effect of running a 38R and 43F on HTA

[Ron]

I am going to try running a 38 R, 43 F GravelKing SS, on my Moots just to see the difference in handling in more loose and washboard. I probably wont run that all the time but after some emails with Moots, they suggested I give it a try. I cant find any calculators on the effect on HTA's. Just wondering if anyone knew. I'm just curious

Fwiw, On CO gravel, 38's seem like the near perfect tire width, actual of 39.5 @ 38psi on 23ID rims. I run that for a mixed ride of tarmac, to mag chloride to dirt/stone rides. GK SS's are my fav' tire!

 

[scott43]

I mean...1/10th of a degree?

 

[Ron]

lol, I dont know.

 

[Jersey Skier]

Maybe you'll be 1/10th of a MPH faster. Or maybe 1/10th of a MPH slower.

What if it's better? Than maybe 45mm is more better. And 47mm even better...................................see fat bike thread.

 

[Ron]

its not speed, its Head Tube angle. Its just a question. I dont think it has any significant impact but It does have some, (and effects trail) and I Was interested in the math behind it. OMG.

 

[scott43]

It's a bit funky with fork offset but pretty close to 1/10th of a degree I'd say.

 

[Ron]

Thanks, I have been swapping emails with Moots lately and this is as a result of some things to try for some of our roads that have loose sand and gravel. the discussion of HTA was discussed and without too much typing, you can change your HTA with larger (taller) front tires vs smaller (shorter) tires in the rear but again, we were discussing there was no calculating tool they knew of. Wolftooth makes a new Headset that can increase or decrease HTA by1* but they really are for small road/gravel frames or MTN bikes.

 

[scott43]

I don't think you'll notice. You're talking 2.5mm higher in the front. That's a very small distance. I'm more interested in how the handling is with the smaller tire in back..reminds me of my XC days..

 

[Doug Briggs]

Too many TLAs. Not enough defs.

 

[scott43]

OMG = Oh My Goodness

MPH = Miles Per Hour
MTN = MounTaiN

Oh and HTA = Head Tube Angle

 

[Ron]

or if you're from the north east. "Oh-my-gawd".

 

[Doug Briggs]

I get it. I geek in other arenas. I'll crawl back under my rock now.

 

[Ron]

I don't think you'll notice. You're talking 2.5mm higher in the front. That's a very small distance. I'm more interested in how the handling is with the smaller tire in back..reminds me of my XC days..

I doubt it too. Lots of people ride with slightly smaller rear tires here. I doubt Ill like it enough to keep a 43 on all the time, it will spin up slower and is probably not as quick feeling and although its only ~70 grams heavier, its rotational weight. Its worth a try when Im riding more loose roads but I really like the ride and feel of the 38's. seem to be the sweet spot for CO gravel and tarmac rides.

 

[Doug Briggs]

But are they running different size tires to affect geometry or to enhance contact?

 

[scott43]

I get it. I geek in other arenas. I'll crawl back under my rock now.

The idea is a wider front tire will increase the height of the front axle relative to the back which effectively slackens the head tube Angle ever so slightly. I don't know that most people could feel half a degree let alone a tenth. Worth a try.

 

[Ron]

View attachment 128646

But are they running different size tires to affect geometry or to enhance contact?

according to my contact at Moots, its really both but most actually do so because of limits on older frames that won't clear a 38 or larger tire. In my case, I can clear a actual 40 tire but I also would like to try the 43 to see how I like them vs a 38 and for roads that I know have loose sand or sections of near single track. My HTA is 72 so I dont really need a slacker head unless I was riding more rowdy roads/trails. the 72 rolls really nicely on mixed surfaces and class 2-3 gravel. the only time I wish it was more slack was on some descents with washboard or loose rocks/sand but skills can cover a lot of that, and running the tires at a lower PSI. My gravel king SS's sit at just under 40 at 36psi (39.65) and really do very well on 90% of terrain.

 

[scott43]

View attachment 128646

But are they running different size tires to affect geometry or to enhance contact?

Contact generally. The geometry change could be net good or bad but it's a very small change. In loose dirt the front can dig in so more float can be good there.

I've done some gravel on my road bike with 25's and typically digging in the front is danger 1 followed by the annoyance of losing traction on climbs.

 

[Ron]

I don't know that most people could feel half a degree let alone a tenth. Worth a try.

100% agree! not sure why im seeing "new" geo's with 71.5. Just go to 72 or 71. My problem with what Im seeing in new gravel Geo's is that they seem to be built for riding single track or bike packing.

 

[Ron]

I've done some gravel on my road bike with 25's and typically digging in the front is danger 1 followed by the annoyance of losing traction on climbs.

not mention the bone rattling after 15-20 miles of rough road. a wider tire at lower pressures will soak up a huge amount of vibration and hits. my neighbor I rode with today has 32's on his bike and I had him ride mine with the 38's, not only do the 38's roll faster but he couldn't get over the difference in ride quality. he's literally putting 38's on his bike today. I gave him a set of Gravelking SS's .

 

[scott43]

Yeah and I'm not suggesting 25s are the best tool for gravel... Even for casual road riding. It's just the tool I have on my rides with the occasional gravel bit..