Winter is over and with it, the riding season (except for you mountain riders) What's that you say? You still feel the need for some speed? You're not ready to put your sled away yet? Well, there is an answer and it is asphalt drag racing. Yes, I said, asphalt drag racing. The popularity of this sport in the last three years has grown tremendously. Three years ago I took a snowmobile to my local drag strip to race. On a typical Sunday it would be a 'big' turnout of there were nine sleds. The following year, twelve or thirteen sleds was the norm. This year, there has been at least twenty sleds at the track every weekend. It's easy to get involved. It is relatively cheap and very easy on your equipment. So what are you waiting for?

Ahh… you're waiting for some guidance on where and how to get started? If you're like most new asphalt racers you will be running your trail machine on the race track in the "bracket" class. But before you can do that, there is some preparatory work that must be done. You will have to use a purpose made track and skis for the asphalt, otherwise your trail machine will get you in the race just fine.

This is what the NHRA rulebook says about suspensions:

"All snowmobiles must have a minimum 1-inch of travel in the track suspension. Only steel springs will be permitted. Externally activated suspension systems prohibited. Hyfax must be removed. Bogie wheels must be used on each side of the rear suspension to prohibit the slide rails from coming in contact with the rubber track surface. Use of any external slide rail lubrication system prohibited. Steel coil over or leaf spring permitted."

Why Change The Suspension Setup ?

Suspension setup is the first thing you should work on when trying find the quickest way to the finish line. All the engine work and clutching in the world won't get the job done if you don't have your suspension dialed in. In working with one racer who was running a stock 97 mach Z, we were able to take his sled from 11.3s to 10.7s (1/4 mile times) by modifying his suspension as detailed in this article. That is a .6 second decrease in elapsed time with NO changes other than suspension. The sled cuts 1.39-1.41 60 foot times fairly consistently. That's pretty good for an essentially stock 750lb sled/driver combination. Before the suspension changes that sled was cutting 1.7x 60 foot times. Rule of thumb is whatever you can reduce your 60 foot time by will result in about twice that amount in ¼ mile ET reduction. So suspension setup is free ET.

This will be a bit different for each sled/rider combination but it demonstrates the importance of proper suspension setup. This particular sled was dipping down into the 10.3s last year after the driver started working with clutching and gearing.

I race with Ski-Doo snowmobiles so this article will give some specifics for the SC-10. However, the same overall principals will apply to other OEM systems.

The main objectives are to lower and stiffen the front as well as add wheels and lower the ride height in the rear, while staying within the NHRA rules. We want to transfer weight, but not yank the skis in the air. Keeping ski lift to an inch is plenty. Any more than that and you will lift your skis out of the timing lights. This will end up giving you a red light start. If the sled is going up, it's not going forward. On asphalt, you do not need a skis-to-the-sky launch.

You don't want the skis in the air as many would think. The highligthed area shows the desired ski lift. Click picture for larger image

Lower the ride height of the sled does three things:

- It lowers the center of gravity, making weight transfer less dramatic.
- It reduces the front "profile" of the sled, thus making it more aerodynamic
- It reduces the approach angle of the front of the skid/track. This reduces the friction of the track rolling around the skid.

Reducing the front profile, or "frontal area" and the resulting reduction in aerodynamic drag is an important factor to consider. The horsepower required to over come aerodynamic drag increases according to the cube of the velocity. What this boils down to is that it will take a lot of hp to overcome aerodynamic drag. So simple changes that reduce drag should be attended to. You can reduce the frontal area of your snowmobile by using a low profile windshield. When a low windshield is installed, you should crouch down behind it, keeping you head up just enough to see where you are going. Keep your elbows tucked inside the front profile of the sled. At 100 mph, the small aerodynamic improvements of lowering the sled, low windshield and crouching behind it can be worth 2-4 mph in top speed as well as a reduction in ET. In a sport where thousandths of second can mean the difference between winning and loosing, every tiny bit counts.

No matter what brand of sled you are working with, before you begin changing parts you should have a copy of the manufacturers race manual. These manuals are packed full of information and part numbers that you cannot get anywhere else.

Front Suspension

First thing here is to consult your race manual and find a shorter spring. Springs are relatively inexpensive, do yourself a favor and purchase a proper spring. Do not cut coils off a longer spring. Cutting down a spring does not reproduce the original manufacturers end of coil configuration.. The end of the coil springs must be flat to seat properly in the spring retainer and to ensure the force is applied equally around the spring seat. You don't want any springs coming out of place at the 1320 foot mark. The pavement is not nearly as forgiving as a snow bank is when you hit it at over 100 mph.

On Ski-Doo snowmobiles, for the F2000/S2000 chassis, the spring of choice for the front end is p/n 415-0207-00. It is identified with 4 red lines. It is a 150lb/in spring 200 mm long. On the F2000 triples the stock front springs were 100lb/in and 265mm long. Using the 0207 spring will drop the front of the sled 2.5". This is about as low as you can go with the front end.

The shorter springs need to be retained in the spring collars. The stock springs were long enough so that at full extension of the front end, the spring kept tension on the shock and would never fully unload. With this shorter spring, if the front end of the sled were to lift up and the shocks extend out, the spring will fall out of the retainer because the shocks are 2.5 inches longer at extension than the spring.

There are a couple of ways to cure this. If you have HPG Take Apart (HPG/TA) shocks, these can be disassembled and a 60mm long spacer can be installed. That will allow the shock to work with the 0207 spring without a limiter strap. For Ski-Doo HPG/TA shocks, the kit is PN 861-7442-00. Note that this kit is for "HPG/TA" shocks, not "HPG" shocks and it is for F2000/S2000 chassis. CK3 and ZX chassis use a different spring and shock kit. Consult your factory race manual for part numbers. Unless you have a full compliment of shock rebuilding tools, this is an option that you should leave to your dealer to do for you. All take apart type shocks (Fox, Works, etc) have this tuning ability. Consult the manufacturer for specific instructions on how to do it.

 

Looking at the enlarged image you can see the limiter strap on the front shocks. A simple but very effective solution to customize your front suspension. Click picture for larger image

If you do not have take apart shocks then you must use a limiter strap that will physically limit the amount of extension travel your front suspension will have. Typically limiting straps are either a chain or an actual section of skid frame limiter strap. I use a section of limiter strap bolted to the top and bottom shock mounts. For F2000/S2000 Ski-Doo sleds, the shock bolt-to-bolt center distance should be 11 inches. This will be just long enough to keep some preload on the spring at full extension so it does not come out of the retainers.

Rear Suspension

The Ski-Doo SC-10 rear suspension is lowered by using a shorter center spring, adjusting the limiter strap length, compressing the rear scissors and adding a limiter strap or chain to the scissors to keep it compressed. Let's not forget that we need to add wheels !

When it comes to wheels, there is no set rule on where or how many to use. I used a combination of oem wheels and Wahl Brothers asphalt wheel kits.

 

A combination of OEM and Wahl Brothers asphalt wheels are installed on the SC-10 skid. Click picture for larger image

In the photo of the SC-10 skid, the rear axle is not installed. However, I recommend you use 4 rear idler wheels. Use the largest rear wheels you can fit. Ski-dDoo SC-10 skids do not have any kits to use larger than stock rear wheels. However, Arctic Cat skids can have their rear wheels replaced with 8" wheels using an inexpensive kit. A couple of tips learned from racing:

- Do not use the oem aluminum wheel with rubber laminated onto it. The rubber will de-laminate from the aluminum from the heat generated during racing. When possible, use all plastic wheels.
- Wahl brothers offers several small size wheels. Use the 3-1/4" all plastic wheels. These are made from medium density Polyethylene. They will not come apart like the small rubber wheels.

When installing your wheels, make sure that they will clear all components when the skid is compressed!

Last year I left the ACM installed in my SC10, however it was set on full loose. This year, the ACM is being removed since it does nothing when the skid is so far compressed. With the ACM removed, there is room for more wheels. Also, I installed 4 OEM wheels on the outside of each rail. Part way through the season I removed them. There was no recordable difference in ET or MPH with and without them. I did leave the small Wahl Brothers wheel on the outside of the rail toward the forward end. Along the attack angle of the rails, the more wheels you have the better to ensure smooth transition of the track off the drivers and down under the trails. The last thing you want is the track rubbing on the rails in this area.

When installing your wheels, set the skid frame up on a flat bench. Make sure it is flat and no twist in the trails. When drilling holes for new axles, make a template of some sort. I used a piece of mylar (clear plastic sheet) that fit inside the I beam shape of the trail. The location of all the holes I needed to drill was marked on the mylar. It was lined up with some factory holes, center punch for new holes. Then transfer the template to the other rail, line up on factory holes, center punch again for new holes. This is as close as you're going to get with hand tools.

Make sure you install some "stab" wheels at the front of your skid. "Stab" wheels are used to prevent the front of the skid from "stabbing" the track. While the sled is under acceleration, the front of the track is actually ballooned out away from the leading angle of the skid. However, when you cross the finish line and left off the throttle,

The Limiter Strap Quick Adjust Kit, p/n 861-7602-00, will allow limiter length adjustment via a hand knob. Click picture for larger image

the sudden reduction in power makes the track suck back up. If the track is loose and you don't have any stab wheels installed, you 'could' catch the skid the inside do the track. Not a good thing to have happen.

The same technique used on the front suspension works on the center spring of the skid frame. Here the spring to use is PN 415-0206-00 with 4 green lines. It is slightly softer than the springs used in the front. This spring is 125lb/in. Again, if you have HPG/TA shocks, you can have your dealer install a spacer. If not, the extended length will be controlled with limiter straps anyway. You can cut down your OEM limiters and drill new holes in them, or if you are using a Ski-Doo SC-10, you should get a "Limiter Strap Quick Adjust Kit". This is p/n 861-7602-00. This kit will allow limiter length adjustment via a hand knob.

The baseline setting on the front of the skid will be to pull it down so there is 1" of travel left in the arm. On an SC-10, this will bring the front arm down to almost horizontal as you can see in the photo to the right.

 

Enlarge to see the horizontal angle of the front arm and the 1" of travel still present. Click picture for larger image

At the rear end of the skid, I used torsion springs from a 97 mach1 p/n 414-9435-00 LH and 414-9436-00 RH. The Ski-Doo Race Manual recommends p/n 486-0785-00 LH and 486-0786-00 RH. These springs are softer than the mach 1 springs. Keep in mind the race manual is addressing grass and ice racing. On grass you need a softer setup to maintain compliance with an uneven race surface. On asphalt, the race surface is as flat as you're going to find. I've tried both springs and my results were better 60 foot times with the stiffer mach springs. When installing the springs, also use spring bushing p/n 572-0861-00. These get installed inside the coil of the torsion spring. They are a plastic bushing that keeps the coil from cocking out of alignment when the skid is compressed.

 

Enlarge the photo and you can see the spring bushing. These are the same bushing used in the MXZ 600 Project Sled when we installed studs to keep the springs from bending over. Click picture for larger image

The hardest part of the entire modification is chaining down the rear of the skid. I have found that it is easier to install the skid with the rear unrestrained, then squash it down and chain it after it's installed in the sled. You'll need two of your biggest friends and one of your smallest. The big guys are used to squash the sled, while the little guy gets to put his hands in skid and attach the chain. Careful here, you can get your hands hurt doing this!

When setting the rear height, I found my best 60 foot times are when the rear scissors arm is at the same angle as the front arm. With the springs I've described this should provide two inches of travel in the skid. The skid will be very firm. It will take considerable force to compress it two inches but it will compress. Shown in the photo is just how close to horizontal the rear arm is on the skid in my sled.

The chain in the photo keeps the suspension compressed. Notice the safety wire on the clevis hooks. ALWAYS safety wire the clevis pins. Loctite is not enough to keep them from coming unscrewed. Click picture for larger image

To hold the rear end down I used threaded clevises. One clevis on the cross member of the ACM and the other on the upper arm, next to the grease fitting. I used a single 1" chain link between them. Make sure you safety wire the clevis pin in place. These WILL vibrate out. If you loose one, the rear end of the sled will pop up instantly.

I run a 1997 ski-doo Mach 1 chassis. This sled, with me on it, weighs in at 800 pounds. I run a 170hp 700cc engine. My Mach ton routinely cuts 1.40-1.42 60ft times with this setup.

 

Comments from the Section Editor. As always some of the pictures or adventures we describe on Snowmobile Online at Off-Road.com are not to be taken lightly, since you can get yourself killed. Drag racing a snowmobile can be as dangerous as drag racing an automobile or motorcyle. In Steve's opening photo, he is shown on the race track wearing blue jeans. Before I get hundreds of email telling me how unsafe this is, blue jeans are legal under NHRA rules. The NHRA rule book states, "Leather jacket, leather gloves, leather boots over the ankles snell 95 or better helmet. Full leathers are required if running quicker than 9.99 or faster than 120 mph." Off-Road.com and Snowmobile Online suggests that full body racing leathers be worn for all drag racing on asphalt for the obvious reasons, but they are not required by the sanctioning bodies. Matthew Baynard

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