Lesta3474 Posted January 7, 2009 Report Share Posted January 7, 2009 Which is easier to spin? A 17 inch diameter wheel weighing 9.6 KG vs a 18 inch diameter wheel weighing 8.7? The reason I ask is because I'm thinking of upgrading my car to light weight racing wheels but not sure if I should bump up to 18 inch but still be lighter then my present wheels. If its gona reduce performance, then I'll go for the 17 inch racing wheels which weight 7.7 kgs. Anyone can shed any light on the calculations involved regarding rotational inertia? Quote Link to comment Share on other sites More sharing options...
Ira Posted January 7, 2009 Report Share Posted January 7, 2009 To be absolutely sure you'd need to know the distribution of the weight, not just the total weight since I'm pretty sure it's not going to be a constant density across it. But let's assume it is for both(They'll have similar weight distributions I think, at least) IIRC the momentum in this case would be Momentum=1/2radius*weight So, ignoring units just to get a relative number(I can't be bothered converting into the right units to get newton meter seconds)...The 17 inch will be 81.6 whatevers, the 18 will be 78.3 and the 65.45. Basically, don't be a ricer, stay with what you have and put the money into something that will have more than a tiny effect. Quote Link to comment Share on other sites More sharing options...
Lesta3474 Posted January 7, 2009 Author Report Share Posted January 7, 2009 Righto thanks for that. I'm just trying to look at it at every angle possible before I blow money on wheels. From what I can read on the internet, 1 pound reduced in the rotational weight is roughly 3-4 pounds of the car''s static weight and that every 10lbs reduced in static weight = 1 HP gain... mulitply that by 4 wheels and it would be a significant increase? So if I were to go for the same wheel size but lighter wheels, I'd be reducing 2kgs or 4.5lbs of weight on each wheel. 4.5lbs X 4 wheels X 4 = 72 lbs. This is 7.2 HP gain so I think its worth it? Then again you can't believe everything you read on the internet. :lol: If I went 18's then rotational inertia plays a big part? Its all so confusing to me. :-? Probably the reason why I was interested in the calculations. Thanks again Ira! Quote Link to comment Share on other sites More sharing options...
Ira Posted January 8, 2009 Report Share Posted January 8, 2009 There's a huge amount of other factors that come into play that makes that 1 pound = xxxx a VERY vague guideline. And if your goal is to have fun rather than Mod=Warren: Swearing removed, warning sent factor I'd say save the $1000 on wheels and spend it going to Manfield for an open day every once in a while, I assume they still have them. Quote Link to comment Share on other sites More sharing options...
Warren Posted January 8, 2009 Report Share Posted January 8, 2009 I'd be less worried about the inertia and more about the increased cornering grip. You should find the 18's corner better and will suit tighter slower tracks like Taupo better. To get the best you'll need to rework your suspension geometry as well as the 18's will be lower profile and deform less requiring less negative camber compensation in the corner. Quote Link to comment Share on other sites More sharing options...
suphew Posted January 8, 2009 Report Share Posted January 8, 2009 I think the law of diminishing returns applies. If you haven't done much else to your car, there are far far better ways to get far more performance for far less. If your at the point of eking out the last bit of performance then go for it. I was told once by a Porsche specialist mechanic that you need to gain round 10% in power before you can really 'feel' any difference. So if your driving a stock 1973 Datsun 1200, then 7.5hp might make a huge difference, otherwise unless your looking to shave a thousand of a second off a 1/4 mile your wasting your time. Quote Link to comment Share on other sites More sharing options...
Ira Posted January 8, 2009 Report Share Posted January 8, 2009 I doubt most people would even notice 10% unless it's accompanied by an increase of at at least 30% mental horsepower. Quote Link to comment Share on other sites More sharing options...
Warren Posted January 8, 2009 Report Share Posted January 8, 2009 Agree totally - if it's for road use then there will ne very little if any noticable difference. If it's for on the track then it could be very worthwhile - it could easily be the difference between 1st and 2nd or worse still, 1st and 5th... It will also depend on the track. If it's just road use then you might find it makes a difference of a few minutes to a 3 hour trip if you keep to legal speeds and a few minutes more if you don't. Not really a lot of gain for the $$. 18's will also be noisier and give a harder ride. You have to weigh up the whole comfort factor as well (coming from someone who used to run 4 x 48mm sidies with no aircleaners and no firewall soundproofing - also no heater cause it got in the way of the engine). If you're young this probably isn't important to you... Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.