Stopping the Vibration - SSWCT-001
This week in our Sprinter Upgrade Series, we'll be focusing on something slightly different: the development of our SSWCT-001 Sprinter Wheel Centering Tool.
After we had finished testing the front strut upgrades on our own 2021 Sprinter Class C, we had a customer with a 2018 Sprinter come into our shop due to a vibration issue. She had been to multiple tire shops to have her tires rotated and balanced and at some point replaced, but the issue was never totally resolved.
We drove the coach with her on our usual Road Performance Assessment test route. This includes back roads and four lane 55 mph highway. Since she noticed her issue at higher speeds we also ran the coach on the interstate to get up to 65 mph.
While driving the coach with her, we noticed a bit of vibration from the front tires at highway speeds, although it was fairly minimal at that time. We got the coach back into the shop and noticed some uneven tire wear (out of round, not due to an alignment problem) which we believed may have been contributing to the vibration. She decided to get new front tires for the coach from a nearby tire shop, hoping they would resolve the vibration issue but also so she would not have to worry about how badly worn her current tires were.
The next day before taking the coach to the tire shop, we decided to take it out on the highway again just to make sure nothing had changed. Surprisingly, the issue was quite a bit worse in the morning - at about 45 mph, a very noticeable vibration in the front tires showed up, causing a fair amount of shaking in the steering wheel.
After the test drive we headed to the tire shop. Since the tires had been replaced in the past without fully addressing the vibration issue, we worked with the tire shop to make sure that these new tires were correctly balanced. After being spun balanced once, each wheel was removed from the balancer, rotated, and remounted before being spun up again to ensure that the machine was still showing the tires were balanced. This eliminated the possibility of improper mounting on the balancer.
With the new tires mounted and no doubt that they were correctly balanced, the wheels were reinstalled on the coach. We then repeated the same test route as had been followed earlier that morning. Once we hit 45 mph, wouldn't you know it - the vibration was back, just as bad if not worse than it had been in the morning!
At this point, we went back to some old-school tech to address the issue. We still have an old Hunter wheel spinner for on-the-car balancing, as was done in the 60s. For tricky vibration issues we'll still use it from time to time, and this issue was definitely a tricky one. We got the front end of the coach off the ground and used the spinner to run the wheels up to highway speed. Unsurprisingly, the wheels shook like crazy once they got going fast enough. We then used a trial-and-error method of adding and moving wheel weights until the wheels were adequately balanced. A follow-up test drive confirmed that we had addressed the vibration issue. Still, the question remained, why were the wheels not balanced when installed on the coach, when we had just confirmed they were correctly balanced at the tire shop?
Later that same week, we had another customer come in the shop with a 2016 Sprinter Class C. Based on our recent testing, he decided to install the 87-2657 front struts along with our recommended rear shock and sway bar upgrades. Once all the work was complete on his coach, we took it out for a drive to compare our results on his coach with that of our own 2021 Sprinter chassis. While we were pleased with the handling of the coach, we ended up feeling the same kind of front end vibration on his coach as on the 2018 Sprinter earlier in the week! The customer was sure that vibration had not been there before getting the work done.
At this point, we had a hunch about what was going on. Nothing had been done to the tires on this coach, but the front wheels had been removed in order to install the new front struts. Was it possible that by simply removing and reinstalling the front wheels, we created a vibration issue?
We again balanced the customer's front tires on his coach and test drove it to confirm the vibration was gone. Afterward, we started examining our own coach. Our Navion came with aftermarket alloy wheels, but the previous two coaches had OEM Mercedes steel wheels. One thing the alloy and steel wheels have in common though is that they are designed to be hub-centric. This means that the hole in the middle of the wheel is designed to fit precisely around the wheel hub, keeping the wheel centered.
A close examination of our own wheels revealed that they were not quite as precise as one would hope - the diameter of the hole in the middle of the wheel was at least 0.009" larger than the outer diameter of the hub. While this may not sound like much, this means that depending on how the wheels are installed, they might end up off-center enough to cause a vibration just like an out of round tire would (possibly worse, since you have not only the out of round condition but also out of balance since the wheel and tire assembly are no longer rotating around their center axis.
To test out whether or not this could happen, we removed one of the front wheels and reinstalled it as an average mechanic might, without paying attention to how centered the wheel is. We had used our wheel spinner before removing the wheel and confirmed no excessive vibration. We spun it again after reinstalling the wheel, and as expected, we now had a vibration.
We now knew that we needed a way to keep the wheel centered when installing it. There are products available for other application, such as Tru-Balance, which address the same type of issue on larger coaches. However, there was nothing that we could find which would fit the smaller wheels of the Sprinter chassis. So, we decided to come up with our own.
We started with tapered lug nuts, and had them machined to a precise taper to come up with our SSWCT-001 Sprinter Wheel Centering Tool. The idea is simple - after getting the wheel on the studs, you thread the centering tools onto the wheel studs at the 12, 8 and 4 o'clock positions. They are then carefully tightened by hand until the wheel is centered on the wheel studs - more precise than what the hub-centric design allows. Three factory lug nuts are then installed and tightened to lock the wheel in place, at which time the centering tools are removed and replaced with the remaining three lug nuts. One set of the SSWCT-001 comes with three machined nuts, which can be carried in the coach and used anytime the front wheels are removed and reinstalled.
We tested these out on our own Navion and found that it resulted in a uniform gap between the hub and wheel, meaning that the wheel was now truly centered relative to the hub as well as the wheel studs. A test drive confirmed that we had finally come up with a reliable solution to the vibration issue!
If you are fighting a stubborn vibration issue on your Sprinter, we hope this article was helpful in giving an idea of what might be causing it and how to solve it.
Please check back with us next week as we continue our series!
Sprinter RV Chassis - Better Steering and Handling (Filmed back in 2022, earlier in our testing process)