it turned out really well. We didnt use pop rivets to do the skin. Instead, the skin was put on with sikaflex, dont know that I spelled it right. One bead, one inch long will hold 600 lbs of strength. I know I didnt phrase that right. It is all smooth. We had a guy there where we live that did it for us. It isnt cheap stuff, but, saved a lot of work. It took 12 tubes of it. MCI uses that to put skin on with. It is almost ready for paint. We are getting all the body ready now. We will be home middle of Nov. and I will send some pics of it. Meantime, if you have questions, give us a call. Ruthi

ruthi · October 12, 2009, 01:21:48 PM

Brian, trying to send a pic of the bus. It keeps saying file is too large. I dont know what to do to post it.

John316 · October 12, 2009, 06:04:57 PM

Quote from: ruthi on October 12, 2009, 01:21:48 PM
Brian, trying to send a pic of the bus. It keeps saying file is too large. I dont know what to do to post it.

Put your head together with Dreamscape (Paul). He is the photo wizard on here. I want to see the pics too!!!

God bless,

John

belfert · October 12, 2009, 08:37:29 PM

Ruthi has posted pics of the bus in the past. You might find them in the archives.

Ruthi, I am going to PM you about maybe flying out to look at your Dina in early December. I want to see how the roof raise was done.

John316 · October 13, 2009, 05:05:28 AM

Eight pages into Ruthi's posts, I found them. http://www.busconversions.com/bbs/index.php?topic=10766.msg111168#msg111168

God bless,

John

rv_safetyman · October 13, 2009, 09:44:32 AM

Hi Brian. The engineering calculations for the force applied to the front of a vehicle by the air movement is pretty straight forward. There are four factors: drag coefficient (how smooth the shape is), frontal area, air density, and speed **squared**.

Your roof raise would affect the frontal area and the drag coefficient (to a very minor degree). The fact that you are changing the frontal area by 10% would suggest that you will increase the force required to move the air at a given speed by 10%.

What is not clear is the impact of the force required on MPG

There is a pretty good article at: http://www.edmunds.com/advice/fueleconomy/articles/106954/article.html

Consider a few things: 1) I would guess that the power required to move the vehicle down a level highway is probably ranges from a 40/60 to a 60/40 balance of the affects of weight and rolling resistance as compared to overcoming resistance to moving the air (lots of big variables in that balance). 2) We don't have level highways very often (always seem to be up-hill

) and the affect of weight would change that balance considerably. 3) The affect of speed has significantly more impact on the air drag than your roof raise.

Just some thoughts.

Jim

ruthi · October 13, 2009, 10:48:05 AM

Brian, just let us know, we will have something for you to do, lol

Hopefully, we will have a lot more work done on it. We just ordered the basement air units. I would love to go to Elkhart to look, but time is NOT on our side, if you know what I mean. We really hope to have it usable by January. We will see. We are debating whether to sell the bus we are using now, cheap, or just use what we want off of it and get rid of the rest. Yep, those are the pics before the fiberglass work was done. I wanted to show the after. The before pics were took with my old camera, and the settings were for small pics on it. It wasnt set that way on the new camera, so, cant figure out the process to make them smaller in i-photo on mac.