Thursday, September 11, 2008

Tuning the Sloter Opel Manta

The Sloter Opel Manta has gotten some negative comments concerning its less than perfect representation of the 1:1 car, the unorthodox layout, and the somewhat weak and overly flexible chassis. These complaints are not without substance, but simply dismissing the Sloter Opel as not worth buying is much too harsh. When I looked at the car, I saw an innovative model design of a “not done before in 1/32” Group B rally car. Although not AWD like most Group Bs, the Manta was very competitive on the paved rallies. And since I collect Group B cars, which are the “Can Am” of rally cars, I thought I would take a closer look. The first thing I noticed was the lack of a traction magnet, which I believe is only correct for rally cars, as they are meant to slide and drift.
The unusual aspect of the cars layout is the wide angle, anglewinder (AW) SCX Pro Speed motor set up. The Pro Speed is one of the most powerful SCX motors (20k rpm), but it is still controllable when running with no magnet. I actually prefer SCX motors for rallying because they have a smooth powerband without too much torque and still have a good top end rev, which is good for rally because it is easy to drive. This also makes for a fun car to just race with a friend. The AW layout moves the center of gravity (CG) back slightly compared to an inline, improving rear grip, but also gives more room for the drop arm guide within the stock wheelbase.

The AW uses what appears to be a standard inline crown gear and pinion. Because this gives a smaller contact between the gears, the gear mesh is critical to avoid damage. Mine was a bit on the loose side, so some adjustment was in order. By carefully squeezing the rear axle just beside the crown with a pair of needle nose pliers, then twisting the pliers slightly, you can move the pressed on gear slightly on the axle adjusting the mesh. Do this in small steps to avoid making the mesh too tight. Mine runs very smooth and quiet after setting the mesh to minimum free play. Since the gear seems to be just a standard inline crown, aftermarket inline crown gears should also work. The mesh would also be much easier to adjust with a set screw type gear.

The drop arm style guide may be new to some people who aren’t familiar with rally cars or vintage slot cars, which also sometimes use this style. The drop arm allows the car to traverse bumps and uneven surfaces, as found on rally tracks, while keeping the guide in the slot. If properly adjusted, this style of guide should have no negative affects on the cars operation. There is a screw mounted on the chassis just above the guide which limits the upward travel of the arm. This should be adjusted so that the guide is fully engaged in the slot when the car is sitting on the track. Use either a set up plate or an extra piece of track to check this adjustment.

The drop arm mechanism includes a small spring which ensures the arm will drop and follow the surface of the track. This spring should be very light and not put excess force on the guide, which might cause the front of the car to lift off the track. I found the spring was a bit too strong and made a slight adjustment by bending the spring a little flatter than it was originally. I also managed to break the chassis were the arm attaches, but made a fix by gluing on an old axle bushing. I have been told that Sloter is aware of the weak chassis and is sending free improved replacement chassis’ to customers on request. I have made a request and will report back when it arrives. This is some very impressive customer service.After adjusting the drop arm, I noticed some slight interference between the guide and the body when the guide was all the way up. The guide was moving through it’s full travel, but it was contacting the body behind the grill which made for some inconsistent drifting action, which is critical for a rally car. It was an easy fix to take the Dremel and make an arc shaped relief on the back side of the lower part of the front valence and grill. The relief did cut through part of the grill, but because the grill is all black plastic, the hole is not visible when the car is assembled.

I also did some standard tuning tricks, such as truing the wheels and tires on some sand paper. I added some lead on either side of the motor because the car had good grip (with stock tires!), but tended to roll at the limit. I installed some low profile front tires to eliminate some interference with the front fenders. With these changes, the Manta went from an erratic mid pack car to one of the quickest 2WD rally cars I have. I hope the new chassis makes the car even better and will report back when I am able to try it. So although it is far from perfect out of the box, with some relatively simple tuning, the Manta joins Sloters other cars as some of the quickest non magnet slot cars available.

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At 12:43 PM, Anonymous Hugo Figueiredo said...

Very interesting stuff to work with...

I've always been eager to buy one, but the issues of the car kept me away from it. Now that I'm more into rallying, and with your suggestions, perhaps I'll throw myself into getting one...

At 5:27 PM, Blogger Robert Livingston said...

I have a Manta, also, and it is a very nice handler, now that I have replaced the motor with an SCX RX-42B, and immobilized the drop arm.

I only changed the motor because it seemed too fast for this kind of car, compared to my other cars. But that swing arm is trouble, as the car deslots with a working swing arm, on a down hill, braked sweeper

At 2:36 AM, Anonymous Anonymous said...

I'm probably going to change the motor to a standard SCX also, as the Pro is a bit much. The drop arms work ok if set up and actually keep the car from deslotting over hills IMO. There is a bit too much play in the Manta arm, so I hoping the replacement chassis helps.

At 9:05 PM, Anonymous Anonymous said...

Why is it that the manufacturer's can't seem to get it right out of the box? This failure to be able to market their own car in a ready to race condition is wearing us all down. We, those of us spending our hard earned dollars on product, shouldn't have to do the basics to get the car in good running condition. Sorry, but only cars that are prepped properly will be getting the green from my wallet.


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