December 20th, 2017 by Matt Holden No Comments
Last week I teased the next batch of articles that will cover the setup process of our own in-house Class C Truck over the Winter season. Since this is week 1 and nobody on the team has raced a truck in over a year and a half, we had to start from scratch. Since the first week is at Myrtle Beach, we also ran into the problem where none of us have raced there before, so we had that to contend with as well. Despite that, we were able to pull from our Class B Xfinity notes and eliminate most of the problems in the truck to get ready for the first week. Do we expect to win with this? I don’t, but anything’s possible!
As of writing, the truck hasn’t been raced yet, so we don’t know where it is on pace and we don’t really have a grasp on how it will behave through the events. Still, that’s part of the process, so if we can get out of Myrtle Beach with information and notes on what happened, we can take that straight to IRP and hopefully implement updates to the chassis for that race due to the similarities between the two tracks.
Front End Configuration
The options for the front end at Myrtle Beach were quite varied, so we had to look at the track characteristics to see which we wanted to prioritize and place higher on the “give this a whirl” list. Without bump springs in the truck, we’re really limited to four configurations possible for any track configuration:
- Double Coil-Bind
- LF Coil-Bind
- RF Coil-Bind
Each of these options has inherent advantages and disadvantages, most dependent on the track itself. We knew the track was relatively slow, it has a rough surface, and it’s very flat. All of these played into what we wound up choosing for the front end.
We were able to eliminate the Double-Bind option immediately due to the track’s rough surface which would cause a lot of headaches in tire life. The solid suspension in a double-bind suspension is great for speed and splitter control, however we knew from the Class B series that it was hit-or-miss on tire life. With the new build seeming to shine a spotlight on tire management, we decided not to go this route.
A conventional setup makes a lot of sense for such a bumpy track since it keeps the suspension moving at all times. Plus, the extra rebound in the front shocks seems to allow larger springs to be used without the front end rising up too far on the straights. We decided this would be a last-ditch effort if we couldn’t get the other two working.
Lastly, we have a single-bind front end configuration, which we’ve actually used before in the Class B series in 2016. When the car had bump springs added, everyone on our team bound one spring but ran a bump spring on the other so we had a lot of notes to pull from. We only ran these at Texas and California before figuring out how to re-work the cars for double-binding, so our notes covered bumpy tracks. Looking through these, the notes showed that cars with LF-bind setups had issues with bouncing excessively at both tracks, while the RF-bind cars didn’t have such problems. A quick test in the truck confirmed the LF-bind setup as being a little too bouncy for us to commit to, so we threw a 300lb/in spring in the right-front, a 600 lb/in in the left-front, and got to work.
Pogo Stick – Solved
We ran into some issues early with bumps towards the center of the corner, especially down in Turn 3. This was bad enough to cause the front end to lose grip and slide, evidenced by a loss of lateral G-force despite increasing steering angle. We then ensured the splitter was not touching the track at all (which could set off an uncontrolled oscillation in the suspension) and isolated the suspension to sort out the problem. We also compared damper position traces to ride height traces to see if either suspension was moving with the bounce and discovered that they weren’t. In fact, both suspension systems appeared locked in place through the whole ordeal. Not an easy fix, but one we’ve seen before. To work through the issue, we removed all rebound from the left-front shock (to eliminate tie-down issues) and went through these steps:
- Spring Rates: Changing the spring rate on either corner would alleviate or exacerbate the bounce if it was being caused by the spring. Changes to the left-front spring did not alleviate the issue at all, however increasing the right-front spring appeared to reduce the severity in the center of the corner. We settled on a 320lb/in right-front spring to allow it to open up without being bound solid through the bumps. This also required a reset on the ride heights (the spring allows more travel than a 300) and only one extra click of rebound to keep it where we had it.
- Bar Diameter: We swapped the solid link for a chain to keep sharp impacts from the left-front wheel from getting to the right-front wheel. It was nice but didn’t help…at all. A reduction in bar diameter stabilized the front end a little bit, but also produced no change in right-front height but huge changes in left-front bounce height. This was a big deal, a sign that whatever was happening stemmed from the left-front suspension.
What finally worked was a reduction in crossweight to preload the left-front spring more and keep the truck a little more solid. We had started the crossweight at 57% but dropped it to 55%, a change that eliminated the repeated “chirp” from the left-front tire, a sign that it was leaving the racing surface, and was worth around 6 tenths, likely from just having four tires on the track instead of three! From there, we simply set the nose height to just touch the track on fresh, low-pressure tires but have it clear the track after a few laps. Nose weight was set to simply balance the truck with these settings, and we wound up with 51.0%.
Shocks, set to just barely hold on to the front end, are 10/25 on the left-front and 20/15 on the right front. Splitter lift out of the corner is perfectly fine at a track like this where corner speed is going to outweigh the top speed, plus having less rebound on a bumpy surface will help with tire wear.
For those interested, we basically carried camber and caster values over from Class B coil-bind values, setting the left-front as high as possible and right-front to where the Inside and Middle of the RF tire match. Your mileage may vary, but this produced -4.5° of camber. Caster was 7.0/8.5, and the front tires are toed-in slightly.
The rear-end is actually where we had no problems at all, since this is fairly straightforward. Short tracks offer the use of a rear sway bar (which we used religiously in the days of the COT), but we opted to leave that in the hauler. Instead we went with a pretty simple rear spring package, a 200lb/in left-rear and a 650 lb/in right-rear, setting the rear heights to put the skirts just above the track at full-load. The left-rear shock is a high-rebound shock with 9 compression and 17 rebound to help de-wedge the truck under braking, but ideally we’d like the rebound to be around 10 or so since this is a bit of a crutch. The right-rear shock is a low-rebound shock with 19 compression and 2 rebound to aid in turn-in when rolling off the brakes. Rear toe is full skew (4/16) on both wheels, mostly because we would rather go with what we know right now instead of running a straighter amount to help drive off from the tight corners. Hopefully we can get that worked out by week 4 at IRP as well. The track bar was full rake, as high as we could get it, so 12” and 15”. Truck arms were at the bottom.
The pressures are, admittedly, a shot in the dark. Nobody on our team has raced this truck in a year and a half, so we went with a comfortable set of pressures to suit the rest of the setup. The left sides are set to 14psi to put the center temperature in good agreement with the outer temperatures, and the right-sides are 39psi to do the same. Test runs are showing the buildup pressures are showing slightly tight, with the left-rear and right-front building up more than the other two which is expected. Should they start showing issues during the races, we can change them pretty easily during a pit stop.
This being the first race on the schedule, the drivers we have will all be on a very similar setup. The problem with this is that Alex, Jeff, and I all drive completely differently from each other, so one of us will likely do quite well while the other two just exist on the race track taking up space. Luckily, we can do quite a lot to tailor the car to each of us with just shocks and crossweight and noseweight adjustments. The springs, however, will likely go un-changed before IRP.
It’s very likely that the three setups start going in a different direction after Myrtle Beach as we figure out what each of us wants the truck to do. Different crossweight/noseweight combinations, different shock packages, and even rear spring choices could send each of us in a different direction, but that only helps each of us in the long run by having a notebook to fall back on. Considering how this truck was built off of notes developed from the Xfinity car, that can only be a good thing!
We should have gotten at least one race under our belt by the end of the week, likely Thursday or Friday, so the article following the races will be about what we’ve found and what needs to be addressed. The schedule for the winter season consists of three flat short tracks (Myrtle Beach, IRP, and Milwaukee) that this chassis will be used on, so developing the chassis quickly and effectively will be paramount.
Of course, there is the chance that it’s trash. If the setup is indeed a total bust, we just rebuild it based on what went wrong during the race by making notes of how the setup didn’t perform well relative to the rest of the field. Simply saying “no good” and starting over with no information won’t help at all, but knowing what went wrong will help to eliminate many issues and give a good starting point going forward.
But hopefully it’s not trash!
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