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Bmw 320 Sports Sedan

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#1 terry

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Posted 08 July 2019 - 01:12 PM

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In desperation I come to you for guidance and comfort!

After some weeks ( or is it months?) I'm no closer to bringing this to a point where I'm happy to race it in our Sports Sedan class.

It should be quicker, similar cars in my stable are turning 4.5 - 4.6 sec laps here at Marsden Park, presently I'm about 2/10 the off the pace.

I've used a HRS chassis the car weighs 90 grams, like the others, PGT tyres(like the others) but its a dog.

Can anyone offer any solutions?

Terry

Edited by terry, 08 July 2019 - 01:13 PM.


#2 munter

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Posted 08 July 2019 - 01:48 PM

Pictures of chassis and underside of body would help with forming an opinion.

I have an HRS under my Datsun 260z and it goes well....inline configuration, very smooth and quiet.on NSR rear tires.
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#3 terry

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Posted 08 July 2019 - 02:09 PM

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This is an early pic, the chassis is attached in much the same way, just a bit tidier now.

The thing is that the bare chassis laps here in 4.45 with the body on I'm almost 4/10's slower, but the car wants to tip, I'm loath to load the car with any more weight as its heavy enough now, I'm now using a NSR guide, replacing the shortened down SlotIt wood guide, I've tried NSR Ultras, loosened off the chassis pod and body screws to get some float going, without any improvement.

It seems whatever change I may do it makes no difference to the car, I'm running the BWA NC1 motor, 9:24 gears.

I suppose its my fault I was thinking of doing a brass and wire angle winder but got lazy and used the HRS, like you John I usually have goos results from the HRS, not this time though,

Could it be the body mounting?

Edited by terry, 08 July 2019 - 02:09 PM.


#4 Wobble

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Posted 08 July 2019 - 03:08 PM

Is there enough power in the motor to run a 90gm car on 9:24 gears? Maybe a lower ratio would give more punch out of the corners and later braking at the end of the straight bits, so you mightn't need to go around the corners so hard.
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#5 munter

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Posted 08 July 2019 - 03:42 PM

How heavy is the body?)
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#6 terry

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Posted 08 July 2019 - 04:04 PM

I'm not worried about the motor, its the spec motor for this class, 9 or 10 tooth pinion is the preferred choice

The body weighs 34 gms, by comparison I have a quite quick Panoz Esperante weighing 28 gms and a Mercedes SLS 31 gms,

The Panoz laps in 4.46 the Mercedes 4.59 the BMW 4.81 the BMW has the shortest wheelbase by about 10mm.

Could this be that the BMW's wheelbase is too short causing it to be unstable?

#7 ZeGas

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Posted 08 July 2019 - 04:17 PM

Short wheel bases do tend to tip a vehicle, longer tend to slide.
Minis tip, Cadillacs slide.

To counteract the tipping try repositioning any weight to the outer limits of the chassis just in front of the rear wheels, in the case of the HRS chassis it might have to go a bit further forward onto the wider section.

That will produce a counter balance in corners, the weight will want to hold the tipping side down.
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#8 munter

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Posted 08 July 2019 - 04:20 PM

It does look short and wide. Coated front tires?
Independent rotating front wheels are always helpful in that situation

How is the body attached at the front?
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#9 Orakei Racer

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Posted 08 July 2019 - 06:14 PM

We are running 1/24 Fujimi Mini kitset bodies on a HRS chassis. The body with a lexan type interior weighs 28g - overall weight of body and chassis is around 100g. Main diff is we went with a sidewinder pod and 16k MRRC motor and went Paul gage urethane tires for slightly less grip. Cars are as stable as and runs at a reasonable pace. I think your challenge is you are top heavy so you need to lower COG with some more weight down low.
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#10 Orakei Racer

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Posted 08 July 2019 - 07:19 PM

As per Zegas's suggestions.
Try gluing some strips of lead between the side mounting knobs on the chassis. They are ideally located parallel to the side of the motor which optimises balance for inlines. Maybe add a bit of weight in the magnet holder behind the motor also. Aim to add some weight to chassis to lower COG but also bias the weight bit more to rear to promote a bit of slide when cornering instead of tires digging in at the limit. Also make sure you round the edges of the rear tires - straight edge will just dig into the corners. and cause tipping.

Double check tires front and back are not rubbing on the insides of the guards on hard cornering - you may have to dremel the inside edge of the wheel openings so they are not so thick.

Also check setup of your front axle and front wheels. Because of the cross beam piece on the guide holder, this limits how low you can set the front axle and the guide depth. Don't make your front tires too small in diameter otherwise you won't be able to get them to touch the track - I think I trued fronts on the mini to 19.3mm. You want the car weight on the front tires, not on the guide, to help with controlling the tipping.

Also try some thin soft braid like Sideways 0.05mm braid. Stiffer braid may be contributing to the guide lifting.

It's a fine balance with weight tuning - more weight you use, the more your brakes and acceleration will suffer but on the other hand, if the car becomes stable to the point you are confident to push it and drive smoothly, the fast lap times will come and you will stay in more often. Also try driving with less attack, sensitivity or power on your hand controller, this will help to keep the car driving smoothly.

Edited by Orakei Racer, 08 July 2019 - 07:35 PM.

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#11 gazza

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Posted 08 July 2019 - 08:06 PM

View Postterry, on 08 July 2019 - 02:09 PM, said:

Could it be the body mounting?

You could try having the rear solid (still with the one screw) and loosen the pod screws a bit at a time to give it some body float and a bit of flex.
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#12 terry

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Posted 08 July 2019 - 08:10 PM

Thanks for all your suggestions,

A couple of points, the fronts don't touch the track very slightly above, I have trued the fronts and I do try to use the softest of braid, usually NSR.The front and rear tyres don not touch the bodywork,

I do add weight strips to a couple of my pre 65 cars and that 'tuning" works well, maybe a couple of strips is worth a go.

The front mounting point is just behind the guide holder

#13 munter

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Posted 08 July 2019 - 10:53 PM

Two screws in line...works but leads to body rock where two at the front as wide apart as possible gives float not rock.


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#14 terry

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Posted 09 July 2019 - 06:20 AM

Added more weight last night, things are a bit tight so its difficult to get the space to add without fouling something else, probably added about 5 gms, not a dramatic improvement and the car still wants to tip, all this at a 4.85 lap, way too slow for a HRS car.

I've tried all sorts of adjustments with body and pod screws with no success, I've even had to adjust my driving style in as much as being more gentle in my approach into and out of the corners, I'm starting to think my options with this car may be a chassis change.

I needed to put this issue out there just to see if I hadn't noticed the bleeding obvious but you've all had a similar opinion to my own.

Its weird because I'd always had the opinion that in difficult applications I'd always thought the HRS was the answer.

I'm halfway through a brass chassis rebuild, when I've finished that I'll most likely start a new chassis build.

#15 lenny broke

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Posted 09 July 2019 - 09:50 AM

Hi Terry,
I've used one of these chassis under a monogram E type Jag that I ran in the IPS for a couple of years. I added some weight in the void behind the guide mount, in this case a piece of 1mm brass sheet. I also made some body mounts, 2 fore and one aft and glued washers into the wheelbase adjustment slots to hold the body in position. Also lighten the body as previously suggested.
Alan

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#16 terry

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Posted 11 July 2019 - 12:34 PM

I remember that car Alan, 2011 IPS,

From memory I thought it went quite well,

work continues with the 320





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