# Extra wheels on trucks **URL:** \ **Category:** Factual Questions\ **Created:** [August 11, 2008, 8:39pm UTC](https://boards.straightdope.com/t/extra-wheels-on-trucks/459655 "2008-08-11T20:39:45Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [August 11, 2008, 8:39pm UTC](https://boards.straightdope.com/t/extra-wheels-on-trucks/459655/1 "2008-08-11T20:39:45Z")
Inspired by the “semi” thread, I want to ask something I’ve been wondering lately. In the past few years, I’ve been seeing trucks with smaller “extra” wheels, mounted just in front of the main load-carrying wheels, and not even touching the ground. Like this: [http://www.bandag.com/download/ImagesLogos/dumptruck.jpg](http://www.bandag.com/download/ImagesLogos/dumptruck.jpg) What are those for? ---
**Author:** ![yabob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yabob/32/2821_2.png) [@yabob](https://boards.straightdope.com/u/yabob)\ **Post date:** [August 11, 2008, 8:50pm UTC](https://boards.straightdope.com/t/extra-wheels-on-trucks/459655/2 "2008-08-11T20:50:26Z")
The lift axle may be lowered to provide extra support and meet weight-per-axle requirements when heavily loaded. At other times, they can be raised for improved gas mileage, or to save wear and tear on the unneeded wheels. ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [August 11, 2008, 8:51pm UTC](https://boards.straightdope.com/t/extra-wheels-on-trucks/459655/3 "2008-08-11T20:51:27Z")
They can be [extended down](http://auto.howstuffworks.com/question26.htm) for greater load bearing utility. ---
**Author:** ![Dog80](https://avatars.discourse-cdn.com/v4/letter/d/6bbea6/32.png) [@Dog80](https://boards.straightdope.com/u/Dog80)\ **Post date:** [August 11, 2008, 8:59pm UTC](https://boards.straightdope.com/t/extra-wheels-on-trucks/459655/4 "2008-08-11T20:59:59Z")
[QUOTE=yabob] At other times, they can be raised for improved gas mileage, or to save wear and tear on the unneeded wheels. [/QUOTE] The main reason these wheels are retractable is that the vehicle cannot make tight manouvers with these wheels extended. Due to their positioning they slide sideways instead of rolling. ---
**Author:** ![Harmonious\_Discord](https://avatars.discourse-cdn.com/v4/letter/h/74df32/32.png) [@Harmonious\_Discord](https://boards.straightdope.com/u/Harmonious_Discord)\ **Post date:** [August 11, 2008, 9:39pm UTC](https://boards.straightdope.com/t/extra-wheels-on-trucks/459655/5 "2008-08-11T21:39:40Z")
Without them many of the trucks wouldn’t be allowed on the county roads during certain times of the year. Adding the extra axle allows them on roads they couldn’t travel otherwise. The bit about why they are retractable has been covered. ---
**Author:** ![KneadToKnow](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kneadtoknow/32/3999_2.png) [@KneadToKnow](https://boards.straightdope.com/u/KneadToKnow)\ **Post date:** [August 11, 2008, 9:43pm UTC](https://boards.straightdope.com/t/extra-wheels-on-trucks/459655/6 "2008-08-11T21:43:33Z")
Damn. I always just assumed those were spare tires. ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [August 11, 2008, 9:44pm UTC](https://boards.straightdope.com/t/extra-wheels-on-trucks/459655/7 "2008-08-11T21:44:17Z")
Thanks, all. I’ve never noticed any that had the extra axle in the lowered position - I wasn’t aware it could do that. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [August 11, 2008, 10:04pm UTC](https://boards.straightdope.com/t/extra-wheels-on-trucks/459655/8 "2008-08-11T22:04:07Z")
[QUOTE=KneadToKnow] Damn. I always just assumed those were spare tires. [/QUOTE] I thought they mounted them just to look badass. Like people put spoilers and spinners on their cars. 😛 ---
**Author:** ![danceswithcats](https://avatars.discourse-cdn.com/v4/letter/d/9e8a1a/32.png) [@danceswithcats](https://boards.straightdope.com/u/danceswithcats)\ **Post date:** [August 12, 2008, 1:56am UTC](https://boards.straightdope.com/t/extra-wheels-on-trucks/459655/9 "2008-08-12T01:56:42Z")
Around here, they’re called tag axles, and are used along with tarps. Heading into the quarry, both are up. Heading out, axle down and tarp over, or a large fine is waiting for you, if an officer is 1/4 mile down the road. Note-many truckers hauling commodities other than stone employ tag axles and tarps. The above was meant to be illustrative. ---
**Author:** ![Santo\_Rugger](https://avatars.discourse-cdn.com/v4/letter/s/e95f7d/32.png) [@Santo\_Rugger](https://boards.straightdope.com/u/Santo_Rugger)\ **Post date:** [August 12, 2008, 2:27am UTC](https://boards.straightdope.com/t/extra-wheels-on-trucks/459655/10 "2008-08-12T02:27:42Z")
A tangential question: is GAWR (Gross Axle Weight Rating) the same as ESAL (Equivalent Single Axle Load)? ---
**Author:** ![x-ray\_vision](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/x-ray_vision/32/351_2.png) [@x-ray\_vision](https://boards.straightdope.com/u/x-ray_vision)\ **Post date:** [August 12, 2008, 2:42pm UTC](https://boards.straightdope.com/t/extra-wheels-on-trucks/459655/11 "2008-08-12T14:42:32Z")
[QUOTE=Santo Rugger] A tangential question: is GAWR (Gross Axle Weight Rating) the same as ESAL (Equivalent Single Axle Load)? [/QUOTE] > [@](#): > > gross axle weight rating > GAWR > Gross axle weight rating represents the value specified by the vehicle manufacturer as the load-carrying capacity of a single axle system, as measured at the tire-ground interface. > [@](#): > > equivalent single axle loads > ESAL > Equivalent Single Axle Load (18KIP) is a measure developed for pavement design. This measure is an estimate of the number of equivalent 18,000 pound single axle loads expected to traverse a section of pavement during the pavement design life. ftp://ftp.dot.state.tx.us/pub/txdot-info/gsd/manuals/glo.pdf ---
**Author:** ![Santo\_Rugger](https://avatars.discourse-cdn.com/v4/letter/s/e95f7d/32.png) [@Santo\_Rugger](https://boards.straightdope.com/u/Santo_Rugger)\ **Post date:** [August 12, 2008, 4:51pm UTC](https://boards.straightdope.com/t/extra-wheels-on-trucks/459655/12 "2008-08-12T16:51:16Z")
So GAWR is a rating for the vehicle, and ESAL is a design estimate for the road. Got it. ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [August 12, 2008, 5:10pm UTC](https://boards.straightdope.com/t/extra-wheels-on-trucks/459655/13 "2008-08-12T17:10:57Z")
[QUOTE=CurtC] Thanks, all. I’ve never noticed any that had the extra axle in the lowered position - I wasn’t aware it could do that. [/QUOTE] But if it was in the lowered position, how would you know that it could be raised? It would just look like another axle. ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [August 12, 2008, 5:29pm UTC](https://boards.straightdope.com/t/extra-wheels-on-trucks/459655/14 "2008-08-12T17:29:59Z")
[QUOTE=CurtC] Thanks, all. I’ve never noticed any that had the extra axle in the lowered position - I wasn’t aware it could do that. [/QUOTE] If you watch, you’ll usually see them lower them as they get on a freeway (or soon after) and they tend to raise them as they are heading down the off ramp. ---
**Author:** ![gotpasswords](https://avatars.discourse-cdn.com/v4/letter/g/c57346/32.png) [@gotpasswords](https://boards.straightdope.com/u/gotpasswords)\ **Post date:** [August 12, 2008, 5:55pm UTC](https://boards.straightdope.com/t/extra-wheels-on-trucks/459655/15 "2008-08-12T17:55:46Z")
A fair number of long-distance motor coaches (eg: Greyhound and Trailways) have tag axles as well. One nifty thing that the bus builders are doing is designing the suspension so the tag is automatically lifted when the transmission is in reverse or if the forward speed is under 15 MPH to improve maneuverability. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [August 13, 2008, 2:49am UTC](https://boards.straightdope.com/t/extra-wheels-on-trucks/459655/16 "2008-08-13T02:49:48Z")
Due to state laws you will see LOTS of these in Michigan. Don’t know the details, but it was something that really stood out when I had to travel there on buisiness.