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  These documentations are excerpts from a three-year LCV field test from 8/93 through 4/95 done by UMTRI.

University of Michigan Transportation Research Institute


A converter dolly, or simply a dolly, is used to support the front end of a semitrailer when it is used as the second or third trailer of a combination vehicle. The semitrailer has suspension and wheels of it's own only in the rear. Its forward end must be supported by other means, most typically by the rear of a tractor. When the semitrailer is used as the second or third trailer it must be converted to a full trailer (which has suspension and wheels at both ends) by the use of a dolly.

Converter dollies typically have one axle, but may have two. (All dollies in this study had single axles.) Dollies are equipped with a fifth-wheel coupler directly above, and slightly forward of the suspension center. This is the coupling to the semitrailer that the dolly supports and tows.

Depending on design style, dollies may have a single- or double-tow-drawbar arrangement for coupling to the towing trailer. In either case, the tow bars terminate in a simple, rugged towing eye. The towing trailer is equipped with one or two pintle hitches consisting of a hook and locking mechanism, which engages and secures the eye(s), thereby supporting and towing the dolly.

Two types of converter dollies, which are distinguished by the number of tow bars, are illustrated in figure below.

A-dolly. The defining quality of the A-dolly is its single-point tow bar. The A-dolly is the most common type of converter dolly; over 99 percent of the dollies in use in the U.S. are of this type. The single hitching point allows the dolly to articulate in yaw (steering), pitch (fore/aft rotation), and roll (side-to-side rotation) with respect to the towing trailer.

C-dolly. The defining quality of the C-dolly is its double-tow-bar configuration. The C-dolly (previously called the B-dolly for reasons too involved too explain here) originated in Canada. Its attractive quality is its ability to improve the stability of multiple-trailer combination vehicles. This is accomplished because the double-tow-bar hitching arrangement eliminates yaw and roll articulation with respect to the lead trailer. Eliminating yaw, in particular, can degrade low-speed maneuverability and produce excessive hitch forces and tire scrubbing during tight turns at low speeds. To mitigate these low-speed problems, the wheels of the C-dolly are allowed to steer by a caster mechanism. However, a centering mechanism provides mechanical resistance to this self-steering action as required for dynamic stability at highway speeds.

Tow bar. The forward structure of the dolly is referred to as the tow bar. It terminates in one or two tow-bar eyes, which are

simple rugged steel rings for attaching to the Pintle hitch. One (for A-dollies) or two (for C-dollies) pintle hitches are mounted to the rear trailer of the trailer which tows the dolly. The hitch, consist of a rugged hook which engages and supports the tow-bar eye and a locking mechanism which ensures that the eye stays in place. A single, hitch and eye results in a joint that allows articulation in all directions.


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The lateral acceleration experience of the trailers of triples relative to the acceleration of tractors, distinguished by dolly type, loading condition, and trailer position (speeds above 45 mph). Results show that the use of C-dollies tends to reduce the exaggerated motions of rear trailers as expressed in terms of rearward amplification, a measure in the frequency domain. The results of this section show that C-dollies also tend to reduce lateral motions of rearward trailers as observed in the time domain.



ARTICLE SUBMITTED BY: Chris Winkler, University of Michigan Transportation Research Institute


The Engineering Research Division of UMTRI has recently completed a three-year field study of long combination vehicles using ABS and double drawbar dollies. Results are reported in:

Winkler, et al. 'An operational field test of long combination vehicles using ABS and C-dollies.' Vol. 1, final technical report, pp 168. Vol 2, appendices, pp 260. Report no. UMTRI-95-45-1,2. December 1995.


The objective of this study was to evaluate the stability enhancing characteristics, practicality, reliability, maintenance costs and fleet personnel reactions to ABS and double-drawbar dollies. To do this, a fleet of double- and triple-trailer LCVs in actual commercial service was equipped with ABS and with C-dollies and monitored for a period of one and one-half years. The fleet of test vehicles was distributed among five commercial fleets operating in the northwestern region of the United States where the use of LCVs is most prevalent. The fleet accumulated 1.4 million miles on trips, and the individual units accumulated over 10.5 million unit-miles. All maintenance work done on the vehicles during the study was monitored, and the physical behavior of the vehicles on the road was measured with on-board instrumentation systems.

Findings include:

(1) ABS can be expected to play a significant, stability-enhancing role in some ten to twenty severe braking events per 100,000 miles (roughly a year for a professional driver).

(2) ABS on LCVs can be powered through the brake-light circuit provided that several important conditions are met.

(3) ABS increases the total maintenance costs of an LCV by about 1 percent but reduces costs due to flat-spotting of tires.

(4) The use of C-dollies on LCVs reduces rearward amplification in normal use.

*(5) Using C-dollies increases the total maintenance costs of an LCV by 3 to 5 percent, due mostly to increased tire wear.

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(6) Drivers, mechanics and fleet managers favor the use of ABS and C-dollies in LCV operations. Drivers especially favor C-dollies.

*NOTE: The tires on the C-dollies used in field test were new customers spec at the start of the study, where the A-dollies had used tires, therefore the initial wear factors were not an accurate comparison. The steering dampening/centering mechanism pressure was increased because of UMTRI's recommendations, which caused increased tire wear. This pressure was more than the manufacturers recommended settings and caused axle to steer less and tires to wear more.

Contact the UMTRI Research Information and Publications Center: Tel: +1 313 764-2171. Fax: +1 313 936-1081. E-mail: The web:

This document is available to the public from the National Technical Information Service, Springfield, Virginia 22161 or by the following link

Acknowledgements: (Chris Winkler, University of Michigan Transportation Research Institute), (Bogard, S. E., Bowen, M. A., Ganduri, S. M., Lindquist, D. J., Corporate Author: Michigan University, Ann Arbor, Transportation Research Institute), (N. Royce Curry of NZR National Zephyr Research, Burlington, Ont).


One hundred ninety-one individuals took least one trip under the field study and, in the process, suffered indignities of UMTRI trip logs and data cards. Although our thanks go out to all of them, there are too many, so we represent them all with the top twenty according to the number of trips they took.

Maintenance and Use: Based on experience with both A-dollies and C-dollies, have you found one to be more difficult to use (maintained)?

Written comments

C-dolly operation is much safer than A-dolly. [Driver 2]
The C-dollies are much (very much) an improvement in tractability in long combinations (triples). They also eliminate sway almost completely. I feel safer in snow or ice when pulling a set with a C-dolly than with an A-dolly. I would hope that, in the near future, these C-dollies would become universal converter gear used with all doubles and triple combinations. [Driver 2]
Noticed difference between A-dollies and C-dollies in the first mile of freeway driving. C-dollies pull straight as a pin--can't get them to sway without really trying. The C-dolly cornering at road speed is much improved over A-dollies. First impression--They're a pleasure to pull. [Driver, DTF]
Snow was rough and rutted. Trailers pulled very well, back box didn't flop around and try to pull the rest of the set out of line like A-dollies. [Driver, DTF]
The C-dolly. I've heard from most drivers they like it better than a regular dolly. The trailers change lanes better and without the whipping you get from a regular dolly. From a service point of view, the C-dolly is not much harder than the A-dolly. As a mechanic and shop foreman I am impressed with the operation of both ABS and the C-dolly. [Mechanic, 2]

C-dolly Use: what is your opinion of using C-dollies in double and triple combinations? All three classifications of fleet personnel were in favor of using C-dollies. The drivers held the strongest opinion in this regard. This result is not surprising since it is the drivers who benefit the most from the positive qualities of the C-dolly. Management's view on this question fluctuated during the study, but always remained favorable. The mechanics consistently had a favorable view of C-dolly use in LCV operations. Again, there were no strong trends toward changing opinions over the course of the study.

Written comments

From what I have seen I am impressed with both (C-dollies and ABS). [Mechanic, 5]
In my opinion the C-dolly is the best thing made for safety of pulling combinations. [Driver, 1]
C-dollies should be mandatory. [Driver, 5]
A-dollies should be outlawed! [Driver, 5]
Both types of equipment will improve the safety and handling of doubles and triples. [Manager, 3]
After learning how the C-dolly works, its the best way to go! Back trailers are a lot more stable in wind, on rutted roads, etc. [Driver, 1]
I feel the C-dolly should be law to pull triples and doubles. (LCVs) would be a lot safer. I have had several compliments pulling doubles and triples down the road on how straight they are pulling! [driver, 3]
The C-dolly equipment is great. I have never driven anything that handled that good in my life! [Driver, 3]

C-dolly Use: What is your opinion of using C-dollies in double and triple combinations?

This question was only addressed to drivers, and was intended to assess whether drivers viewed C-dollies, overall, as a burden or an aid in their jobs.

Despite the additional burden of a more complicated and sometimes difficult hitching mechanism, the drivers consistently indicated that, overall; the C-dolly did make their job easier. This rating rose early in the study and then fell again at the end. However, it always remained distinctly positive.

Written comments

After using a C-dolly for so long, going back to the A-dolly is like going from the space shuttle to a horse and buggy. It takes longer to do my work; more hooking and unhooking, switching trailers around. Going down the road there is a considerably more trailer sway. Also, I believe that when the rear trailer sways and moves the dolly, it also allows the front trailer to move more. The C-dolly provides stability to both front and rear trailers. [Driver, DTF]
The C-dolly made for the most stable combination of trailers that I have ever pulled. I was highly impressed with the way the trailers handled. It also made it very easy to maneuver when backing into a dock. [Driver, DTF]
Liked the way they pull down the road. First trip with C-dolly made job a little because of hook-up, but can see no real change in job or time with more trips. [Driver, 3]
My third trailer was to be left at Springfield. I was able, because of the C-dollies; to back all three trailers into the dock and disconnect as usual, leaving the dolly under the trailer. I have been able to save a considerable amount of time using the C-dolly. [Driver, DTF]


shopko2.GIF - 2369 BytesThe Wisconsin-based Shopko has one of the largest fleets of C-dollies/trailers in the U.S. About 190 of its trailers are currently designed to use C-dollies, of which 140 were manufactured specifically for C-dolly use, and 50 were modified for C-dolly use.

Herman Miller, fleet equipment manager for Shopko Stores, Inc., Shopko's decision to try the C-dolly configuration to minimize swaying and maximize driver control in its longer combination vehicle (LCV) operation. Shopko's experience with the C-dolly shows it solves all these problems and "the tail can't wag the dog".
"Our experience has been mostly positive, and I believe based on our experience in the use of the C-dolly that there is a definite gain of safety," he said. "We have had no problems in pulling the dollies and there have been no injuries involving the C-dollies. We have received many compliments from both the public and law enforcement officials about how straight our trailers pull in the wind. Our drivers tell me they could make any evasive maneuver with (LCVs) using C-dollies that they could make with a 48 ft. trailer."

Productivity gains

Miller also said the C-dollies have provided increased productivity. For example Shopko has never had a single workers' compensation claim among its C-dolly fleet drivers. This is largely attributed to the fact that drivers do not have to maneuver the C-dolly manually during couplings they do with A-dollies. Instead, drivers back their vehicles directly into the dolly eye, rather than stop three feet short of the dolly and manually roll it in place.
C-dollies also give Shopko about five percent additional trailer floor space in certain LCV operations. For example, Idaho law allows 85-ft.-long LCV operation with 5.5-ft.-ft. off-track. However since Shopko's C-dolly-equipped LCVs have a 5.4-ft. off-track, Idaho allows the carrier to operate LCVs that are 86-ft. 10-in.-long. The reduced off-track also allows Shopko to minimize the space between trailers for better fuel economy. (Reference, Go-West 3/94)

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