Hello Autopians. Welcome to another edition of Ask an Engineer. Let’s start off this week with a question from Pieter about why his car was only available with 20-inch wheels, then we’ll chat about whether crossovers are tougher than cars and about the AE86 Toyota’s handling.
Pieter lives in Belgium and he has got himself a very nice new Renault Grand Scenic as a company car. It’s not a car he would necessarily have chosen himself but a free car is a free car after all. His question relates to the tires, which are P195/55R20. Apparently the new Scenic comes only with this tire size regardless of trim level and he thought that was odd. He also wondered why his car might have been equipped with steel wheels instead of aluminum alloys and what difference that would have made.
Sticking With Your OEM Tires, Some Notes About Wheel Sizes
Well, Pieter, I too find the choice of 20” wheels across the board odd, but there could be a very good reason for this. I can’t look into the minds of the engineers and product planners at Renault but one reason to have a single tire size is that tire development is expensive and time consuming. If you can release a new vehicle with the same tire on all trim levels it saves a lot of time and money during development. A lot of effort is put into making sure the tires a car is sold with perform at the high levels a manufacturer specifies. This process can take over a year to complete and requires the tire manufacturer to make several “submissions” to the car company.
In each submission, the tire company will provide several hundred tires with different types of construction and rubber compounds. The constructions may vary in the way the steel and nylon belts are oriented, or what rubber compounds are used in the tread blocks. Each design will behave differently and the vehicle development engineers will then spend several weeks testing each of the tires to see which one works the best in the car. They will test each tire for cornering, stopping, and acceleration performance in dry and wet conditions. They will test the amount of noise a tire makes over various types of road surfaces. They will see how well the tire communicates road conditions through the steering system. Does the tire wander over minor road imperfections? Is it sensitive to ruts in the road? They may also test it on special machines to get an estimate of tire life.
Stick With OEM Rubber
All this can take several months to complete and it will usually happen about three times, with each successive submission refining the tire construction until hopefully by the third submission there is at least one construction that meets the targets set by the company for that particular vehicle. These targets, as well as many other vehicle targets, like cost, weight, fuel economy, vehicle size, etc. are set very early in a vehicle program to meet the expectations of the expected customer. The overall vehicle targets then get broken down into targets for each individual system, including the suspension and tires. Often the targets are based on vehicles made by the company’s competition, and also by doing a bit of guesswork on how the competition’s future products might improve — a process called “futuring.” You have to make some assumptions about how everyone else’s products are going to get better in the years it takes to bring a new car to market and make sure your product is as good or better than that. You don’t want to get into a situation where your new car is just as good as what your competition had four years earlier.
Once the engineers find a tire submission that meets (or even exceeds) the vehicle targets, that submission will be released for production. Since so much work has gone into choosing the right tire construction for a particular car, it is almost always advisable to replace your car’s tires with the same ones it came with. Round and black is not sufficient. All tires are absolutely NOT the same and choosing a non-OEM specified tire can have significant effects on the ride and handling of your car. It can also have a very strong effect on steering feel.
The tuning of the tire and the tuning of the steering system go hand-in-hand, as the rubber plays a big role in communicating the steering feel the manufacturer wants. That great steering feel and handling confidence your BMW 3 Series has can quickly disappear if you put some generic aftermarket tire on it. More importantly though, if the replacement tires don’t have the same amount of traction as the OEM tires, you could increase your stopping distances and hurt braking performance. Replace your car’s tires with the same brand and model it came with and you will likely be much happier in the long run.
Now let’s get back to Pieter. The thing I find odd about the choice of a single tire size is that auto companies like to make money. Preferably lots of money. And one way to do this is to get your customers to buy extra things for their car. It’s called up-selling and one of the best up-selling items is wheels. Sell a base car with 18” dull wheels and your customers will spend thousands on optional 19” and 20” wheels because they just look better. If all your cars come with 20” wheels the look of the optional wheels is not nearly as different from the base wheels as it could be. Most manufacturers have calculated that the expense of the added tire development is more than offset by the profits from selling optional wheels and tires. Renault clearly felt differently in this case.
What Renault HAS done here is offer a steel wheel as base and have an optional aluminum alloy wheel. Steel wheels are cheap to make so they help keep the base vehicle cost down but they have significant aesthetic limitations. That’s why most steel wheels come with hubcaps that make them look like alloy wheels. Hubcaps can fall off though, and they can crack and discolor, ultimately tending to look like cheap versions of the real thing. Steelies are also less stiff than aluminum wheels, which is bad for handling — perhaps not an issue for a Renault Grand Scenic, but it might be for some other cars.
So there you have it, Pieter. Enjoy your new free Grand Scenic. As they say, “drive the wheels off it”! En tot ziens.
Crossover Versus Car: Which Is Tougher?
Andrew P. asked a question regarding the “rugged” look of many crossovers and if this actually represents additional capacity in terms of durability versus the regular cars many of them are based on. Well, Andrew, the answer is “it depends.” It depends on the manufacturer and on the vehicle. Some manufacturers do use different durability testing cycles for their crossovers and some don’t. At one of the OEMs I worked for, SUVs, especially the large ones, had a durability test that was a hybrid between cars and trucks. It meant they were tougher than cars but not as tough as trucks. In other cases, though, there was a recognition that the crossover would see no more than mall duty and there was no need to build in the extra weight and cost required to meet a higher durability standard. It really depended on who the target customers were.
Where these vehicles really benefit from a durability standpoint is with their tires. The first line of defense against potholes and other road impacts is the tire, and the larger the tire, the better it is at absorbing those impacts and at protecting the rest of the car. The part of the tire size that matters here is the sidewall height since this is the rubber that gets deformed when you hit a pothole or other bump in the road. It’s very easy to figure out how much sidewall you have by looking at the tire size numbers. Take the first number in the tire size and multiply by the second number and then divide by 100. For example, a P225/50R19 tire will have a sidewall height equal to 225 X 50 / 100 = 112.5 mm.
Now take the 2022 Toyota Camry, which comes with P215/55R17 tires in base trim. That tire has a sidewall of 215 x 55 / 100 = 118.25 mm. The same year Highlander has a base tire of P235/65R18 which has a sidewall height equal to 235 X 65 / 100 = 152.75 mm. That extra 34.5 mm represents a significant extra cushion that the Highlander has before the edge of that pothole hits the rim and does some serious damage. It means that the Highlander tire will protect the rest of the car much better than the Camry tire driving over the same road. That’s not to say that the Camry tire will do a poor job. 118 mm is still more than enough to protect the car on most roads. It just means that it will take a much bigger impact to damage the Highlander than the Camry.
Why Can The Toyota AE86 Out-Handle The Honda Accord?
Jonathan asked me a question about the AE86 Toyota which has what appears to have a very basic suspension setup — MacPherson strut front and live axle rear, while the same era Honda Accord had double wishbones. The AE86 is by many considered to be a much better handling car than the Honda and Jonathan wonders why that might be given the difference in technology between the cars.
Well, Jonathan, what you are seeing is there is more to car ride and handling than just suspension choice. The AE86 may have had a basic suspension but it was designed from the beginning as a rear wheel drive (RWD) car while the Honda was always meant to be front wheel drive (FWD). Designing a FWD car means you make vehicle level decisions that favor a front heavy car. The heavy engine and transmission are all in front with the majority of the engine ahead of the front wheels. Since the transmission ends up slightly behind the engine and the front wheels must be inline with the output of the transmission, they end up further rearward. That’s why FWD cars often have so much front overhang. All this means a weight distribution around 60% front, or worse. That does not favor a well handling car. The AE86 on the other hand could push the front wheels forward because they weren’t tied to the transmission. The weight of the engine would be placed further rearward and the transmission was behind that. All that meant that the AE86 only carried 53.5% of its weight in the front. That’s a big difference and will definitely be felt in the handling of the car regardless of how sophisticated the suspension is.
Well that’s it for this week. Please keep those questions coming to email@example.com and I’ll do my best to keep this discussion lively and informative.