Date: Thu, 15 Feb 2001 13:10:44 -0700
Reply-To: John Klun <jklun@GJ.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: John Klun <jklun@GJ.NET>
Subject: Re: How Much Hosepower Is Lost In Wheel Size?
Content-Type: text/plain; charset=us-ascii
Derek,
Are you also taking into consideration the air pressure of the tire? The more air
in the tire, the less rolling resistance (smaller footprint)? Narrow tire vs.
wider tire? Weight distribution of the vanagon? Passenger, cargo weight (if
any)?
Derek Drew wrote:
> As some of you may know, I am preparing an article about changing to larger
> sized wheels and tires on the Vanagon, particularly with reference to Syncro.
>
> Among the issues that has come up is the limited amount of horsepower that
> the 2.1 litre motor has to push the vehicle, and how much of this
> horsepower gets eaten up driving larger tires. I have seen reference in
> various places to "increased rolling resistance" of larger tires, and the
> consequent apparent decrease of power to the driver of using larger wheels.
>
> The situation I am looking at right now is one where we are switching from
> 26" tall tires to 31" tall tires, and the question arises as to how to
> quantify the apparent horsepower percentage loss of using these larger
> tires. I can compensate for this apparent horsepower loss by making sure
> the gearing stays within a certain range or in a range similar to the stock
> range in order to stay conservative, but I have no data to go on as far as
> what the correction factor should be.
>
> In 4th gear and 4,000rpm the stock tire travels 70.95mph wherease the BFG
> 27 x 8.50 / 14 tires travel 74.32 mph. One situation I am looking at is
> whether to set the 31" tall tire so it is traveling in 4th gear at
> 77.27mph, which seems like a nice welcome bump up in effective top cruising
> speed on the highway. But if the rolling resistance is much higher with
> these larger tires on it, it is possible that it would be better to stay
> conservative and set 4th gear at 72.56mph.
>
> To put this into perspective one can chart:
>
> 70.95mph is stock
> 74.32mph is BFG
> 72.56 is the Conservative Option (better for having power to push the vehicle)
> 77.27mph is the Aggressive Option (better for higher top speed)
>
> Can I have a discussion from somebody knowledgeable about the issue of
> trying to quantify the additional rolling resistance in terms of
> horsepower? I am not sure if the sheer additional weight of the larger
> wheels and tires also degrades the apparent horsepower as well, and needs
> to be calculated separately. Possibly neither of these two elements of much
> effect and we can forget them. And can I also hear opinons about whether
> the 4000rpm-in-4th speed for this scenario should be set at 72.56mph or
> 77.27mph?
>
> I can say from personal experience that the BFGs seemed to tucker out the
> motor a little bit to my experience, and so I did not experience the
> mileage per gallon gains one might have expected from bringing the rpms
> down. That is, it is my experience that the BFGs are toward the top end of
> a good user experience. If you disagree I need to know this now because it
> will influence further calculations. Ideally, I would have set 4th and 4000
> at 81mph, but after thinking about it, I sadly feel that the stock motor
> cannot handle this very well and will poop out the benefit that would
> otherwise be achieved.
>
> I am speculating that it is possible one can make up for some or all of the
> hosepower losses by means of the ratio rocker/ECU chip and other Lilley
> mods, but I can't even address that question until quantifying the loss
> from the larger tires.
>
> Needless to say, if you already installed your subaru motor or TDI or
> whatever, you are less sensitive about this issue, and would likely go for
> the 77mph gearing or even higher is possible. Depends how much those motors
> like to be run above 4000rpm and what their power is so it is an individual
> case there.
>
> For the record, the situation here involves either 7.50 x 16 tires or
> 235/85 x 16 tires running 6.17 ring and pinions and a 4th gear of either
> 0.82 or 0.77 which are alternate to the factory 0.85 4th.
> _______________________________________________
> Derek Drew New York, NY
> CEO & Co-Founder
> http://www.ConsumerSearch.com/
> ===========================
> "Best Expert Review Site"
> for product reviews on the Internet
> Jan. 2001, PC World Magazine
> ==========================
> 80 South Street, 2nd Floor
> New York, NY 10038
> derekdrew@rcn.com
> 212-580-6486
>
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