Tire question for racers: Bridgestone GP Moto3 Slicks? - Honda CBR250R Forum : Honda CBR 250 Forums
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post #1 of 27 Old 05-17-2016, 12:35 PM Thread Starter
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Tire question for racers: Bridgestone GP Moto3 Slicks?

I have a question for you other CBR250R racers: My Bridgestone sponsor convinced me to try out a set of their GPR125 slicks on my CBR250R at Hallett this last weekend. I only got one practice session on them (and then I crashed out on my CBR600RR - cracked rib, wadded bike, concussion; lots of fun).

The tires are 120/60/17 rear / 90/58/17 front mounted on the stock OEM 140/70/17 rear / 110/70/17 front. I snuck up on them slowly in the practice session, and I never felt like I got them up to speed. But when I checked my times, at the end they were the same as my times on full size DOT tires. So I think there's some speed advantage to be gained.

My rear felt solid and planted, but the front felt "bouncy". I'm not sure if that is the result of a more flexible carcass or changes that I need to make to my suspension. (My suspension guy wouldn't help me, stating that running 125 tires on a 250 was dumb and he wouldn't be involved.)

So my questions - Is it dumb of me to even try these out for Bridgestone? If not, can I work them into the same kind of corner speeds as the full size DOTs? If any suspension changes are need to the front, what do you suggest? Any general advice?

Thanks in advance to any racers with relevant experience! (Non-racers, please don't just speculate on this topic - I know the dangers - I race.)

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post #2 of 27 Old 05-17-2016, 09:19 PM
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Too much grip can be a problem.
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post #3 of 27 Old 05-17-2016, 10:48 PM
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We wondered if we could fit a set of rains in the same sizes while at Talladega a couple of weeks ago. From what our tire guy said it wasn't a great idea, So we didn't. I would get in touch with Derek "Stickboy" Bennett on FB. He is the Bridgestone rep at all of our races and is very knowledgeable with all things tire. It's interesting that you tried it out though. Got any pic's of it mounted up?
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post #4 of 27 Old 05-27-2016, 12:35 PM Thread Starter
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Just a quick update. I found out that some very quick folks out in CA won championships and set track records using the Bridgestone GP3 slicks on a CBR250R last year. So, if they are good enough for that... I'm going to try them, but haven't been back to the track yet (still healing from my introduction to the tire wall in turn 1 at Hallett two weekends ago).


However, the lower profile of the tire makes the bike lose height and requires some geometry changes. Raising the front is pretty easy - just move the clip-ons from above the triple to below it, and drop the forks by 16 centimeters or so. That should give me the lost ground clearance, and also help with a little of "jittery" feeling I had by increasing the rake a little.


But raising the rear looks harder. I'm checking with my suspension guy, but right now I'm not sure how to do it (other than buying a longer shock). Any thoughts would be appreciated. Also, how much do you increase the shock length to get a 15mm or so overall rear height raise?


Tyand - I have run the same size rains during a race, and they seemed fine - although I'm not fast enough in the rain to push the tire to its absolute limit. I'll try to post pics once I get everything set up - hopefully for NOLA in a couple of weeks!

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post #5 of 27 Old 05-30-2016, 02:56 PM
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I would jack the bike so that the rear suspension is unweighted and off the ground. Put a piece of tape on the bodywork 90 degrees above the axle and make a horizontal line as a reference. Put a jack under the swingarm and unweight the swingarm so you can remove the lower shock mounting bolt. Then drop the jack so the swingarm is 15mm below the reference mark. Now measure how far the shock raised in respect to the mounting hole. It is usually around a 3 or 4 to 1 ratio.
You could oval the holes in the shock mount to raise it some what.
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post #6 of 27 Old 05-31-2016, 04:06 AM
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Buy some adjustable dog bone links

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post #7 of 27 Old 06-06-2016, 05:25 AM
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Very interested to see the responses to this as our local racing is on a "short circuit" 1 mile layout, very tight and twisty and sharp handling is essential.
CBR150s do very well running Moto3 slicks, never thought about it on the 250 though.

Any further developments to report kfox?...

Sorry, I know you warned against speculation but here goes anyway. What about addressing the loss of ground clearance simply by raising your rearsets 20mm?... that'll save the hassle of lengthening the rear shock.

Also, the reduced front end trail should make turn in and steering super sharp... would this not be one of the major benefits of running the thinner moto3 slick?... why negate this by lengthening the front end back to stock trail?
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post #8 of 27 Old 06-06-2016, 04:55 PM Thread Starter
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Not much further to report. I'm going to try them June 17-19 at NOLA. The GP3s lower my front by 7.5mm and my rear by 12.5mm (assuming my math is right - OD of GP3s are 576mm front, 602mm rear; OD of normal race tires are 591mm front, 627mm rear). I can gain the 7.5mm in front by simply lowering the forks in the triple. (The Vortex clip-ons are about 7mm slimmer than the OEM, so no need to even put the clip-ons below the upper triple.) I'm not yet sure how I'm going to gain back the rear - I have YSS rear shock and YSS USA has been completely unresponsive to my inquiries. Lemmy99's suggestion of adjustable links is interesting, but I haven't looked into it yet. Another option is just to replace the rear shock with something that has actual after-market support. (Pretty frustrated with YSS right now.)


Raising the rearsets will help with dragging my foot in the corners, but doesn't put the bike back to my original geometry. The steering is now super sharp - but a bit too sharp, almost jittery. I'm not sure if that's geometry or just a narrower tire. I know that in the stock geometry the bike handled better than any other bike on the track.


I'll report again after NOLA.

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post #9 of 27 Old 06-21-2016, 03:58 PM Thread Starter
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The after NOLA report - I was able to practice on the GP3s all day Friday. Great weather, great track. The GP3s are fantastic, even without the geometry changes yet. Great feel and grip, even at full lean. The steering is really touchy, but once I got used to it, I was running well. A little bit of tearing in my front tire, right at the edges. Tried different pressures to compensate, but nothing stopped it. Not sure what the problem is - maybe I need to play with suspension, but most likely I'm having geometry problems (front is either over-weighted or under-weighted; I'm not positive which) - here is a great guide on tire tearing Race Track Motorcycle Tyre Wear Guide - Types and Causes.


Then the engine blew during practice Saturday morning. I've already shipped it off to SmBkRdr for analysis and repairs. Hopefully it'll be ready for racing at Cresson July 22-23.

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post #10 of 27 Old 06-24-2016, 02:59 AM
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A good tires for racers must be a racing slick that have a smooth thread. As far as reviews are concern, Bridgestone has a good performance and has one of the most advanced tires.
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