Author Topic: Weight and Tyre Size  (Read 6071 times)

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Offline Paul K

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Weight and Tyre Size
« on: November 08, 2016, 11:08:16 pm »
A bit of guidance required for a newbie four-wheeler.  I've competed regularly on two wheels so I'm probably as good as I'm ever going to get on those. I've also competed on three-wheels with similar competence.  I'm now trying four wheels in the shape of a 1300 cc Beetle in class 4.

My daughter and me recently competed in the Falcon Motor Club's Guy Fawkes Trial, which is similar to an MCC Testing Trial from what I remember.  The Beetle was set-up as for an MCC main trial; e.g. bash plates front and back; two spare wheels overhanging the rear; a spare wheel on the rear seat; rear parcel shelf stacked with trolley jack, tools, bead breaker, spares, oils; and 165 mm wide tyres on the rear wheels.  There were times when the engine was labouring to pull the car up a slope without slipping the clutch and increasing the revs, and times when the wheels got no grip and just spun (or smoked).

Accepting that the driver could well be at fault and wanting to stay in class 4, would my chances of cleaning the sections have improved if:

1.  I had discarded all this weight and had the car as light as possible, i.e. remove bash plates, spare wheels, tools, etc?
2.  Fitted the standard 155 mm wide tyres on the rear wheels, effectively lowering the gearing and reducing the rolling resistance of the wheels?
3.  Got my daughter to drive and me acted as the passenger?  :-\

One other factor was the engine would not idle and I had to maintain a fast idle using the throttle pedal, though, I don't think this was much, if any, of an influence on our performance once we were in the sections.

Offline Alastair Queen

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Re: Weight and Tyre Size
« Reply #1 on: November 09, 2016, 03:13:29 pm »
<<<One other factor was the engine would not idle and I had to maintain a fast idle using the throttle pedal, though, I don't think this was much, if any, of an influence on our performance once we were in the sections.>>>

Ignition timing might be out?  Inlet manifold might be leaking? These will affect overall engine performance to an extent.

Class 4 is really the only class where power-to-weight ratio is of importance.....being the only class whereby engine size is limited.

Obstructing the rear seat with stuff, limits how a passenger can transfer their weight, from one side to another.

I'm not sure simply going down one size in rear tyre will affect the overall gearing much..  especially if tyre deflation is taken into consideration?  I feel the size of the footprint made by the tyre is important.....one can go right up to 185 section...providing  {I think???] there are no more than 2 sections difference twixt rear & front tyres?

Offline Paul K

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Re: Weight and Tyre Size
« Reply #2 on: November 09, 2016, 07:49:09 pm »
Ignition timing, valve clearances, spark plugs are spot-on; well, they were about 500 miles ago.  Inlet manifold leaking; that's possible.  The inlet manifold seals are a bugger to do if they are leaking, but I can test with some highly flammable aerosol sprayed over them  :) .

As regards tyre footprint, there's a Yamaha PW200 dirt bike with a 180 mm wide rear tyre that's reputed to be rubbish for grip because the tyre is so wide.  I'm sort of buying into that line of thought.  The 165 mm tyres on the Beetle do make a difference to the gearing; you can see the difference when you stand the 165 and 155 side by side.  Plus on the road, the car feels like it's labouring at times, depending on the slope your ascending, and acceleration is not what you'd call mind-blowing.

I wouldn't go up to a 185 mm on the rear with my 1300 cc; the fronts would also have to increase and that would reduce the steering lock.

I noticed that not many cars on the event had spare wheels overhanging the back.  The Hillman Imps especially had nothing added that I could see.

Hence my question; would a lightened Beetle have done better on grass than a heavier Beetle; and even better with 155 mm tyres that would have lowered the gearing and 'bitten' into the ground?  And finally, the unthinkable, even better with a different driver?

Offline Simon Woodall

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Re: Weight and Tyre Size
« Reply #3 on: November 10, 2016, 11:24:50 am »
Leaking manifold boots are easiest checked by spraying WD40 on them.   If the idle speed increases when sprayed, you have a leak.

The Imps probably didn't spare wheels out back because they are set up for PCT's (sorry, Car Trials) where such configuration is not allowed.  The usual reason for reducing weight on the back is to improve steering, which is an important factor on this type of event.

I think I am right in saying that your engine is completely standard.   Roughly 44 bhp from the factory.   Bear in might that the "hotshots" in this class have nearly double that - 70-75bhp.   It still should not struggle as you describe.  Do you know what the final drive is?
You can tell the final drive from the gearbox serial number;  you should have a 4.3 final drive which is determined by a serial number starting AB or AM.   You can find the serial number on a flat part of the casing on the right hand side almost total obscured by the mounting forks.  Other final drives are 4.1 or 3.8 both of which are too high for a 1300.

Offline Paul K

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Re: Weight and Tyre Size
« Reply #4 on: November 10, 2016, 04:59:58 pm »
I didn't know that spare wheels out the back were not allowed for PCTs; the scrutineer never mentioned anything about removing them.  The lack of weight at the front end would explain some of the collisions with the course gates  :) .

I'll have to get under the car to find out the gearbox prefix  ??? . It came from the factory, I believe, with a 1200 engine, had a 1600 for a bit and now has a 1300.  I suspect  whoever put in the 1600 engine would have kept the 1200 gearbox for the lower-ratio final drive. But that is all guesswork; a poke about underneath should let me know for sure.

Offline Jonathan Laver

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Re: Weight and Tyre Size
« Reply #5 on: November 18, 2016, 10:52:54 pm »
Hi Paul,
Who knows ?  Having driven a wide range of cars in trials and having not been very effective at getting up sections, one always questions the machine and its set up. It is fair to say that the practice of having to slip the clutch to climb the majority of inclines is only going to end in mechanical failure. What it is saying is that your machine does not have the power/torque to take the weight of the machine up the hill. Without getting more power or more torque by making mechanical changes you have other options 1/Lighten the machine  2/increase the tyre to surface slippage by increasing the tyre pressure 3/ Change your driving technique and maintain your initial momentum (drive it like you stole it). Or all 3.  I suspect that the wheel spinning issue is down to driver/tyre pressure. However much truth there is in the above, most drivers are far more confident with their ability towards the end of an event than at the start. One gets to how to get the best out of your car and tyres. Stepping into a vehicle that you are not familiar with and expecting to do well is only for the likes of a few.
Jonathan L.

Offline Paul K

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Re: Weight and Tyre Size
« Reply #6 on: November 20, 2016, 08:06:47 am »
I think I've got the hang of this now  ::)

For the MCC sections, the beetle seemed ok; any deficiencies were down to the driver and a still slightly reticent accelerator pump in the carburetter (soon to be resolved).

For PCT events, lighten the car by chucking out all the spare wheels, tools, spare parts, oil, etc, but keep the passenger.

For all events, don't admit that the driver is past their best   :-[.

Offline Myke Pocock

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Re: Weight and Tyre Size
« Reply #7 on: November 20, 2016, 11:21:53 am »
Thing is Mark, in my case Ive never had a 'Best' so cannot be past it!!!

Offline Paul K

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Re: Weight and Tyre Size
« Reply #8 on: November 21, 2016, 09:46:06 am »
Perhaps your best is yet to come  :D

Offline Arron Homewood

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Re: Weight and Tyre Size
« Reply #9 on: November 21, 2016, 09:02:12 pm »
Some days we can climb everything with ease...the next day we cant get up anything...but everyone else does.

It's a funny game this Trialing...if it was easy I would not do it.

Many years ago driving home from a Trial I starting telling my brother Gary (my long term faithful passenger) what modifications I was going to do next to the car. He replied "leave the car alone, its you that needs to improve".

I was not sure whether to be offended, hit him, sulk or do all 3!

Looking back, he spoke some wise words that day.

Offline Paul K

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Re: Weight and Tyre Size
« Reply #10 on: November 22, 2016, 11:18:58 am »
He replied "leave the car alone, its you that needs to improve".

Sounds like sibling rivalry to me.