Caster, Camber, and Other Neat Stuff

If you're new here, you may want to subscribe to our E-mail Digest. We will then send you the stories that are posted each day in an e-mail digest. We use a service called Feedburner for delivery of these emails. You will receive an e-mail from Feedburner after you subscribe and you must click on that email to activate your subscription. Thanks for visiting and enjoy all the information!

RV.Net Blog Admin

Before we get to the front end stuff, a comment on Labor Day Weekend.  We spent the week before the weekend parked on the shores of Lake Winnipesaukee in New Hampshire. Friday we headed for Wiscasset Maine to attend a wedding and meet up with daughter Becca, her husband Tobin, and an eight week old grand daughter Elise. Elise is a keeper. But the other eye opener was the lack of folks, traffic, and campers in Maine on Labor day weekend. There was not a No vacancy sign to be seen and the camp ground that we stayed in, a Passport America affiliate, West Side Campground in Wisscasset was only a third full. The economy is taking it’s toll.

Like I said last week, reading a tire is not rocket science as Sean and Kristie Michel, authors of the Long Long Honeymoon blog, recently found out. They wacked a curb with the Airstream and then began a wear tire problem resulting in blown tires. Sean published a photo of the ruined tire and it plainly showed a camber condition. Excessive wear on one side of the tire.

Camber wear will very seldom extend down onto the side wall. When wear does that it is usually low pressure. Camber when excessive will create a pull to one side or a drift. It is simple. When a tire is rolling down the road and the center line of the tire is parallel with the ground the tire will roll straight ahead. If we tip the wheel in, one side of the center line will point up into the air while the other side will extend down and the intersect with the pavement. This makes the tire think that it is an ice cream cone rolling and will roll around the point of the cone. Meaning the open end of the cone (tire) will not roll straight but rather around in a circle, pulling to one side.

In the old days with bias ply tires we used to off set the camber when aligning front ends to over come the tendancy of the car to drift down the crown in the road. Radial tires tend to track truer making the vehicle handle these problems better. So camber is first a tire wearing angle with positive camber being the wheel tipping out at the top while negative camber the wheel tips in at the top. When camber changes the contact point between the tread and the road surface does not change, the tire tilts around this point. There are several angles that also use this patch of contact as the meeting point. These angles are called the Included Angle and is a major part of steering geometry.

Caster can best be illustrated by looking at a shopping cart the next time that you are in a grocery store. If you drew a line through the pivot point for the front wheels of the cart it would intersect the ground in front of the wheels. This leaves the wheels to trail the pivot point and follow. If we try to make thw wheels stay out in front of the pivot, they will swivel right around and follow. In a vehicle caster is changed by tilting that line forwards or backwards so that we can control the angle and the tendency to provide more or less of a tendency to track straight ahead. In other words more positive caster, more straight ahead stability. And as a result uneven caster will create a pull in the direction of the wheel with the most negative caster.

The problem with tilting the pivot line front to back will cause the wheel to lay over when going around a corner. This means that tire edge wear will occur but it will be on both the inner and outer edge, like camber wear but on both sides, so there is a limit to the amount of caster that you can dial into the front end.

Well enough for this week, we will wrap all this up next week and then move on to more stuff. have a great week and keep camping.

Brad

More Fuel Saving Technology and Upcoming Events

A lot of you will recall that I talked a few posts back about how different tires can affect fuel economy. Well, as it happens, a representative from Goodyear came into our shop last week to tell us about a line of tires and retreads with what they call “Fuel Max Technology”. They’re designed for commercial truck fleets as of right now, and as such are available in drive, steer, and trail configurations. However, some of these Goodyear tires will also fit some motor home applications.

Read more

Care for a Truck and Camper

 

We have been working on new suspension products for pick-ups for quite some time now, and last week we had the opportunity to work on a Dodge heavy-duty truck that would allow us to use much of the knowledge we have gained working on motorhome chassis. This truck hauls a camper, and although it was already equipped with airbags, the handling was still far from ideal. Read more

Can You Realy Tell Anything About Your Alignment By Looking At Your Tires?

The simple answer is yes you can. The first step in gauging tire symptoms is tire pressure. A tire that is improperly inflated will show wear that may partially mask the alignment wear patterns. Underinflation will tend to allow the center of the tread to squish up and leave the two shoulders to carry the weight of the rig. This, on careful examination, will be evident as the wear pattern extends down onto the side walls of the tire as they are now in contact with the pavement. An over inflated tire will show more wear on the center of the tread as the tire balloons lifting the shoulders off the pavement on both sides of the tread.

Under inflation gives the rig a squiggle squishy feeling and lots of imprecise steering input as the tires tend to flop under on turns and absorb steering. This squishiness is what causes the excessive flexing in the tire building an overheating condition resulting in tire failure.

Over inflation will give a harsh ride and tend to create a dartiness in the steering as when turning the tire tends to roll off the high center created by the ballooning effect of the tire and cause a lead in steering. On the plus side heat build up is less but over inflation can result in decreasing the tread contact surface with the pavement effecting handling and braking. There is a vacuum cleaner commercial out that shows a vacuum cleaner using a ball for a front wheel, and it touts the ability to turn corners.

And of course the proper inflation for a tire is best arrived at by weighing the rig and then consulting a tire chart for the maker of your tires. There is a bit of wiggle room in the pressures but reading your tires will tell you the answer to that.

Next week lets start with caster. Caster is a directional control angle first off and then when caster is way off it can become a tire wearing angle. Camber is primarily a tire wear angle and can be a directional control problem as well. Toe in is a tire wear angle first of all and then a handling angle. Notice that I have used a different term here as there are several different names for the way a vehicle reacts to steering input, wind, and road conditions.

So, tune in next week when we get to the nitty gritty.

Brad

A Weighty Issue

Weight and loading are perhaps the biggest issues affecting the safety, handling and longevity of RVs. But if you’re new to RVing, you’re probably wondering why. After all, there doesn’t seem to be so much concern over weight in a car or even most pick-ups and SUV’s, so why RV’s?

The simple reason is that RV’s are both a vehicle and a home, and they are subject to build considerations not found in any other industry. If you want ideal balance in car, you do things like shift the weight rearward, use lighter components in the front, etc.-and side to side balance isn’t in issue unless your passenger is significantly heavier than you are. In an RV, manufacturers usually don’t enjoy that kind of freedom. Think about what sells an RV. Is it the performance or handling? Nope-it’s the floor plan! Manufacturers have to make an nice floor plan that attracts customers when they walk into the coach on a dealer lot. The problem is, the layout that draws customers in may not be perfect for weight balance. After all, you can’t have the appliances staggered from side-to-side throughout the galley, for example, to keep weight balanced…no one would like your floor plan. Then there’s the placement of the tanks, the generator and the storage bays, and they can all conflict with one another. It’s all about compromise, and some coach builders are better at it than others.

Read more

Don’t Forget The Trailer

In “Minimize Resistance, Maximize Fuel Economy, Part II” post, we discussed the importance of checking your trailer’s alignment. We recently had a customer come in who took this advice to heart, and decided to have his double-axle fifth wheel trailer checked out. Indeed, it was out of alignment, but that was only a small part of the story.

Read more

More On Tires

Well I really stepped in it last week and as Sven pointed out about a minute after I posted that I did not answer the question in the title. That was what is the correct tire pressure? Well, if we hang in a bit I promise that I will answer the question. A hint is that there is no one answer but perhaps we can clear up the formula.

Larry wrote that the “air limit is stamped on the side wall of the tire”. That is basicly true but there is also a weight stamped with the PSI number and usually more to the statement. larry has it half right. The Firestone T559 tires sized 9R22.5 12PR on my old Foretravel says the following: “max load single 4540 LBS at 105 PSI cold,” and then a second sentence, “max load 4200 at 105 PSI cold.” First of all why the difference in the tire pressure between single and dual? According to my sources it is the clearance between the dual wheels to make sure the tire side walls do not touch during deflection. 

The statements on the tire side walls says that you can carry 4540 pounds of weight at a specific pressure and the tire will have the correct foot print and side wall deflection for a combination of low heat build up, good ride and handling and good tread wear. The entire statement must be taken in context and does not mean that this is the correct tire pressure under all conditions for this tire.

Jerry wrote, “OK under inflated is not good.  So where do I get an accurate tire gauge?  Where do I find a air pump that will properly fill my tires?” Well Jerry I use a digital tire gauge that I bought from the Northern Tool catalogue. My compressor is a Campbell Hausfield 110 volt 100 PSI that came off the shelf at Wal-Mart. It is a bit slow but it does do the job.

Roger, a tire designer, wrote, “I am a tire design engineer with 38 years experience. I teach tire failure analysis ( Think CSI for tires) so let me correct a few minor points and answer some of the questions.

The correct inflation is based on the actual load you are running. The inflation information in your owners manual is based on some assumptions on how much weight you are carrying. I have heard of some vehicles being only capable of adding 5oo# over their empty weight before the tire is overloaded so the only way for you to know the real answer is to get your vehicle weighed for each position. Then contact a tire dealer and ask if he has a load inflation chart (or check the manufacturers web site). If he doesn’t find a different tire store. The correct inflation should be something below the max infl on the sidewall of the tire. Only check your inflation when it is cold - not in the sun and not driven on for at least 4 hours.”

Roger says it all. Weight the rig, at least each end and preferably each corner with your travel load, and then compare the weight with the makers tire chart and determine the correct pressure for conditions. 

Now stay tuned for next weeks blog when we will hear Gordon say, “I recommend doing a Google search on Nitrogen in tires.  The information will make the choice a no brainer”.

The Age Old Question, How Many Pounds Of Air Shall I Put In My Tires?

Well sir, I have never been accused of being terribly smart and politicaly astue, so here I go jumping from the frying pan into the fire on a subject that some have said in the forums is a dead horse and beaten to death. But there is more to the tire pressure story than just when will it fail or blow up. Or how much pressure to ad and when.

We all know that a tire that is under inflated will use more power to make it roll than a properly inflated tire. That is because of these factors:

Read more

Valid Mileage Comparisons

The mail bag is getting full and it would be the polite thing to do to answer the questions and make room for more. Santa Skip wrote the following after I used the term Tommy Knocker:

“I spent my younger years exploring the ghost towns of CO, to me a tommyknocker was a little elf who lived down in the mine, if he liked you he would show you where the gold, (was) if he didn’t he would cause all kind of problems.”

One of the neat things about traveling is that you get to learn that an expression that you grew up with have different meanings regionally. But this is a new meaning of Tommy Knocker to me.

Chuck wrote the following after my comments on driving at 55 miles per hour:

“I’m posting this as a reality check. I’m getting better MPG at 68-72 mph (2200 rpm) than at 55 mph. I drive a F250 with a V10. With or without the 5th wheel trailer I get better mileage at a higher speed and higher rpm. At my last fill up I got 10.8 mpg. I traveled 107 miles with the 5th wheel trailer and 190 miles without the trailer. In the past if I drove at 55 I would get around 8 to 9 mpg. Has anyone else seen this type of mpg driving faster instead of slower?”

Under real world conditions it is almost impossible to get accurate fuel economy numbers. Back in the dark ages, about 1980 something, the EPA was in the process of certifying fuel savings devices. They invited a group of automotive writers to a test drive to provide the EPA with numbers of fuel economy improvement of the new Volkswagen E Shift light system. It was a computer controlled device that would turn a light on on the dash that would signal the driver when to up shift for best economy. The route was from the tip of Long Island to Newport, Rhode Island. Lots of rolling hills.

VW teamed up the drivers and it happened that Dan Holt, then a writer/editor with the Society of Automotive Engineers publication, and I drew the duty as partners. Now not to say that we were heavy, but between the two of us we exceeded the load carrying capacity of the car, not counting our luggage or our camera gear. Now as every one knows writers have imagination. Dan and I being both engineers decided that we would show some real fuel economy, and here is how we did it.

The first thing that we did was to eliminate rolling resistance. We stopped and pumped the tires up to 80 PSI. The car rode like it had Freddy Flintstone Hard Rock tires. Then we applied some simple junior high school physics. Gravity. We figured with our combined weight, the weight of the car, and gravity, that we could use the engine on down hill runs to get the speed up, and then coast up the next hill. Foot to the floor in high gear going down hill, put the tranny in neutral and shut the engine off for the uphill coast. In several instances on the run we were able to leave the engine off for several hills as well as seeing the tach hit red line in high gear.

VW controlled the record keeping of fuel in each car with a burette attached to the front bumper and at each fuel stop they measured fuel used and filled the burette to the full line and we ventured out again. Well Dan and I placed second in this 50 car run with around 100 miles per gallon. A couple of guys from one of the magazines out did us by a couple of miles per gallon. I think they ran a bit more air in their tires but they used the same techniques. We did show VW that we could get good mileage but it did little for the certification of the Up Shift Light. And we broke speed laws and really had a ball.

The moral of the story, which is a true story all the way, is that determining a difference in fuel economy means measuring apples against apples under like conditions with only one variable at a time. Driving cost over an extended period of time is a valid number.

“Drive your Motorhome Like a Pro” DVD released

RV.net echoes the announcement of Mark Polk’s brand new DVD titled “Drive your Motorhome Like a Pro”, co-hosted with Lorrin Walsh, author of the book by the same name. As indicated in the press release, this DVD provides the tools to increase one’s awareness of driving motor coaches, and demonstrates unique driving techniques needed to become a safer and a more confident driver.

We thank our bloggers for their dedicated efforts to keep the RV community informed and entertained. Therefore, when we find out that one of our bloggers has released a new book or product, we help them spread the word.

Next Page »