Login required to started new threads

Login required to post replies

Prev Next
Re: Chipseal Crr test... interesting result [rruff] [ In reply to ]
Quote | Reply
rruff wrote:
lanierb wrote:
rruff wrote:
Oh, I'm sure. As I stated earlier I dropped down to 100 psi with those tires later. We weigh the same. How did you arrive at 93 psi?

I did what you did -- took my known CdA that I've tested a zillion times and then ran a section of the course over and over at different tire pressures.


That's great! I guess you lived close? And trail got the same result (edit: different course I guess...); very nice to have confirmation. How low a pressure did you try and was it significantly smoother?

Adjusting tire pressure to find the optimum isn't so tedious as adding in different sizes. I may get ambitious enough to try that.
I actually just went there the day before and did tire testing during my openers ride. I know I have the data somewhere but didn't find it in a quick search to get you the exact numbers. I honestly don't remember how low I went other than it was lower than I thought was safe to run in the race, just to see what would happen. I do remember that lower pressure made hardly any difference -- ~0.0001 higher CRR. I presume there was a tradeoff where it was better in some parts and worse in others. Higher pressures (than 93) were a lot worse. It's different from rollers where lower is predictably slower at any pressure. The road was pretty rough though.
Quote Reply
Re: Chipseal Crr test... interesting result [grumpier.mike] [ In reply to ]
Quote | Reply
grumpier.mike wrote:
That can depend on the tire as well. I don’t know what Conti did to their 700x20 GP4000, but that was an incredibly slow tire compared to the 700x23. Too bad. That tire looked great on the old Specialized and HED trispokes, but it was almost Gatorskin slow.

At least at testing on a wooden track and on a smooth drum „our“ GP4000II in 700x20 are not that bad. And in the velodrome the aerodynamics were good.
Quote Reply
Re: Chipseal Crr test... interesting result [lanierb] [ In reply to ]
Quote | Reply
lanierb wrote:
rruff wrote:
lanierb wrote:
rruff wrote:
Oh, I'm sure. As I stated earlier I dropped down to 100 psi with those tires later. We weigh the same. How did you arrive at 93 psi?

I did what you did -- took my known CdA that I've tested a zillion times and then ran a section of the course over and over at different tire pressures.


That's great! I guess you lived close? And trail got the same result (edit: different course I guess...); very nice to have confirmation. How low a pressure did you try and was it significantly smoother?

Adjusting tire pressure to find the optimum isn't so tedious as adding in different sizes. I may get ambitious enough to try that.

I actually just went there the day before and did tire testing during my openers ride. I know I have the data somewhere but didn't find it in a quick search to get you the exact numbers. I honestly don't remember how low I went other than it was lower than I thought was safe to run in the race, just to see what would happen. I do remember that lower pressure made hardly any difference -- ~0.0001 higher CRR. I presume there was a tradeoff where it was better in some parts and worse in others. Higher pressures (than 93) were a lot worse. It's different from rollers where lower is predictably slower at any pressure. The road was pretty rough though.

" 'Tis far better to err on the side of too low of pressure, than too high" - me ;-)

http://bikeblather.blogspot.com/
Quote Reply
Re: Chipseal Crr test... interesting result [rruff] [ In reply to ]
Quote | Reply
This is really great work and fun to read!!

We have a bit of a file on this topic, if you search 'bituminous asphalt' in the research journals you can find some pretty great info on the different strategies employed by different municipalities, private contractors and even countries that can have some really big effects on Crr. I've talked to a lot of civil engineers and some course contractors over the years in an effort to better understand and here are the basics:

In an attempt to maintain flexibility at cold temperatures, this stuff is generally modified with plasticizers, rubber, and other stuff that makes it very high hysteresis when warm or hot. Some formulations are still pliable at -20C/-5F and have the consistency of a gummy bear at 90F. Different strategies are also applied in terms of thickness of the coating, I've seen it nearly 2mm thick in some applications, and when thick, it behaves very visco-elastically with damping coefficient in the 0.8+ range.

The rate of hardening is dependent on chemical makeup and heat, but it will continue to harden asymptotically approaching some maximum over it's lifetime.. it's likely 95+% hard at 1 year according to most.

Ironically, you are seeing the benefit of the roughness of the coarse stone chips on that part of the road.. the seal cannot build on top of the stone chip and rather drains into the gaps, leaving a much thinner effective coating that you are riding one.. so effectively you are decreasing surface hysteresis at the expense of increased surface impedance. The rather thick coating on the side of the road will be due to the large fill size of the stone (which is cheapest) and then needing more bitumen to fully fill the negative space and best stick it all down.

The strategy for both of your surfaces is to present a larger contact patch with lower pressures at the interface.. so yeah, bigger tires, lower pressures!

There were some surfaces laid down/sprayed/repaired for Tokyo that presented challenges in this area and were not this bad, but presented some challenges/opportunities to exploit...and now with the delays will be quite a bit faster as the games will be run 14-15 months after the work was complete!

http://www.SILCA.cc
Check out my podcast, inside stories from more than 20 years of product and tech innovation from inside the Pro Peloton and Pro Triathlon worlds!
http://www.marginalgainspodcast.cc
Quote Reply
Re: Chipseal Crr test... interesting result [joshatsilca] [ In reply to ]
Quote | Reply
So your summary was roughly "bigger tires lower pressure" (which was what I was getting at since the OP's original test was 23's at 120 psi). While I understand it is all rider plus pavement specific dependent, I guess the hard part is knowing how wide to go and how low to go, because in most cases we cannot field test before our race (we just show up and the pavement is the pavement). So with really wide tires you are trading off aero vs Crr so you're stuck having to guess the tradeoff, not really knowing what the pavement is really like, and you're also maybe limited by rim and cutout issues too. So then it boils down to gut feel before the race since you don't have all the data before (you pick tire and width and then if you can pre ride some of the course on a Saturday before a Sunday race you then guestimate the best psi for the tire installed on you rims).
Quote Reply

Prev Next