SLROC Forum

Best size tyre/wheel for comp safari racing?

cheers

dave

Most people will use 205/16 as that's what size the vast majority of Diamonds come in!  This also depends on the surface as quite a few people run a different tyre for somewhere like Forrest (gravel) as the rest of the rounds (finest Scotch clarts).

A 205 will spin up, and have less gyroscopic effects, than a 750.
Alloys are lighter (see above) and used to be the preferred rim.... however they shatter rather than bend, and after a couple of nasty incidents, people have returned to steel rims.

As for treads.... the limits are defined by the blue book (Section L, List 5) and the event regs (ATs for the BCCC, MTs for the Borders Hill Rally, and Agressives for the SCCC).

As Mac says.... the "best" tyre is often determined by the terrain, and as the SCCC uses a wide variety of terrains, there is no single "best", there is just the least compromised... and thereafter it comes down to driver's preferences & driver-style.

Walk around the pits, and speak to people

what about tyre make??

cheers

dave

Dave
1.

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Apart from the good advice from Ian regarding eligibility for different events I think your choice may be influenced by your budget....and whether gearing or unsprung weight is an issue......
You could go for braid alloys or split rims with bead locks but braids will set you back about ?150 a corner.  You could stick BF Goodrich Mud terrains on these at another ?140 a corner for 235/75R16 for example.  Expensive perhaps cos you will need spares....
Or you could do what we do and buy a secondhand set of steels or alloys off ebay for ?10 to ?20 per rim and stick a set of diamonds (Maxsport, Kingpin or Bronco if you can get them) at ?50 each or ?25 echa like we did and race on them for 3 years....Doesn't hurt the wallet quite so much when you bend them!
2.

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I think your choice may be influenced by the weight of the car or where the engine ie weight distribution is.  trial and error.
3.

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I think your choice (of wheels) will be influenced by the axles you have and whether you think that wheel spacers are required or a good idea. ie for wider track (stability or caliper clearance). Disco2 wheels are 5x120 PCD and the defender type are 5x165 or something like that.  centre bore of the wheels may be different too.  Could also depend what brake calipers you intend to use for clearances etc.
Good luck!
In general most folk seem to use 206 R16 diamonds or 235/75R16 directional diamonds like the Insa Sahara turbo although diamonds have arguably the toughest sidewalls although they weigh a lot.  Hence 205 rather than 236 or 7.50R16 to save weight and lower gearing.

Among many offer places.
www.kingoffroad.co.uk/ortyres.html
www.tyresdirectuk.co.uk
www.camskill.co.uk
http://www.stores.ebay.co.uk/JCS-4x4-Wheels-and-Tyres

george

Among many offer places.
www.kingoffroad.co.uk/ortyres.html
www.tyresdirectuk.co.uk
www.camskill.co.uk
http://www.stores.ebay.co.uk/JCS-4x4-Wheels-and-Tyres

george

thanks for everybody's input,  these websites will also be very useful

cheers

dave

Teamwasp -

Thanks for some great info Brian, I can give some addtional info on parts been used in my build so far

disco 2 axles strenghten tomcat way/standard disco 2 calipers and disc's

Mach 5 Competition Off Road wheels

Gearing - 5 speed tremec close ratio gearbox, 4th at full revs will be just over a 100 mph

weight of tomcat - light as possible, using the gm ls3 alloy engine, 180 kg approx, engine/gearbox will get mounted as far back as possible, but not sure exact placement yet

like you say, it will come down to trial and error

many thanks

dave

whether you think that wheel spacers are required or a good idea

No wheel-spacers please.... not allowed (SCCC regs, and [if memory serves] blue book too)

Hi Dave
Looking forward to seeing your finished racer and having some new competitors and previous chaps returning to the SCCC for 2012.

You are right to strengthen the D2 axle with the external bracing and internal tubing from Tomcat.  That is what I need to do the next time as I'm needing a new D2 front axle cos I seem to bend them quite easily with my 5.7 chevy mounted 8" back from standard.  You might be advised to fit Ashcroft shafts and CVs which are about ?600 for the front axle.  My 290bhp hasn't managed to break either the CVs or shafts in three years so I would recommend them.  I run standard rear shafts.
You will probably know about uprated diffs as well.  Various ratios are used and also different ratio transfer boxes too.  You may find that 4th at 100mph is too high geared for SCCC but might better suit gravel rallying (otherwise known as the BCCC). SCCC can be rough and twisty and acceleration in the lower gears is essential.  Uprating the diffs can be with Quaife or Eaton Detroit true trac LSDs or locker.  Some people run open diffs and 4 pin are popular but for bigger bhp an LSD is stronger.  You really don't want to be changing a diff at a race so better to put in the best you can afford.  Steve Haworth at Crown Diffs was a big help to me and supplied/built the two 4.11 ratio eaton LSDs that we run.  They are pegged too.  Without pegging, the crown wheel can skip off the pinion and strip a few teeth.  therefore you need and new crown wheel and or pinion.  With pegging the crown wheel and pinion are kept in constant mesh and if something goes wrong under stress, loads of the teeth will be broken.  So you still need a new crown wheel and pinion anyway but it is much less likely to break.  Depends on the bhp and planted right foot and whether you are airborne a lot and land heavy!
Steve Haworth also built our front diff in reverse so the drive is on the stronger side.  Arguably a rover faux pas.  Something worth considering. 
Totally digressed off your original tyre question.  Still think that 206R16 Diamonds or 235/75 directional diamonds are the best all round choice.  We ran a couple of BFGoodrich MT's but they clog too easily in the deeper mud compared to the Kingpin highlander diamonds.
Have you decided on 100" wheelbase with front engine or 106" wheelbase with rear engine?
Cheers Brian

Thanks for the great info Brian, and I dont mind you digressing atall

Paul at Tomcat, recomended the ashcroft stuff, so will be going with ashcroft shafts/cv's and the stronger diff internals(hd ring and pinion), pegged etc, not fully decided on which lsd, I was thinking either Quaife's ATB or Ashcroft ATB(Automatic Torque Biasing), it all seems to cost a fortune, but do it right the 1st time, will save me money in the long run, interms of diff ratio just not sure, could be abit of trial and error, but as you suggest maybe start looking at 4.11 and above

yeah going with the 100 inch chassis and therefore front mounted engine, Iam lead to believe if you use a light engine, mount it back the 8 to 10 inch, you can still achieve a 50/50 weight

what is your top speed in 4th gear? and what rpm is your chevy running up to?, I based my 100mph on using a 4.11 and 1.2 transfer box and 6000 rpm, but it will rev to 6600 rpm

here's each gears top speed, reving to 6000rpm

1st - 34
2nd - 52
3rd - 78
4th - 100

how does that look??

many thanks

dave

Hi Dave
The October issue of LRO International magazine has a two page article on 'Choosing a Diff Locker' and mentions Quaife ATB, Ashcroft and eaton.  Air lockers and electrics are best left for the challenge guys I would suggest.

I originally ran 3.54 diffs and when being rapidly overtaken by David Bartlett (Scottish Champion) one day decided to investigate his ability to rapidly accelerate out of corners.  4.11 diffs said Alan Agnew.  So I went and got a couple.  Big improvement.  But David still thrashed(es) me out of corners so I enquired again. 4.7 diffs said Alan. Damn!  Davids car is rapid!

Sorry I can't comment on your in gear speeds as that is not my forte.  You see we 'race' for fun and enjoy looking at the scenery and interesting flora and fungi.  If we go too fast we might miss something.  By the same token I haven't time to look at a speedo or rev counter.  Which is just as well as we don't have either....

Apparently with the 4.11 diffs my car is geared to around 90mph flat out, according to some info I had given to Steve Haworth.  Prior to this with the 3.54 he reckoned it would be geared to 110mph.  I still find it high geared and have only managed to have it flat out at the Borders Hillrally at Forrest.  Never had it flat out in a comp.  I don't find the bent front axle inspires much confidence in the cars straight line ability!

Back to the tyre thing.  Colin who runs an LS3 in his 2011 championship winning car uses split rims and bead lockers.  The lockers are meant to retain the tyres even if punctured which may give some control and steering and may save the rim.  You do find that with a puncture the car is about as useless as a two wheel drive if its muddy.  We used to get a lot of punctures and following some good advice (DB) upped the pressures from 30psi to 45psi which has reduced the number of punctures remarkably.  Quite funny that the only one so far this year was on the drive round lap at Drumclog2.  And it was a new tyre!  haha.

At the time you enquired Brian we were running on 4.11 diffs, but then you must have started catching us and we moved down a ratio to 4.7! Bouncing off the rev limiter in 5th is just touching 100mph, but as you said, in our championship you only get to that speed once a year, on one bit of straight track, at Forrest Estate and its blooming scary!

The MSA have been doing surveys of speeds at events (so far, we've managed to keep the MSA, and their Cross Country Committee, out of our events)... however they have sampled speeds at BCCC & AWDC comps, and the Borders Hill Rally.

Amusingly, when asked how fast they were going, drivers responded with "Oh, well over the ton" and other such things. The BCCC is generally reckoned to be the fastest events, and the fastest car at Sweet Lamb was tagged at 92mph with a speed gun, at the fastest point on the course. The driver reckons he was doing 110

As others have said.... top end is mostly irrelevant. There are three main criteria for good lap times:

• How fast you can accelerate out the corner (600bhp is no good if you have a 5-second turbo-lag),
• How well you can grip when accelerating out a corner (300bhp is no good if all you are doing is spinning your wheels), and
• How well you can ride the rough (flat out on the flat is fine.... when its flat - Cross Country is rarely flat)

Ian, from personal experience I would have to add a fourth criteria to good lap times.  Staying the right way up...haha.  I did however experiment with this at Bathgate by rolling it on my side first, rather than Patricks which is the norm (although after 270 degrees it did finish up on his side....).

I agree that top speeds are fairly irrelevant as we are not the BTCC on tarmac etc.  I think to be honest the highest speed my racer obtains is usually on the truck to and from events....