wheel size-might be the dumb question of the year

Impulse24,

Just a couple of points, the change in breaking is due to the radius of the tire. The larger the diameter, the longer the moment arm of the friction force at the road to the brake. (Think of it like a longer wrench on a socket...your hand is the road friction and the torque at the nut is your braking force.) So for a given velocity, to decelerate the vehicle at a give rate, the greater the force the brakes must exert. Same works the other way for engine torque for acceleration...

Next, the braking friction between the tire and the road is a function of the coefficient of friction for the two materials (the rubber and pavement) and the force between them, which is essentially the weight on that tire. The contact area does not figure into the equation in its most basic sense. When you consider local deformation of the materials, then the frictional force decreases with more surface area. With small contact area, things "dig in" due to local deformation. That's why you put a big pad under a small leg to get it to slide easier or why big wide tires don't have much traction on snow.

Bottom line, larger tires will hurt braking.

Jeff

Impulse24......If you take a 5' tall road grader tire and fit it on your coach, please explain how more tire surface will make your braking better and the additional weight will have no impact. I understand you want something better, but you may be asking more of your coach than its capable of giving. If Yugo's road like a Lexus, they might still be around. I'm not calling your coach a Yugo, but the newer suspensions are more exotic and have 22 years of ride enhancement over your coach.

I would guess that a Trac bar would be your best bet for improvement. Try and find a 10' length of 2" square tubing. Insert it in your hitch and have a strong person push it side to side while you lay under the coach and watch your rear suspension. You may see it move all over the place. If it does, a Trac bar will probably give you some relief.

For years Ford MH chassis had 16 inch wheels while the Chevys had 19.5.

Impulse24 wrote:
Dually tires are secured by studs and lug nuts and do not revolve separately, so there is no friction generated between the tires.

I'm only going to respond to this statement as I'm on break at work and I'm a bit time constrained.

Your statement is only true IF the tire size allows for sufficient gap between the two tires. All the way around, including at the bottom where a tire will typically bulge a bit due to weight. If the tires come together at any point, then there IS friction as each tire is flexing. It's not friction due to one tire rotating at a different speed than the other, but ANY contact will produce some friction and rubbing and when you remember that those tires are doing many revolutions per minute, that little bit of friction and rubbing add up. It adds up into heat, which is a destroyer of tires.

Even if they don't quite rub, each tire is producing heat, and having one tire right next to the other will cause that sidewall to get hotter than the one open to the air. Too much heat and the tire goes BOOM. A bit of a gap allows air movement to move the heat away and keep things at safe temps.

So, make sure that the width of tire you go to keeps a safe gap between the duallies.

DSDP Don,
I thank you for your response and fully understand what you say. However I don't think you really understand my predicament.
1. The Coach is a 1992 Gulf-Stream, spring suspension without ride height adjusters.
Since I have had the coach, I have had the steering checked and aligned according to Spartan specifications, I have new shocks all round (Monroe Magnum) I have installed Safe T plus steering bar and rear anti sway bar stabilizer. Although Wandering has decreased I still get pushed around by the wind on the freeway not so much by passing vehicles. On reading the forums I find miss leading information and contradictory posts which do not help.. I have talked to 3 different "Experts" and been given 3 different answers.

One tech say's I can't do anything its the nature of the beast.

2nd tech says add a Henderson Track Bar and up grade tires from the Sampson 8R 19.5's to 225/70 19.5. When asked about width his reply was as I stated, the wider tire will give a good seal on thre rim and the tire pressure will hold the tire walls stable.

3rd tech says the tire size is the problem, but to increase the size will result in Rubbing and Higher MPG. To replace the tires and wheels alone is approx $7000.

Dually tires are secured by studs and lug nuts and do not revolve separately, so there is no friction generated between the tires. Please note the wheel size would still be 19.5 ! Inaccurate speedometer wouldn't be a factor if I only do the front wheels. And my MPG averages 9.23 (pen and paper) that was achieved recently in the White Mountains of AZ with grades or 6 and 7 % towing a tow dolly with Chevy Equinox at an average speed of 55 mph.

I know a lot of people would say trade the coach, but this coach is in excellent condition and has only done 126000 miles on a Cummins 5.9 with a AT 542 Allison transmission which the previous owner had rebuilt less than 1000 miles / 9 months ago. The only problem is the wandering. When we bought the MH we new we would have to put some money into it, but we need to know where the money should be spent to correct the wandering.

As to braking, I understand your points however I have to say what stops the vehicle is the friction between the road surface and the tires. Therefore the wider the tire the more area is in contact with the road giving better braking. I also realise weight and speed plus road surface makes a differance and taking into account manufactures specifications leaves a contradicting circle of what to do to fix the problem.

I have read comments about Monroe Shocks, but I think it is a personal preference, any new shock replacing a bad one will cause the user to think the replacement is the best. Monroe use these shocks on their Bajo race cars, so they can't be that bad..


I appreciate your help Don Thank You

Impulse24.....You can change to whatever will fit, but it won't necessarily improve your ride or handling. Have you ever seen the guys who take a large car or half ton truck and put on 22" or 24" tires and wheels because they look cool. The problem is that the brakes were not designed to stop something that large. Just one issue, but probably not the biggest where you're concerned.

Rims are designed for a certain width tire so that the TIRE performs correctly. The tire manufacturer designs the tire to have a certain amount of flex and bulge for weight handing and ride. When you throw an oversized tire (width wise) on a rim, there is too much bulge. Your view that they are bolted and spaced by the rims and lug nuts doesn't make sense. If you put a wider tire on, it's going to hang farther over the rim. In some cases you wouldn't be able to even tighten the lug nuts because the tires would rub against each other. They're also spaced for cooling and to prevent pinching debris between them.

Your diesel was designed so that it sits at a certain height in the front and rear. This is why you have adjustable ride height valves. If you start messing with the ride height geometry, you'll suddenly realize that you'll have all types of issues,

There is a whole myriad of reasons a vehicle is designed with a certain tire and rim size. In the past 30 years RV's have become bigger and heavier....and the chassis manufacturers have increased the carrying capacity of tires and rims to meet the challenges of larger RV's.

Just because one company puts on 22.5 rims on their coach doesn't mean they're a better manufacturer. If you compare 22.5 tires through the entire 22.5 size range, there is a lot of difference in height and width, just like cars. Have you ever seen an aluminum 19 RV rim......no....so some manufacturers will go to 22.5 to dress up their wheels. The 22.5 tires on a gasser are much smaller in height and width than those on a 45000 pound DP.

This is not a hard concept to understand. You wouldn't put wheelbarrow tires on your motor home and you wouldn't put motor home tires on your wheelbarrow. Each vehicle is designed to ride on a certain size rim and tire. RV manufacturers go cheap on a lot of things, but rarely do you hear someone complain that they got too small a tire and wheel on their coach. Overloaded axles yes, tires and wheels....no.

Sorry Duplication

Sorry Duplication.

OK, I've been reading these forums for a long time and none of my questions have been answered to my satisfaction, So here is a list of queries I would appreciate some ansers to.

And please if you don't know or just want to voice off do it somewhere else, there is enough BS posted from so called experts who know nothing.

1. I have a Diesel Pusher, why can't I put a bigger size tire on the front wheels?
2. My wheels are 19.5 present tires Samson 8 R 19.6. Everything I read say's the wheel is designed for a max 6 inches.
3. However a bigger tire will give a tighter fit against the rim? And common talk is the larger tire will cause the walls to soften, But if a tire has a correct pressure, logic says it will hold its strength and shape unless punctured?
4. Many comments say that you can't increase the tire size on the rear because the Dually wheels will rub? The wheels on a Dually can't rub against each other bother wheels and tires are secured by the same stud and therefore will not cause friction ? I agree space is important to allow air circulation for cooling but friction.....
5. If you put bigger tires on the front you have to adjust your speedometer? As there is no drive train on the front of a Diesel Pusher and the speedometer cable is connected to the transmission which is connected to the rear axle (Drive Axle). Yes I know about different sizes and revolutions changing speed, but come on does your tag axle change your speedometer if it had bigger tires. No it doesn't and neither does a steering axle with no drive train attached.

So If I change my present front tires (8R 19.5) to 225/70R 19.5 what can I expect in handling and control?

I will add tires are the only thing I haven't done to correct slight wandering, which I only suffer from in windy conditions.

Thanks in advance..

It's also an age thing. Back in the 90's and before, most gas motorhomes had 16" wheels. In the late 90's those were phased out completely in favor of 19.5's

We had a pace arrow, a 1999 26,000 lb F53 36', it had 19.5's.

In more recent years The heavier versions of the F53 have gone to 22.5.