IMCA Modified Front End Questions part one

rickd39 Avatar rickd39
Pit Crew
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Nov 13, 2017 at 11:24am
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While at the duel in the desert at the Las Vegas dirt track this weekend we saw some cars running what appear to be some very soft springs in the front.
One person from the midwest told me that some of the top running cars have gone to front springs as light as 300 lbs.

My questions are:

Is anyone running that soft?
I personally have not seen anyone do this myself but I am pretty sure I know why they are doing this. My first guess is that they are coil binding the front with assistance from a fairly expensive shock package that falls within the rules.
The softer the spring the more it’s going to need to be compressed at the same ride height as a stiffer spring, which brings it closer to a state of coil bind. Pavement teams do this, and that’s what bump stops were invented to mimic because of how costly and difficult and time consuming coil binding can be to make work.
The idea is to pass ride height rules on as soft of a spring as you can run. Once you get onto the track the strong rebound shocks let the front end settle down further and further. More emphasis is on the right front, than left, due to the importance of reducing camber change and steering geometry stability, but the principle is the same on both sides. Left sides usually get tied down harder on a stop and go track so they can pull the left front up on exit and increase left side % putting a lot of weight on the left rear.
The thing with coil binding is once the spring is compressed to a state of bind, with certain spring designs there is usually a portion of the spring that is left uncompressed, like if one end is ground and the other is pig tailed, once the spring gets down and binds, it doesn’t quite bind all the way, it actually runs on a tiny portion of that springs coil that doesn’t compress which makes it operate at a much higher rate, at a lower ride height, so it’s not completely solid as some people people think. Here both your compression and your rebound control are absolutely critical to maintaining your wheel rates. You still want to have weight transfer to the back, but if the coil begins to unwind the rate is reduced and you lose drive out of the corner. There’s a really small window there to control that spring. Anyways, there’s many benefits to this.
First is that it loads your contact patch much earlier into the corner. This allows the car to maintain a more consistent level of grip on the track earlier on. It stabilizes the front end geometry which helps the car find its equilibrium sooner, improving overall handling. Secondly it gets the front end super low to the track which has obvious aerodynamic benefits as well as weight transfer reduction in the front of the car.
Depending on the track and the demands of the car, you might go about this a few different ways.
For example if you wanted to have it so the car would tighten up on entry, you would run a regular stiffer spring on the left, and softer on the right, and have higher compression setting on the RF, so that on entry you have big spring split causing the car to de wedge, and generate side bite by loading LF and RR tires more, but once you reach the acceleration portion of the corner the RF still compressing, it reaches its bind state, this will tighten the car up by increasing the cross % a lot, this will give the car more LR loading for exit. The trick here is to have the RF corner eventually come back out of the bind state gradually before the next corner, so that you could do the same thing, otherwise you would just be running on the bound coil and then your entry would actually be much looser when you got onto the brakes. It’s tricky because you’re playing with a really small window on that coil, so you need good compression and rebound control in the shock. The thing with this style of set up is that now you need to start stiffening the RR corner, either through j bar, spring, bar angle and/or indexing adjustments. The best way to do it, is to figure out the springs rate when you put into a bind state, and see if it’s linear or progressive. This will tell you what you need to do in the rear to make the car neutral and if you need to increase RR indexing so that RR loading is progressive or to reduce indexing and keep it neutral at a higher rate.
Remember what you do to the front must be compensated for at the rear. The main purpose for coil binding is aero and weight transfer reduction side to side, and, allowing more LR hike, however it can also be used to change the handling characteristics of the car in different parts of the corner by changing to a shock with a different compression setting, and the corner exit handling and straightaway handling characteristics can be adjusted using the rebound side. Because you can’t run adjustables in IMCA, you need to know your car and driver and the track, and bring a bunch of extra shocks with you. I’ve been working with some guys on a coil bind setup for some time now and it not quite ready because we don’t have the shocks we need due to funding. The math is cheap to do. Lol.
As far as bar angles go, building the front end to be stable is much more important than anything. I wouldn’t go so far as to put ridiculous amount of anti dive in the car, or to make the car resist rolling. I stabilize the camber curve of the right front, using different bar angles and lengths, and then use the left side of the car to keep the overall moment lever arm the same or similar, at least for when the car is at ride height versus when it is entering the corner, and mid corner. Because the right side in this case won’t move around as much, I find it better to stabilize the front end using left side geometry.
When you’re coil binding a RF now... it’s important to pay attention to bump steer and Ackerman because that tire becomes really sensitive to steering inputs. If you’re binding both sides, you really need to step up your game lol!
You’re going to find the teams with more money will do a lot better using this method. It will become harder and harder to beat them because they have the time and money and access to resources. If you have the know how, and are able to do the math and figure this stuff out, which is relatively easy to do, you might get them once or twice a year if you’re lucky, but with limited funding to buy lower control arms, frames, shocks and tires, you’ll be playing catch up with the big guy. This is why I think even at least being able to run schrader valves in the shocks would allow a smaller race team to compete. Instead of changing the shock and needing an expensive shock package you could assemble what you needed out of what you had for much cheaper, you could even just adjust the gas pressure if you needed to instead of changing the shock. Or being able to run bump stops also, would decrease the cost a bit, but then you start to open another can of worms. So I understand to a degree why they don’t let guys run bumps, but the trend moving towards coil bind is much more expensive. I’m 99% certain this is why top teams are moving this direction, now if they really wanted to say that these linear springs were linear only in normal range of operation but in bind became non linear, technically this is against the rules. I doubt these guys would run in full bind and if they are, then they’re doing so simply to get around the technicalities of the rules and deal with the drawbacks as well as the gains.

Could it be a dynamic front setting vs a static setting of front end?
A softer suspension would let the front end work more. Go through it's movement, changing the camber, caster settings?
The camber push going it? More of the tire at that point has more contact with the track. ( Less camber in, body rolls more camber is put into RF. Also as the body raises camber is pulled out of the LF. )
Rough track? It's easy to swap out a shock or two.
I might be wrong, I have no box to think in.
Phil