Jack & Danielle Mayer

Choosing and Modifying Your Class 8 Tractor

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In this Section:
  • Which Class 8 to Convert
  • Deciphering Volvo Models
  • Our Analysis
  • Our Truck
  • The Engine
  • The Air System
  • Singling-out Your Truck
  • Solving Vibration Problems
  • First Things to Do To Your New Truck
  • Salesmen

This section discusses our truck, and the factors we considered when selecting it from the many available trucks. Although we ended up buying our truck through Larry Zeigler, we considered all our options before accepting one of the trucks Larry acquired. It took us over a year to find our truck, and we spent a year before that researching our options. We were pretty particular, and as it turned out, some of the things we thought we wanted on the truck we overlooked when we bought from Larry. During this process Larry offered free and willing advice to us. Over three years later I still find his advice was “right on” in every respect. Some of the tradeoff’s you have to make are personal choices/preferences; such as wheelbase. Only YOU can decide on these, but you need to understand your present and future requirements well in order to make an informed decision. Your truck will last a long time, so choose accordingly. Near the end of this section is a link to salesmen you might consider, and some other helpful info. Also, check the Resource Listing section for various sources of parts, places to have your truck painted, etc.  Obviously, much of the info that follows is our opinion. I try to present the logic we used to reach our conclusions. Your preferences may differ.

Which Class 8 to Convert

We obviously selected a Volvo, but that does not mean other class 8’s are not as good. As with RV’s there are tradeoffs in every model. Personally, we wanted an aero sleeper, but others find a sleeper to be too big and a waste of space. Some buyers convert the sleeper to passenger seating. This is certainly doable. You add windows to the back and sides if your truck does not have them (try http://www.peninsulaglass.com/ for a great selection of high-quality RV-style windows). You can also add a jackknife couch in place of the bed (and still retain the storage compartments). If you want to be able to drop the couch into a bed then you will have to remove the storage cabinets in most trucks (definitely in a Volvo 610/660). We chose to leave our interior intact since we sometimes use the Volvo for short overnight trips instead of hauling the 5er. We also like having a fridge/microwave in the truck, which would be difficult in a Volvo 610 if you added a jackknife couch. You can see some pictures of Volvo's with windows and couches added here: http://www.powerhousecoach.com/. Look in the "Toters" section.

There are 2 primary tradeoffs that will drive all other choices when considering which tractor to convert. These are length of cab, and height of cab. Full height tractors are easy to find (full-height tractors are about 13’ tall) and much easier to find with an autoshift than mid-height tractors like a Volvo 610. Mid height tractors have the advantage of being easier to drive off the interstate. There is less tendency to hit tree limbs, or to have to deviate because of low overpasses. There is only one popular mid-height aero tractor and that is the Volvo 610. Other manufacturers make mid height units but they are much harder to find. (You can find more mid-height conventional tractors - those with the long nose - but we wanted an aero sleeper.) Full height tractors do have additional storage in the extra vertical space. Most of the full-height tractors are also longer than the mid height tractors.

The length of the tractor (wheelbase and cab length) is probably the most critical measure. Your wheelbase will be driven by the BBC (Bumper to Back of Cab) measurement. Basically, longer sleepers have longer BBCs. So the Volvo 610, with its 61-inch sleeper, will be much shorter to the back of the cab than a Volvo 770, with its 77-inch sleeper. The longer the BBC is, the longer the minimum wheelbase. So, you need to decide how long of a wheelbase you can tolerate. Anything over around 195” is going to be significantly harder to drive around town and park than shorter ones. I’ve kind of chosen 195” arbitrarily, but it is based on my experience driving our truck (182”) and longer wheelbase MDTs. Of course, you really need to consider what kind of deck cargo you will be carrying. If you don’t intend to carry anything on the deck, then stay on the short side. Our 182” will carry a single motorcycle, but (probably) not in a loader. On a Volvo, 182” is about as short as you can go. If you look in the section describing our body you will find the spreadsheet we used to play with different wheelbases and body layouts. It has a truck body calculator for Volvo 610, 770 and Kenworth T2000. This should help you decide what wb truck to buy.

What you give up with the shorter cab trucks is the nice large sleeper. The Volvo 610 is certainly livable, but a Volvo 770 has a lot of extra room and storage, as does the T2000, with its 75” condo. As an example of the length tradeoffs, consider a T2000 compared to our 182” wheelbase Volvo 610. Our truck body is 9’ long (behind the vertical cab fairing). There is a box in between the cab fairings that is 14”. To put the same body on a T2000 (9’ body behind the vertical cab fairings) you end up with a 204” wheelbase, assuming you can find the 230” wheelbase tractor to start with. (Subtract 26” from the tandem wheelbase to get the single axle wheelbase.) Most T2000 wheelbases (unconverted) are in the 232-240” range. So you will have at least 22” more wheelbase. But you will have 14” more condo space, if that is important to you. Note that our deck is the shortest that is usable for carrying much of anything, so your deck might have to be longer than our 9’.
Deciphering Volvo Models 

Some history of Volvo models, and evolution can be found here: http://www.volvo.com/trucks/global/en-gb/aboutus/history/1990s/VN_and_NH.htm


Here is how Volvo model names work:

VNM = 112" BBC 'bumper to back of cab' (daycab)
VNL = 123" BBC
VHD = construction truck
VT = new muscle hood model

42T = 4x2 tractor
64T = 6x4 (tandem drive axle) tractor

1997-2001 models
420 = 42" nominal sleeper length, flatroof cab
610 = 61" sleeper, midroof cab.
660 = 61" sleeper, raised roof cab (upper side windows)
770 = 77" sleeper, great dinette, raised roof cab (upper side windows + lower side windows)


VN models with new emissions:
VNL models have new headlights, new grille, tank fairings
VNM models have old headlights but new grille

200, 300, 800 = daycabs
420 become 430 42" sleeper
610 become 630 61" sleeper
660 become 670 61" sleeper
770 become 780 77" sleeper

engines, Volvo D12 and Cummins ISX only

VT muscle hood D16 625 HP 2250 torque
880 = VT hood with 780 cab



VNL64T730 (780 sized sleeper with a mid roof 630 roofline). Supposed to be a mid year 2006 availablity. Also since mid 2005 you can get a D16 in the VNL models in several different HP/Torque ratings.

Our Analysis

Our candidates for conversion were the Volvo 610, Volvo 770, Pete 387, and Kenworth T2000. All are full height tractors with large condos, except the 610. We did not consider a Volvo 660 because it has the condo of a 610, with the height of a 770. For us, the worst characteristics of each. We also did not consider any Freightliner product. We have many friends with MDT FL's and they have nothing but problems. Danielle's friends in the heavy trucking industry recommended staying away from Freightliner unless cost was a significant factor. Your opinion may vary on this point. (Update: the newer M2 FL's are pretty nice trucks, and if I was building new I would consider them.)

We discarded the Volvo 770 and Pete 387 because of their height. They are just taller than we wanted for every day driving. They are fine on major highways, but on secondary roads, and especially in some campgrounds, they are taller than is convenient. Even with our 10’10” Volvo 610 we have to be observant. Most of our time is spent in every day driving, not while towing, so it was more important to have a tractor that was good around town.

L-to-R: Volvo 610, T2000, T2000
Middle T2 has upper fairing removed

The T2000 usually has a cab extender (airfoil) in place to match standard freight trailers. If this is removed, the height of the T2000 is 11’ 6”. Only 8” higher than a Volvo 610 (at 10’10”). That is pushing it, but acceptable in our view.

The larger T2000 has a 75” condo, and a 50” setback front axle. Its BBC is 184”. With a suitable body, the minimum wheelbase of a T2000 is in the 204-208” range (assuming you want to carry a motorcycle). Along with the longer wheelbase, the T2000 only has a 45-degree wheel cut; the Volvo’s have a 50-degree wheel cut. The combination of these two factors means that a T2000 will be a much harder vehicle to live with on a daily basis. Our limited experience with T2's also led us to the conclusion they are noisier than Volvo's, and the driving position (especially for short people like Danielle) is not as comfortable as the Volvo. The driving position does vary somewhat based on the installed seats, and the availability of telescopic steering (this is an option in T2's and standard in Volvo's). In favor of the T2000 is its very nice 75” condo, and its looks. We consider it one of the best looking tractors on the market. It is also fairly easy to find one with an autoshift transmission.

In the end, all these considerations led us to a Volvo 610 as the right tractor for us. The final deciding factor was the Volvo had a driver’s side air bag, the engine was designed to dive below the cab in an accident, and the dash was designed to be crashworthy. We would love to have a larger condo, but the 61” condo is fine, even for overnight trips. Despite the negatives of the T2, it is a fine truck, and we would consider owning one in the future. It would have to have telescopic steering, though.

An in-depth analysis that came to a different conclusion, ending with the purchase of a Volvo 770, can be found at http://www.dmbruss.com/ which is the website of Mark and Dale Bruss.

Our Truck

The Volvo we ended up with is a 1999 610. It has a Cummins ISM (M-11) with 400 hp and 1450 lb/ft of torque. One of our “must-haves” was an autoshift transmission – more about that later. We have a 182” wb, after conversion. The frame was cut at 39” behind the rubber of the rear tire (see the body-building section for our recommendations on this). We added Accuride aluminum wheels on the fronts and outer rears. We had the truck dyno’ed, DOT inspected, an overhead (tune-up) done, all fluids changed, and synthetic added to the transmission and rear. We also had Cummins do an engine inspection and bought the extended warranty that they offered (200,000 miles/2 years).

Larry did all of the above, and added a shore power hookup and new carpeting to the interior. In a moment of “weakness” he also agreed to put on tank fairings for us. He says he will never do this again. It was a lot of work and expense (of course, I did pay for it - I couldn't talk him into throwing it in).

I did not have Larry do any additional electrical work, or add any appliances. I'm very particular about electrical modifications and the “draw” of appliances, so chose to do those myself. You can see what I did in the “Electrical Center” portion of this website. I did have Larry add the shore power connector because I was picking the truck up in Kansas City and driving directly to Livingston, TX to register as a motor home. I did this without having all the appliances in. If required to go through an inspection in order to obtain the motorhome designation, I wanted to be able to go to Wal-Mart, buy the appliances, and then complete the registration process for a motor home. Having shore power already in would make this much easier if I needed to do it “on the road”. As it turns out, it was not necessary; I registered as a motorhome without any inspection. I added the appliances later.

The Engine

Our requirement for an engine was Cummins, Detroit, Cat in that order and at least 400 hp/1450 torque. Variable hp engines were fine, as long as they met these minimum requirements. As it turns out, 400 hp is sufficient to pull a heavy RV. I would buy another 400 hp engine, but I would prefer a 430-ish/1550. The extra torque would be handy sometimes in the big hills of the west.

When looking, I was more familiar with Cummins engines, so that was our preference, but at this point I would take any of the big 3. Actually, I think I prefer the Detroit Series 60 to the Cummins at this point. They have a longer median time to rebuild.

Originally, I said I would not  buy a Volvo engine. Not that I thought that there was anything wrong with them, in fact they have a good reputation; it is just that I thought they would be harder to get worked on by “experienced” mechanics than the others. I've since changed my view on this and would buy a Volvo engine, assuming the truck met my other requirements. The experience of others with the Volvo engines has been good, in the service area.

We have routine service (oil change, fuel filter, lube, etc) done at Speedco, primarily. It is easy in and out, we can run through with the 5er attached, if required, and they are consistent in price and service level. They also don’t object to you standing and watching them; in fact they insist on you watching them torque the oil drain plug. I always get an oil test done. It is relatively cheap, and lets you know if anything is starting to go in the engine or cooling system. Once a year or so, when I am in a Volvo dealer for Volvo-specific work I have them do just a lube. That way I have a better chance of getting all the lube points done. You would think a Volvo service person would know where they are (perhaps wishful thinking).

So far (after over 3 years), we have had no engine-specific problems with the truck.
A note about the "big block" engines (like the ISX/ N14 Cummins).  These engines are heavy. When used in a bobtail tractor, with a good fuel load onboard, you could overload the front axle (typically 12k lbs.). To counter this problem, Volvo, at least, moved the fuel tanks rearward on some versions of their trucks. This interferes with having a body with forward storage compartments (just forward of the axle), since there is at least one fuel tank positioned back in this area. You see this most often on 770's but it can be on 610's with the ISX/N-14 (or any "big block"). You need to be aware of this when converting to RV use - you want as much weight on the rear body as is practical to "unload" the front axle and better distribute weight. Many people add weight (up to 1500 lbs, in some cases) behind the rear axle in the form of plates. I think this is a good idea if you have a front axle close to overload.  Remember, you use the tractor "bobtail" more than hauling. If you are having a hauler bed built, then you can move  the tanks forward to regain some of the space lost to the tanks. The body will more than balance out the front axle loading.

The Air System

The air systems, and associated parts, are quite different from a regular truck. The braking system depends on a reliable, and clean, air supply. When selecting a truck you need to make sure the air system is in good working order, with no major leaks. All trucks will leak down over time, when left sitting. “Normal” behavior falls in the range of dropping air to 75psi (the alarm point) in a 24 hr period. When checking a truck for acquisition, I personally would not accept a drop of more than 5 psi (more or less) in a 2 hr period. If it is leaking down that fast you need to discover why. The leak could be anywhere, and it might be hard to find. Some of the component parts of the air system are very expensive to replace, so you need to find out why it is leaking.

On all modern trucks there are three air tanks; a “wet tank” that the air compressor supplies with filtered and dry air (you hope), and a tank and associated lines for the primary brakes (rear) and secondary brakes (front). The wet tank needs to be checked and drained on a periodic basis. It might contain water (through condensation) or a small amount of oil. You don’t want this oil and water to get into the braking system. If the filters that are on most trucks are working properly, the amount of oil/water in the wet tank will be minimal. Our truck NEVER has any moisture or oil in the wet tank. When used in commerce, the driver is required to check once a day. I generally check once a week, and have never found anything. Some trucks have automatic drains on them, but they should still be checked manually on a periodic basis.

When looking at a truck I would be suspicious of lots of water/oil, but it is in the normal range to have some. Depending on how much it has been driven once the dealer acquired it there might be significant condensation. You know the salespeople are not draining the tank.

Using air brakes is a little different than hydraulic brakes. With air brakes, there is a lag between pushing the pedal and brake take-up. This is because the air in the lines takes time to activate the brakes (about .5 second, according to CDL manuals). There is also no direct feedback through the foot pedal; what you feel is the resistance of a spring, not hydraulic modulation. Some trucks have an application air gauge, where you can see how hard you are applying the brakes. This is really only useful for long grade descents, so you can tell if your brakes are fading. It is not real usable for normal stops.

Singling-out Your Truck


Most RV haulers have one of the rear tandem axles removed. This process is called “singling”, or “singling-out” the truck. There is no real requirement for the second rear axle in an RV application. The amount of weight carried when hauling an RV is not enough to justify tandem axles. If carrying a car, in addition to pulling the 5th wheel, it is probably best to leave the tandems in place. It will give you better weight distribution front-to-rear. Otherwise you may overload the front axle, depending on the vehicle, and which axle position you choose when singling the truck.

Pro’s of Singling


  • Better Fuel mileage.
  • Less length – less parking space required.
  • Tighter turning radius.
  • Less wear and tear on parts, fewer u-joints to go bad, one less drive line.
  • Fewer brake parts to maintain and replace.
  • Fewer air lines to develop leaks.
  • Fewer air bags to go bad and need replacing.
  • More room for tool boxes built into a bed.
  • Perhaps less costly to build a hauler bed.
  • Fewer Aluminum wheels to keep polished.
  • Fewer tires to check air pressure each day.
  • Less PressurePro sensors to buy.
  • Higher resale value/easier sale as RV hauler.


Cons of Singling


  • Less weight carrying capacity.
  • Cost about $1500.
  • Aggravation of getting the driveline angles correct.
  • Potential complications with the ABS, which can be a pain to correct.
  • Can not use as commercial vehicle in most cases.
  • Less braking capacity.
  • (Maybe) less traction. Will vary based on circumstances.
  • Shorter “bobbed” tail may not look as good to some.

The cost of singling a truck will vary, based on local conditions and the market for axles and tire/wheel sets. In some cases people have traded the axle and tires/wheels for the work, with no out of pocket costs. However, this is unusual – in most cases you will have to pay to have the truck singled. Prices vary widely, but range from $800 to $2000, with the parts taken as trade. The typical price, and what you should strive for, is about $1500. For that price, you should take the best of the 8 tires and wheels – the rest will be traded for the work. Any good axle/frame shop, and some dealers, can single the truck for you. Ask at your local dealership and they can refer you.


The proper way to single the truck is to utilize the rear axle. It is the main drive axle and is designed for constant service. It is either left in place, or moved forward to replace the most forward of the tandem axles. A new drive shaft needs to be fabricated, and possibly a new carrier bearing. Sometimes a truck is singled using the front axle by just leaving it in place. The rear axle is simply removed. This requires engaging the power divider at all times, and leaving an exposed yoke at the rear. This yoke was the termination point for the rear drive axle. It is designed to be loaded, and running it unloaded can cause issues - you will most likely have early seal failure as well as other problems. The front axle is not designed to be used as the primary drive axle, it is designed to be engaged in low speed, slippery conditions, not to be run at highway speeds. At the 2006 HDT Rally the service manager, a group of mechanics and Kenny Doonan (the owner of Doonan Truck, the Wichita Peterbuilt dealer) all advised against singling the truck using the front axle. Other sources advise against it as well. Personally, I would not buy a truck that is singled this way. Other people have differing opinions, but bear in mind, this is not within the design parameters of the truck. It does work, however. An excellent write-up on correcting a "front axle singling job" is on the website of Mark Shelley - "DIYGuy". His 770 was singled by just dropping the rear axle. It caused him many problems and a lot of work and expense to correct. Figure on a minimum of $4000 to fix this - after the fact. Look here: http://mysite.verizon.net/vze1nh79/diyguyrvinfo/id52.html


The axle position chosen (forward position, or rear position) is based on your cargo carrying needs. If you need a large deck – say to carry two motorcycles – then you will probably consider the rearward position. Otherwise, to maximize maneuvering and shorten the overall length of the truck, most people choose to move the rear axle into the front axle position. It is unusual to create a new axle position that is different from the original locations. It is a difficult and expensive process to re-drill the frame for a new position, so it is not usually done.


On a Volvo the process of singling the truck is fairly straightforward. The suspension and axles are totally independent in operation. Typically, the rear axle is moved forward into the front axle position. No new holes have to be drilled the suspension and axle move intact. One major issue with all axle relocations is the driveline angle. The original driveline angles must be maintained, or you will likely have vibration problems, and potentially driveline failure – usually a u-joint. On a Volvo, there are shims on the original front axle that establish the driveline angle. These must be removed from the original front axle, and moved to the rear axle when it is moved forward. They are inserted between two saddles that are attached to the axle with U bolts. In most cases this will establish the proper driveline angle, but it must be measured to ensure it is within specifications. In some situations additional shims must be added. Make sure this is done, or you could have vibration issues or even u-joint failure in severe cases. If your truck has not had this done, and is operating OK (with no vibrations), then I would not worry about it. The most likely outcome will be early u-joint failure, so just keep an eye on things during yearly preventive maintenance inspections. My Volvo 610 is not shimmed, so I do keep an eye on the u-joints.


All trucks now come with ABS brakes. There are two brake systems commonly used: Wabco and Bendix. The Bendix three axle ABS can be reprogrammed to ignore the removed axle. This is a simple job – any shop can easily do it. The Wabco system can not be reprogrammed and must be switched out to a 2-axle system (cost is about $650-$700, plus 1-2 hours labor). Make sure your ABS is properly set up for the single rear axle. To identify which system you have, you need to find the ABS controller. It is generally under the cab, near the transmission. Verify that any shop work is done correctly. It has happened that the bulb has been removed from the ABS fault light on the dash to "fix" the ABS. Turn your key on and verify the ABS bulb is still there, and that it does not come on once the truck is running.


Kenworth trucks, with the AG suspension, may prove more difficult to single. Most of the AG suspensions have a running beam that ties the suspension elements together between the front and rear axle sets. The AG 380 suspension is an exception – they are totally independent. Some people single Kenworths by completely removing the AG suspension and substituting a “conventional” suspension.

Singling considerations:

  • Make sure the driveline angle is maintained.
  • On a Volvo, make sure the U bolts are torqued properly – 550 lb/ft is the usual amount. If not, your axle will shift, introducing vibration. Same on other trucks, but the setting may differ.
  • Consider replacing the brake cans. They are cheap and now is the time to do it.
  • Make sure that the ABS is moved, and reprogrammed, if required. Establish up-front that the ABS is required to be properly installed and working. In trucks with 2-axle ABS the ECM may throw codes because of the missing ABS, even if the ABS that is present is working. Only you can decide if this is acceptable.
  • Specify that the driveshaft is to be balanced. In many cases the driveshaft on HDTs is not balanced unless it is specified (hard to believe, but true).
  • Do not just drop the rear axle and run with the power divider engaged.
  • On a T2000 or other Kenworth, consider replacing the suspension with a “conventional” suspension. Or negotiate the singling as part of the sale.
  • Have a 4-wheel alignment done after singling.

Solving Vibration Problems


If you just singled the truck, and have vibration problems, look first to the driveline angles. You must verify they are to spec. Also, verify that the axle is not shifting at its attachment point. This will happen if the U bolts are not torqued to specifications. After that, look at the following:


  • Drive shaft balance.
  • U Joints, and U joint orientation.
  • Drive shaft phasing. See the driveline manual elsewhere on this site for further info.
  • Carrier bearing.
  • Tire balance. The only sure way to balance tires is on the truck. Find someone who does it that way. Removing the wheel and balancing will not result is an exact balance on an HDT.
  • Tire trueness. Tires on most trucks are not round. Check for trueness, and have the tires trued if out of round.
  • Wheel bearings.

First Things to Do On Your New Truck


So you got your new truck home. If you bought it from a dealer or broker then you likely have no idea what maintenance items have been done to it, and when. The best thing to do, in my opinion, is to perform all the routine maintenance at the beginning, so you know where you stand with the truck. Yes, it is dumping more money into it, but you probably intend to keep the truck for a long time. Skimping here may bite you down the road. Of course, if you can, get the dealer to do some or all of this stuff as part of the purchase price. Some dealers are open to this, and some are not. If the dealer you buy from is going to charge you for this, then I would consider having it done at my local dealer. This will help you establish a relationship with your dealer.


Here is what I would do to any new truck:


  • Have an overhead run.
  • Change the fluids – oil and filter, fuel filters (primary and secondary).
  • Lube the entire truck.
  • Change the transmission fluid. If an autoshift, make sure it is synthetic.
  • Change the fluid in the rear. I’d add synthetic, but that is your choice.
  • Change the desiccant canister in the air dryer.
  • Change the air filter and the cab filter.
  • Test the coolant and DCA levels. I’d seriously consider flushing the system at this point, but if it looks and tests well you can consider putting this off. Change the coolant filter if you have one.
  • Take a careful look at the power steering fluid. If you have any whine in the steering than have it flushed.
  • Check the front hubs for oil level. Consider having it replaced.
  • On a Volvo, check the headlights to see if they are “hazed”. You might as well replace them if they are – polishing them does not usually work for more than 4-6 months.
  • The U-joints should have been checked during inspection. Check them again for looseness, and lube them. Lube the carrier bearing if you have one. Do these two things yourself, even if you normally have the truck lubed.

This will get you started.

Probably as important as the truck itself is the dealership and salesman you use. Unless you have a lot of heavy truck experience you are somewhat at the mercy of the salespeople. Hundreds of trucks have been bought by posters on the HDT section of the Escapees forum. The best sales people are in the PDF file that follows. If you want a tractor already converted, then check with Larry Ziegler or Randy Butler. Central Carolina Trucks will also convert the tractor for you - if they sell it to you. Most other truck dealers will not.
"Trade terms", is industry jargon for what a dealer would expect a trucks condition to be if aquiring it from another dealer. You often see this in advertisements. You can see a definition of this at the Used Truck Association website: http://www.uta.org/tradeterms.html. The best advice is that you have to make sure the deal is right, and the truck is right. You can not depend on sales people to represent your interests. They need to sell the truck.

Truck salesmen.pdf